ALBERT

Description: Quasi-static (0.0033 s −1 ) and dynamic (10 3  s −1 ) compression experiments were performed on single crystal copper along ⟨100⟩ and ⟨110⟩ directions and best-fit parameters for the Johnson-Cook (JC) material model, which is an important input to hydrodynamic simulations for shock induced fracture, have been obtained. The deformation of single crystal copper along the ⟨110⟩ direction showed high yield strength, more strain hardening, and less strain rate sensitivity as compared to the ⟨100⟩ direction. Although the JC model at the macro-scale is easy to apply and describes a general response of material deformation, it lacks physical mechanisms that describe the influence of texture and initial orientation on the material response. Hence, a crystal plasticity model based on the theory of thermally activated motion of dislocations was used at the meso-scale, in which the evolution equations permit one to study and quantify the influence of initial orientation on the material response. Hardening parameters of the crystal plasticity model show less strain rate sensitivity along the ⟨110⟩ orientation as compared to the ⟨100⟩ orientation, as also shown by the JC model. Since the deformation process is inherently multiscale in nature, the shape changes observed in the experiments due to loading along ⟨100⟩ and ⟨110⟩ directions are also validated by molecular dynamics simulations at the nano-scale.

Description: We report multiple first order magnetic transitions in TbMn 2 Si 2 as evidenced by the thermal hysteresis in the M-T data and the Arrott plots. Metamagnetic transitions are observed at various temperatures as a result of the antiferromagnetic to ferromagnetic transition of the Mn sublattice. Very interestingly, the compound shows significant exchange bias field of about 600 Oe at 5 K, which is attributed to the formation of small domains or regions with ferromagnetic and antiferromagnetic interactions. Furthermore, a large magnetocaloric effect has been found at relatively low fields at both the transition regions. Maximum magnetic entropy changes (−ΔS M ) of 7.2 and 5.4 J kg −1  K −1 have been observed at 68 K and at 48 K, respectively, at 20 kOe.

Description: Growth process and magnetic properties of PbO-type α-Fe x Se nanostructures with shape changing from nanocacti to nanopetals and then to nanosheets are investigated. With iron acetylacetonate [Fe(acac) 3 ] and Se powder as raw materials, the diffusion process of Fe atoms dominates the synthesis of α-Fe x Se nanocacti following phase transitions from FeSe 2 to Fe 3 Se 4 and finally to α-Fe x Se. When a mixed solution containing Se precursor and Fe(acac) 3 was used as the raw material, the formation of FeSe 2 and Fe 3 Se 4 can be avoided and, bended α-Fe x Se nanopetals can be prepared at 345 °C, which became flat nanosheets with a [001] preferred orientation as extending the reaction time from 1 to 4 h. No superconducting transition occurs in the α-Fe x Se (0.84 ≤ x ≤ 1.05) nanostructures due to composition heterogeneity or size effect. Magnetic measurements indicate that an antiferromagnetic component with a Néel point at about 45 K dominates the magnetic properties of the α-Fe 0.87 Se nanosheets.

Description: To develop high performance microwave noise suppressor, the microwave noise suppressors based on a microstrip line using FeCoB based magnetic thin film are presented, whose microwave noise suppression effects have been investigated. It was found that suitable low argon pressure is beneficial to the improvement of microwave noise suppression. In addition, the microwave noise suppression properties of microwave noise suppressor could be tuned by controlling the geometric dimension of FeCoB based magnetic thin film and SiO 2 dielectric layer, resulted from the ferromagnetic resonance loss and eddy current loss. The maximum power loss ratio (P loss /P in ) of thin-film microwave noise suppressor (the length, width, and thickness of FeCoNiB thin film are 25 mm, 10 mm, and 250 nm, respectively) achieves 0.75 at 3.4 GHz. These results show that the presented film noise suppressors have potential for the electromagnetic interference design in the GHz frequency range.

Description: We study the dynamics of skyrmions in a metallic chiral magnet. First, we show that skyrmions can be created dynamically by destabilizing the ferromagnetic background state through a spin polarized current. We then treat skyrmions as rigid particles and derive the corresponding equation of motion. The dynamics of skyrmions is dominated by the Magnus force, which accounts for the weak pinning of skyrmions observed in experiments. Finally, we discuss the quantum motion of skyrmions.

Description: The full Heusler alloys Ni 2 MnGa 1− x Z x were prepared by rf-induction melting, with Z = Sn over the range from x = 0 to x = 1, and with Z = Zn for x  = 0.0 to x  = 0.1. Bulk magnetic properties were measured from 77 K to 450 K. The substitution of Ga with Sn is shown to strongly suppress the martensitic transition. Martensite transition temperatures decrease 25 K per percent Sn. Four Sn bearing samples with x  

Description: Hard magnetic Sm 2 Fe 17 N x nanoflakes were prepared by surfactant assisted ball milling (SABM) at low temperature using liquid nitrogen cooling (surfactant assisted cryomilling). 2-methyl pentane was used as a milling medium and oleylamine as a surfactant. The morphology, structure, and magnetic properties of Sm 2 Fe 17 N x were investigated using scanning electron microscopy, X-ray diffraction, and vibrating sample magnetometer, respectively. By comparing Sm 2 Fe 17 N x nanoflakes produced by SABM at low temperature and at room temperature, we found that the former has many interesting features: more homogeneous morphology, smaller size, especially markedly increased remanent magnetization (M r ), saturation magnetization (M s ), and remanent magnetization ratio (M r /M s ). These Sm 2 Fe 17 N x nanoflakes have great potential to be used for preparing high performance bonded permanent magnets.

Description: The spin-valley currents in silicene-based normal/sublattice-dependent ferromagnetic/normal junction are investigated. Unlike that in graphene, the pseudo Dirac mass in silicene is generated by spin-orbit interaction and tunable by applying electric or exchange fields into it. This is due to silicon-based honeycomb lattice having buckled structure. As a result, it is found that the junction leads to currents perfectly split into four groups, spin up (down) in k- and k ′ -valleys, when applying different values of the electric field, considered as a perfect spin-valley polarization (PSVP) for electronic application. The PSVP is due to the interplay of spin-valley-dependent Dirac mass and chemical potential in the barrier. The PSVP also occurs only for the energy comparable to the spin-orbit energy gap. This work reveals potential of silicene for spinvalleytronics applications.

Description: A model for the description of the transient regime leading to steady-state sound in a quarter-wavelength thermoacoustic prime mover is proposed, which is based on the description of the unsteady heat transfer in the system, coupled with an ordinary differential equation describing wave amplitude growth/attenuation. The equations are derived by considering a cross-sectional averaged temperature distribution along the resonator, and by assuming that both the characteristic time associated with heat diffusion through the stack and that associated with the thermoacoustic amplification are much larger than the acoustic period. Attention is here focused on the only mechanism of saturation due to heat transport by sound within the stack. The numerical solving of the governing equations leads to the prediction of the transient regime, which is compared with experimental results for several values of the heat power supplied to the system and for several positions of the stack in the resonator. The model reproduces the experiments quite well, notably showing that a small diminution of the temperature in the vicinity of the hot end of the stack is associated to an overshoot of wave amplitude growth, while heat diffusion through the whole stack impacts the subsequent evolution of wave amplitude leading to steady state. Additional experimental results exhibiting complicated regimes of wave amplitude evolution are provided, which are not reproduced by the present model.

Description: High pressure thermal-mechanical treatment (4 GPa) is introduced to improve hardness of a Mg-7 wt. % Li alloy. The improvement in hardness is strongly temperature dependent and the highest hardness occurs in a narrow temperature range around 700 °C. The main reasons for improved hardness are mainly related to the increased phase relative abundance of hcp phase and the formation of superfine { 10 1 ¯ 1 } compression twins in hcp-Li 0.92 Mg 4.08 phase which effectively inhibits dislocation movement during deformation process. This phase transformation is consistent with the first principles calculations. It demonstrates that high pressure treatment is an effective approach to achieve higher strength Mg-Li based materials

Description: Coupling and extinction of light among particles representable as point dipoles can be characterized using the coupled dipole approximation (CDA). The analytic form for dipole polarizability of spheroidal particles supports rapid electrodynamic analysis of nanoparticle lattices using CDA. However, computational expense increases for complex shapes with non-analytical polarizabilities which require discrete dipole (DDA) or higher order approximations. This work shows fast CDA analysis of assembled nanorings is possible using a single dipole nanoring polarizability extrapolated from a DDA calculation by summing contributions from individual polarizable volume elements. Plasmon resonance wavelengths of nanorings obtained using extracted polarizabilities blueshift as wall dimensions-to-inner radius aspect ratio increases, consistent with published theory and experiment. Calculated far-field Fano resonance energy maximum and minimum wavelengths were within 1% of full volume element results. Considering polarizability allows a more complete physical picture of predicting plasmon resonance location than metal dielectric alone. This method reduces time required for calculation of diffractive coupling more than 40 000-fold in ordered nanoring systems for 400–1400 nm incident wavelengths. Extension of this technique beyond nanorings is possible for more complex shapes that exhibit dipolar or quadrupole radiation patterns.

Description: The crystallization properties of the prototypical binary phase change material (PCM) germanium telluride (GeTe) are investigated by combining crystallization theory with ab initio molecular dynamics simulations. The temperature dependence of the physical quantities that determine the crystallization properties is calculated and analyzed. It is shown that the critical nucleus radius of a crystalline cluster is smaller than 1.4 nm when the annealing temperature is lower than 600 K, indicating an extremely promising scaling scenario. Our analysis reveals that the elastic energy, which is largely ignored in existing PCM crystallization studies, plays an important role in determining various crystallization properties and the ultimate scaling limit of the PCM. By omitting the influence of elastic energy, the critical formation energy (critical nuclei radius) will be underestimated by 41.7% (22.4%), and the nucleation rate will be overestimated by 74.2% when the annealing temperature is 600 K.

Description: Very less attention has been paid recently to the electrochemical properties of graphene films with intrinsic flat structure prepared by chemical vapor deposition (CVD). In this work, button supercapacitors were fabricated using ionic liquid as electrolytes and layer-by-layer graphene structures as electrodes. The specific capacitances of the supercapacitors increased with the increase of layer number. The areal specific capacitance of ten-layer graphene supercapacitor was 0.29 mF/cm 2 at the scan rate of 50 mV/s, which was about three times of that of monolayer graphene supercapacitor (0.1 mF/cm 2 ). The sandwiched multi-layer structures with oxide deposition further improved the device performance. However, the polycrystalline nature of CVD-grown graphene films introduced structural instability during charge-discharge process, resulting in degraded capacitive performance and cycling stability. Our results suggest that graphene films with intrinsic “in-plane” structure might not be ideal candidates for electrode materials.

Description: We report a combined study of hydrostatic pressure ( P ≤ 25 kbar) and chemical substitution on the magnetic pair-breaking effect in Eu- and Mn-substituted BaFe 2 As 2 single crystals. At ambient pressure, both substitutions suppress the superconducting (SC) transition temperature ( T c ) of BaFe 2– x Co x As 2 samples slightly under the optimally doped region, indicating the presence of a pair-breaking effect. At low pressures, an increase of T c is observed for all studied compounds followed by an expected decrease at higher pressures. However, in the Eu dilute system, T c further increases at higher pressure along with a narrowing of the SC transition, suggesting that a pair-breaking mechanism reminiscent of the Eu Kondo single impurity regime is being suppressed by pressure. Furthermore, Electron Spin Resonance (ESR) measurements indicate the presence of Mn 2+ and Eu 2+ local moments and the microscopic parameters extracted from the ESR analysis reveal that the Abrikosov–Gor'kov expression for magnetic pair-breaking in a conventional sign-preserving superconducting state cannot describe the observed reduction of T c .

Description: We investigated the effect of interlayer exchange coupling parameter on switching current density and switching time in the [CoPt-ML]/Ta/CoFeB composite free layer. The fundamental parameters for the micromagnetic model were extracted from experimental results and ab-initio calculations of the Fe/MgO and Fe/Ta interfaces. We found that the critical current density and switching current decrease with decreasing interlayer exchange coupling. It was observed experimentally that perpendicular magnetic anisotropy (PMA) increases with increasing thickness of Ta insertion due to enhancement of CoFeB/MgO interfacial anisotropy, whereas the interlayer exchange coupling strength decreases. Therefore, our modeling and experimental results indicate that the optimized Ta insertion in the composite layer leads to improved thermal stability via combined interface and bulk anisotropies, lower critical current density, and reduced switching time as compared to the composite layer without Ta insertion.

Description: Recently, tin has been identified as an attractive electrode material for energy storage/conversion technologies. Tin thin films have also been utilized as an important constituent of thermal interface materials in thermal management applications. In this regards, in the present paper, we investigate thermal conductivity of two nanoscale tin films, (i) with thickness 500 ± 50 nm and 0.45% porosity and (ii) with thickness 100 ± 20 nm and 12.21% porosity. Thermal transport in these films is characterized over the temperature range from 40 K–310 K, using a three-omega method for multilayer configurations. The experimental results are compared with analytical predictions obtained by considering both phonon and electron contributions to heat conduction as described by existing frequency-dependent phenomenological models and BvK dispersion for phonons. The thermal conductivity of the thicker tin film (500 nm) is measured to be 46.2 W/m-K at 300 K and is observed to increase with reduced temperatures; the mechanisms for thermal transport are understood to be governed by strong phonon-electron interactions in addition to the normal phonon-phonon interactions within the temperature range 160 K–300 K. In the case of the tin thin film with 100 nm thickness, porosity and electron-boundary scattering supersede carrier interactions, and a reversal in the thermal conductivity trend with reduced temperatures is observed; the thermal conductivity falls to 1.83 W/m-K at 40 K from its room temperature value of 36.1 W/m-K. In order to interpret the experimental results, we utilize the existing analytical models that account for contributions of electron-boundary scattering using the Mayadas-Shatzkes and Fuchs-Sondheimer models for the thin and thick films, respectively. Moreover, the effects of porosity on carrier transport are included using a previous treatment based on phonon radiative transport involving frequency-dependent mean free paths and the morphology of the nanoporous channels. The systematic modeling approach presented in here can, in general, also be utilized to understand thermal transport in semi-metals and semiconductor nano-porous thin films and/or phononic nanocrystals.

Description: We report on the concentration, chemical bonding, and etching behavior of N at the SiC(0001)/SiO 2 interface using photoemission, ion scattering, and computational modeling. For standard NO processing of a SiC MOSFET, a sub-monolayer of nitrogen is found in a thin inter-layer between the substrate and the gate oxide (SiO 2 ). Photoemission shows one main nitrogen related core-level peak with two broad, higher energy satellites. Comparison to theory indicates that the main peak is assigned to nitrogen bound with three silicon neighbors, with second nearest neighbors including carbon, nitrogen, and oxygen atoms. Surprisingly, N remains at the surface after the oxide was completely etched by a buffered HF solution. This is in striking contrast to the behavior of Si(100) undergoing the same etching process. We conclude that N is bound directly to the substrate SiC, or incorporated within the first layers of SiC, as opposed to bonding within the oxide network. These observations provide insights into the chemistry and function of N as an interface passivating additive in SiC MOSFETs.

Description: Capacitance-voltage (C-V) and Deep-Level-Transient Spectroscopy (DLTS) measurements were performed on Metal-Oxide-Semiconductor (MOS) capacitors fabricated on 4H-SiC with the SiO 2 layer grown by Sodium-Enhanced Oxidation. This technique has yielded 4H-SiC MOS transistors with record channel mobility, although with poor bias stability. The effects of the mobile positive charge on the C-V characteristics and DLTS spectra were investigated by applying a sequence of positive and negative bias-temperature stresses, which drifted the sodium ions toward and away from the SiO 2 /4H-SiC interface, respectively. Analytical modeling of the C-V curves shows that the drift of sodium ions in the SiO 2 layer during the voltage sweep can explain the temperature dependence of the C-V curves. The effects of lateral fluctuations of the surface potential (due to a non-uniform charge distribution) on the inversion layer mobility of MOS transistors are discussed within a two-dimensional percolation model.

Description: An analytical model for analyzing the current-voltage ( J-V ) characteristics of bulk heterojunction (BHJ) organic solar cells is developed by incorporating exponential photon absorption, dissociation efficiency of bound electron-hole pairs (EHPs), carrier trapping, and carrier drift and diffusion in the photon absorption layer. Modified Braun's model is used to compute the electric field-dependent dissociation efficiency of the bound EHPs. The charge carrier concentrations and hence the photocurrent are calculated by solving the carrier continuity equation for both holes and electrons in the organic layer. The overall load current is calculated considering the actual solar spectrum and voltage dependent forward dark current. The model is verified by published experimental results. The efficiency of the P3HT:PCBM based solar cells critically depends on the dissociation of bound EHPs. On the other hand, cells made of a blend of the conjugated polymer (PCDTBT) with the soluble fullerene derivative (PCBM) show nearly unity dissociation efficiency, and their cell efficiency strongly depends on the charge collection efficiency. The effects of carrier lifetimes on the performance of PCDTBT solar cells have also been studied. The model is also used to investigate the effect of titanium oxide (TiO x ) layer (at the back contact) on the J-V characteristics of PCDTBT solar cells. The results of this paper indicate that improvement of charge carrier transport in PCDTBT:PCBM blend and dissociation of bound EHPs in P3HT:PCBM blend are extremely important to increase the power conversion efficiency of the respective BHJ solar cells.

Description: We theoretically propose a scheme for the quadrature squeezing of the cavity field via dissipative processes. The effects of the electron-phonon interaction (EPI) on the squeezing are investigated, where the cavity is off-resonantly coupled with a coherently driven quantum dot (QD) which is allowed to interact with an acoustic-phonon reservoir. Under certain conditions, the participation of the phonon induced by both the EPI and the off-resonant coupling of the cavity with the QD enables some dissipative processes to occur resonantly in the dressed-state basis of the QD. The cavity-mode photons emitted or absorbed during the phonon-mediated dissipative processes are correlated, thus leading to the squeezing of the cavity field. A squeezed vacuum reservoir for the cavity field is built up due to the EPI plus the off-resonant coupling between the cavity and the QD. The numerical results obtained with an effective polaron master equation derived using second-order perturbation theory indicate that, in low temperature limit, the degree of squeezing is maximal but the increasing temperature of the phonon reservoir could hinder the squeezing and degrade the degree of the squeezing of the cavity field. In addition, the presence of the photonic crystal could enhance the quadrature squeezing of the cavity field.

Description: We have examined the structure of Cu filaments in Cu/amorphous-Ta 2 O 5 (a-Ta 2 O 5 )/Pt atomic switch from first principles. We have found that the Cu single atomic chains are unstable during the molecular dynamics (MD) simulation and thus cannot work as conduction paths. On the other hand, Cu nanowires with various diameters are stable and can form conductive paths. In this case, the Cu-Cu bonding mainly contributes to the conductive, delocalized defect state. These make a sharp contrast with the case of single Cu chains in crystalline Ta 2 O 5 , which can be conductive paths through the alternant Cu-Ta bonding structure. A series of MD simulations suggest that even Cu nanowires with a diameter of 0.24 nm can work as conduction paths. The calculations of the transport properties of Cu/a-Ta 2 O 5 /Pt heterostructures with Cu nanowires between two electrodes further confirm the conductive nature of the Cu nanowires in the a-Ta 2 O 5 .

Description: We present the structural and magnetic properties of Mn 1− x Fe x V 2 O 4 ( x = 0.1, 0.2, and 0.3), and investigate the magnetocaloric effect in those compounds. The ferrimagnetic spin ordering is enhanced with the Fe doping at Mn site of MnV 2 O 4 , while the orbital ordering is suppressed. Large magnetic entropy changes up to 3.8 J/kg K as well as the relative cooling power up to 110 J/kg at the field change of 0-2 T for Mn 1− x Fe x V 2 O 4 are calculated from the isothermal magnetization measurements. The large orbital entropy change of MnV 2 O 4 is suppressed by the Fe doping, while the spin entropy contribution arising from the strong spin-orbit coupling remains. Moreover, the doping of Fe broadens the temperature span of the large magnetic entropy change and increases the relative cooling power of MnV 2 O 4 by 2.4 times.

Description: In this study, the formation mechanisms of typical dislocation patterns in fatigued copper single crystals were systematically investigated and summarized. The formation of persistent slip band (PSB) ladders depends on the width of extended dislocation and the trap distance of dipole. When the actual capture distance d trap is less than the critical capture distance d critical , a group of relatively stable multipoles are obtained from many dipole segments, followed by the formation of the veins and PSB-ladder structures. Further evolution will bring about more and more ladder-like PSBs appearing in one local zone until 100% PSB, which is corresponding to the formation of developing deformation bands (DBs), and then the developing DBs will be converted into the well-developed DBs by merging ladder-like PSBs. After the secondary fatigue, the veins, PSB ladders, or walls can be regarded as the nucleation source of new slip bands (SBs). No matter how the surface slip morphologies change, the evolution of dislocation patterns always keeps the dynamic equilibrium and essentially reflects the constant deepening of strain localization.

Description: Many magnetomechanical models fit the data sets, they were originally developed from very well but are not transportable to different data sets or geometries. In order to test the portability of a previously developed Della Torre-Oti-Kadar (DOK) stress-dependent Preisach model for high strength steels, a numerical model was implemented to replicate data taken on a non-trivial geometry made of the same material. The data used for comparison were measured previously by the National Institute of Standards and Technology on a toroidal-like sample which allowed for the simultaneous application of longitudinal stresses and transverse magnetic fields. The geometry was modeled in a finite element modeling package and coupled with a DOK model via material parameters. A coupling framework was developed and B-H loops were modeled and compared to the NIST data available with some agreement. In the future, this modeling approach will be extended to incorporate more complex field and stress interactions and applied to additional data sets as available.

Description: Nanoporous graphene holds great promise in the application of filtration such as seawater desalination, gas separation, and ionic channels. In this paper, we study the mechanical properties of nanoporous graphene with different size, shape, and density of nanopore. The strength decreases as the size and porosity of the nanopore increases. However, the rough edges of the nanopore has significant influence to the strength where the blunt tip perpendicular to the loading direction has higher strength. The effective tensile modulus is only determined by porosity of the nanopore as Δ E  ∼ - p 0.64 , while the strength is determined by the size, shape, and porosity of the nanopore, for the same type of nanopore the strength scales with the porosity as Δ σ s  ∼ − p . In contrast, the effective fracture strain increases as porosity increases for small and moderate porosities. The work is a first study of the relation between mechanical properties and porosity of nanoporous graphene and is helpful to the design of high performance nanoporous graphene membrane.

Description: The early stages of InGaN/GaN quantum well growth for In-reduced conditions have been investigated for varying thickness and composition of the wells. The structures were studied by monochromated scanning transmission electron microscopy–valence electron energy loss spectroscopy spectrum imaging at high spatial resolution. It is found that beyond a critical well thickness and composition, quantum dots (width 〉20 nm) are formed inside the well. These are buried by compositionally graded InGaN, which is formed as GaN is grown while residual In is incorporated into the growing structure. It is proposed that these dots act as carrier localization centers inside the quantum wells.

Description: The strength and equation of state of NaCl were determined under nonhydrostatic compression up to 27 GPa using an energy-dispersive radial x-ray diffraction technique in a diamond-anvil cell using the lattice strain theory. Together with estimation of the high-pressure shear modulus, it is suggested that NaCl could support a maximum differential stress of 0.980 GPa at 22.6 GPa under uniaxial compression. The differential stress rapidly drops at 27.2 GPa due to the phase transition from B 1 phase to B 2 phase for NaCl. The hydrostatic compression data of B 1 phase yield a bulk modulus K 0  = 25.6(8) GPa and its pressure derivative K 0 ′ = 5.16(20) using Pt pressure scale. In addition, a comparative study of the observed pressures from Pt scale and ruby-fluorescence scale shows that the ruby-fluorescence pressures may reflect the lower stress state under nonhydrostatic compression compared with hydrostatic compression.

Description: A variety of methods extracting spatially resolved information of solar cell parameters using luminescence imaging on silicon solar cells have been introduced in the past years. Nearly, all methods base on calibrating the local luminescence intensity to local junction voltage. The different methods, however, use different approaches to calibrate the luminescence images to local junction voltage. The assumptions made by the voltage calibration approaches are fundamental for a correct voltage calibration and hence for a correct extraction of other relevant local solar cell parameters. In this work, we review the different voltage calibration approaches and analyse the validity and the effect of the underlying assumptions carefully using circuit simulation results. We present experimental results of voltage calibrated luminescence images of a multicrystalline silicon solar cell using different voltage calibration approaches. We show the relevance of accurate voltage calibration by performing a sensitivity analysis investigating the sensitivity of local cell parameter results obtained by a method known from literature with respect to changes in the calibration constant. We conclude proposing an approach for voltage calibration which is the most reasonable trade-off between accuracy and sufficiently low data acquisition times.

Description: The role of internal field and exhaustion during ferroelectric switching was investigated by studying the dynamics of polarisation switching during the application of static electric fields at different points of the polarization-electric field (P-E) loop of a ferroelectric ceramic (PZT 5A). By simultaneously measuring polarization and strain changes during creep an insight into the evolution of the internal field during polarization switching was obtained. Electric field partial unloading tests were performed to estimate the magnitude of the effective internal field at different points of the P-E loop. Results show that the internal field increases proportionally with polarization reversal. A rate model that includes the effect of thermal fluctuations, internal field and exhaustion of nucleation sites during switching has been developed and applied to fit the polarization creep curves. The fitting of the creep curves in the sub-coercive region suggests that polarization hardening (increasing of internal field) is the mechanism controlling the polarization rate decay. At higher static applied electric fields the mechanism gradually changes and the exhaustion of the available nucleation sites becomes the main process responsible for the creep rate decay.

Description: The structural and physical properties of conducting amorphous Zn-doped SnO 2 ( a -ZTO) films, prepared by pulsed laser deposition, were investigated as functions of oxygen deposition pressure (pO 2 ), composition, and thermal annealing. X-ray scattering and X-ray absorption spectroscopy measurements reveal that at higher pO 2 , the a -ZTO films are highly transparent and have a structural framework similar to that found in crystalline ( c -), rutile SnO 2 in which the Sn 4+ ion is octahedrally coordinated by 6 O 2− ions. The Sn 4+ ion in these films however has a coordination number (CN) smaller by 2%–3% than that in c -SnO 2 , indicating the presence of oxygen vacancies, which are the likely source of charge carriers. At lower pO 2 , the a -ZTO films show a brownish tint and contain some 4-fold coordinated Sn 2+ ions. Under no circumstances is the CN around the Zn 2+ ion larger than 4, and the Zn-O bond is shorter than the Sn-O bond by 0.07 Å. The addition of Zn has no impact on the electroneutrality but improves significantly the thermal stability of the films. Structural changes due to pO 2 , composition, and thermal annealing account well for the changes in the physical properties of a -ZTO films.

Description: In support of efforts to develop multiscale models of a variety of materials, we have performed a set of eleven gas gun impact experiments on 2169 steel, a high-strength austenitic stainless steel. These experiments provided carefully controlled shock, reshock, and release velocimetry data, with initial shock stresses ranging from 10 to 50 GPa. Both windowed and free-surface measurements on samples ranging in thickness from 1 to 5 mm were made to increase the utility of the data set. Target physical phenomena included the elastic/plastic transition (Hugoniot elastic limit), the Hugoniot, any phase transition phenomena, and the release/reshock paths (windowed and free-surface), with associated strength information. The Hugoniot is nearly linear in U S – u p space. Reshock tests with explosively welded impactors produced clean results, by contrast with earlier reshock tests with glued impactors which showed gap signatures. The free-surface samples, which were steps on a single piece of steel, showed lower wavespeeds for thin (1 mm) samples than for thicker (2 or 4 mm) samples. A preliminary strength analysis suggests the flow strength increases with stress from ∼1 GPa to ∼2.5 GPa over this range, consistent with other recent work but about 25% above the Steinberg model.

Description: The recently introduced plasmonic Brewster transmission through free-standing perforated metallic screens (metascreens), which offers ultrabroadband total light transmission has been demonstrated both theoretically and experimentally. This anomalous phenomenon is attributed to impedance matching between the guided modes supported by ultranarrow straight slits and transverse-magnetic impinging waves at a specific oblique incidence. However, this impedance matching mechanism is significantly influenced by the presence of realistic substrates leading to reduce the plasmonic Brewster transmission. To circumvent this substrate influence, the author proposes to carve periodically tapered slits on metascreens to enable the impedance matching at the input and output surface and thus enhance transmission at the Brewster angle. This finding is applied to realize ultrathin perfect absorbers with a broad bandwidth of operation.

Description: The recent demonstration of single-atom thick, sp 3 -hybridized group 14 analogues of graphene enables the creation of materials with electronic structures that are manipulated by the nature of the covalently bound substituents above and below the sheet. These analogues can be electronically derived from isolated (111) layers of the bulk diamond lattice. Here, we perform systematic Density Functional Theory calculations to understand how the band dispersions, effective masses, and band gaps change as the bulk silicon (111) layers are continuously separated from each other until they are electronically isolated, and then passivated with hydrogen. High-level calculations based on HSE06 hybrid functionals were performed on each endpoint to compare directly with experimental values. We find that the change in the electronic structure due to variations in the Si-H bond length, Si-Si-Si bond angle, and most significantly the Si-Si bond length can tune the nature of the band gap from indirect to direct with dramatic effects on the transport properties. First-principles calculations of the phonon-limited electron mobility predict a value of 464 cm 2 /Vs for relaxed indirect band gap Si-H monolayers at room temperature. However, for 1.6% tensile strain, the band gap becomes direct, which increases the mobility significantly (8 551 cm 2 /Vs at 4% tensile strain). In total, this analysis of Si-based monolayers suggests that strain can change the nature of the band gap from indirect to direct and increase the electron mobility more than 18-fold.

Description: Silicon nanocrystals (SiNCs) embedded in a silicon oxide matrix were studied by 3D atom probe tomography (APT). The distribution of the SiNC diameter was found to have a mean value of 3.7 ± 0.8 nm. The elemental composition of these particles was determined by employing two different approaches: (i) The proximity histogram method and (ii) a cluster identification algorithm based on maximum-atom separations. Both approaches give very similar values in terms of the amount of P, O, and Si within the SiNCs: the mean atomic concentrations are c P  = 0.77% ± 0.4%, c O  = 12.3% ± 2.1%, and c Si  = 85.3% ± 2.1%. A detailed cluster analysis implies that, on average, a 4.5-nm SiNC would contain around 30 P atoms, whereas a 2.0-nm SiNC would contain only around 3 P atoms. Radial concentration profiles obtained for these SiNCs indicate that the P content is inhomogeneous and possibly enhanced at the boundary as compared to the interior of the NCs. About 20% of the P atoms are found to be incorporated into the SiNCs, whereas roughly 30% are trapped within the interfacial layer (with a thickness of ∼ 0.8 nm); the remainder resides in the surrounding matrix. Cluster-size dependent P concentrations support the view of self-purification in the Si nanostructures.

Description: Eight percent of Mn was successfully diluted into nonpolar ZnO films deposited by pulsed laser deposition on single crystal (100) SrTiO 3 substrates. X-ray diffraction patterns and energy-dispersive X-ray spectroscopy confirmed high crystallinity of the films and excluded unintentional magnetic doping. A unique surface domain structure was observed by scanning electron microscope and atomic force microscope, which might play a vital role to strain release induced by lattice mismatch between nonpolar (11–20) ZnO film and (100) SrTiO 3 substrate. In addition, the films showed strong ferromagnetism with a large coercivity H C  ∼ 180 Oe at room temperature. The large magnetic moment is ascribed to carrier-mediated exchange interaction between the Mn ions, where the majority of the carriers are oxygen vacancies.

Description: The usual skin effect observed in magnetically linear medium ( μ  = const.) is absent in a magnetically non-linear medium, leading to wrong predictions of the eddy current field using the classical approach. For this reason, this paper proposes a thin sheet model, improving the eddy current field description on the basis of physical ideas in the framework of the saturation wave model, which describes the dynamic magnetization of the material with rectangular hysteresis loop. Therewith, the layer-to-layer nature of the magnetization reversal is taken into account. The hysteresis is modeled by means of a static history-dependent hysteresis model. This leads to a simplified model of conducting ferromagnetic sheet, which describes magnetization of isotropic electrical steels.

Description: We studied magnetic properties and domain wall propagation in Fe 77.5−x Ni x Si 7.5 B 15 (0 ≤ x ≤ 77.5) microwires considering effect of applied and internal stresses. We observed rectangular hysteresis loops and fast domain wall propagation for the Fe 77.5−x Ni x Si 7.5 B 15 (0 ≤ x ≤ 38.75) microwires. We measured dependences of domain wall velocity on applied magnetic field for the Fe 77.5−x Ni x Si 7.5 B 15 microwires with x ≤ 38.75. In Fe 77.5 Si 7.5 B 15 and Fe 62 Ni 15.5 Si 7.5 B 15 samples, we observed domain wall velocities up to 2.5 km/s. In all studied samples exhibiting fast domain wall propagation, we observed reduction of domain wall velocity under application of tensile stresses.

Description: The critical current density J c is a crucial parameter to establish the actual technological potential of a superconducting (SC) material. Furthermore, being proportional to the SC gap parameter, it can reveal important information about the microscopic nature of the SC state in a given material. The FeAs-based class of SC materials has been a focus of intense scientific investigation lately, but direct investigation of J c by transport measurements is rather scarce in literature. For these materials, it is very interesting to map J c as a function of their distinct SC tuning parameters such as applied pressure and chemical substitution. In this work, detailed investigation of the field, temperature, and pressure dependences of transport critical current density J c for Cu-substituted BaFe 2 As 2 single crystals is reported. In this particular material, Cu-substitution has a strong magnetic pair breaking effect. However, with increasing pressure, this sample shows an almost twofold increase of T c , from 3.2 K to 6.9 K, which is followed by an increase in J c . These observations are discussed considering the presence of magnetic pinning centers in the Fe-As plane, which, in principle, could suggest effective routes to increase J c in the this class of materials.

Description: We have investigated carrier transport in SiO 2 /nc-Si/SiO 2 multi-layers by room temperature current-voltage measurements. Resonant tunneling signatures accompanied by current peaks are observed. Carrier transport in the multi-layers were analyzed by plots of ln(I/V 2 ) as a function of 1/V and ln(I) as a function of V 1 / 2 . Results suggest that besides films quality, nc-Si and barrier sub-layer thicknesses are important parameters that restrict carrier transport. When thicknesses are both small, direct tunneling dominates carrier transport, resonant tunneling occurs only at certain voltages and multi-resonant tunneling related current peaks can be observed but with peak to valley current ratio (PVCR) values smaller than 1.5. When barrier thickness is increased, trap-related and even high field related tunneling is excited, causing that multi-current peaks cannot be observed clearly, only one current peak with higher PVCR value of 7.7 can be observed. While if the thickness of nc-Si is large enough, quantum confinement is not so strong, a broad current peak with PVCR value as high as 60 can be measured, which may be due to small energy difference between the splitting energy levels in the quantum dots of nc-Si. Size distribution in a wide range may cause un-controllability of the peak voltages.

Description: The paper deals with assessment of microstress state of martensite P91 steel using three complementary techniques: mechanical Barkhausen emission, magnetoacoustic emission (MAE), and X-ray diffraction (XRD) profile analysis. Magnetic coercivity Hc and microstructure were investigated with inductive magnetometry and magnetic force microscopy (MFM), respectively. Internal stress level of P91 steel was modified by heat treatment. Steel samples were austenitized, quenched, and then tempered at three temperatures (720 °C, 750 °C, and 780 °C) during increasing time (from 15 min up to 240 min). The microstrain level ε i was evaluated using Williamson–Hall method. It was revealed that during tempering microstrain systematically decreases from ε i  = 2.5 × 10 −3 for as quenched state down to ε i  = 0.3 × 10 −3 for well tempered samples. Both mechanical hardness (Vicker's HV) and magnetic hardness (coercivity) decrease almost linearly with decreasing microstrain while the MAE and MBE intensities strongly increase. Tempering leads to evident shift of the MeBN intensity maximum recorded for the first load towards lower applied strain values and to increase of MAE intensity. This indicates that the microstress state deduced by magnetic techniques is correlated with microstrains evaluated with XRD technique.

Description: Bulk nanostructured bismuth telluride (Bi 2 Te 3 ) composite with silicon nano-crystallite inclusions was synthesized via sintering approach. The effect of the composite structure formed by the addition of miniscule quantity (5 at. %) of silicon on the thermoelectric properties of bulk nanostructured Bi 2 Te 3 is shown via a 50% drop in thermal conductivity accompanied with a simultaneous enhancement in the Seebeck coefficient. We demonstrate that the addition of silicon nano-inclusions to the nanostructured compound combined with a systematic thermal treatment beneficially reduces the thermal conductivity to less than 1.0 W/mK over the entire temperature range of 300 K to 525 K. It is shown that the combinatorial techniques of nanostructuring, nano-inclusions, and annealing are effective in reducing thermal conductivity by a significant magnitude. This low thermal conductivity is comparable to that of Bi 2 Te 3 based superlattices and significantly lower than that of bulk Bi 2 Te 3 . The technique is extendable to (Bi,Se) 2 (Sb,Te) 3 based thermoelectric alloys for enhancing the figure-of-merit.

Description: Nd-Fe-B melt-spun ribbons with optimized composition for hot workability have been hot-compacted together with low melting DyCu, DyNiAl, NdCu, and NdAl powders to enhance coercivity. Annealing at 600 °C leads to an interdiffusion of Dy and Nd at the interfaces between the Nd-Fe-B flakes and the Dy-rich phase. This modifies the grain boundary phase and thus further enhances coercivity without decreasing remanence. The higher coercivity for DyCu compared to DyNiAl was attributed to the lower melting point obtained by differential scanning calorimetry. For NdCu and NdAl, annealing was found to be ineffective.

Description: Propagation of soliton-like excitations along spin chains has been proposed as a possible way for transmitting both classical and quantum information between two distant parties with negligible dispersion and dissipation. In this work, a somewhat different use of solitons is considered. Solitons propagating along a spin chain realize an effective magnetic field, well localized in space and time, which can be exploited as a means to manipulate the state of an external spin (i.e., a qubit) that is weakly coupled to the chain. We have investigated different couplings between the qubit and the chain, as well as different soliton shapes, according to a Heisenberg chain model. It is found that symmetry properties strongly affect the effectiveness of the proposed scheme, and the most suitable setups for implementing single qubit quantum gates are singled out.

Description: Novel analytical models have been proposed in this study which extends current available fluid-structure interaction (FSI) theories for explosion induced shock loading on monolithic and laminated composite plates to sandwich composite panels, featuring core compression. The proposed models have been asymptotically validated against other FSI existing theories in low pressure range. A qualitative comparative analysis of the proposed models has been made with other existing FSI theories from the viewpoint of energy conservation. Core compression as predicted by the proposed models can be utilized for more economical, robust design of blast resistant sandwich composite structures.

Description: The effect of rapid thermal processing (RTP) on the formation of copper precipitation in p/p + silicon (Si) epitaxial wafers was systematically investigated by defect etching and optical microscopy. After RTP preannealing at high temperature (1250 °C/60 s, with cooling rate 30 K/s) followed by the 750 °C/8 h + 1050 °C/16 h low-high (L-H) two-step annealing, it was revealed that the bulk microdefects were found only inside the p + substrate, manifesting no defects generated in the epitaxial layer. However, it was found that the width of denude zone (DZ) in samples only subjected to L-H two-step annealing was narrower than that of epitaxial layer, which meant that oxygen precipitation was formed in epitaxial layer. It can be concluded that RTP was beneficial to the formation of DZ. Additionally, it was found that the width of DZ has a sharp dependence on the introducing temperature of copper contamination, that is, the corresponding equilibrium concentration of interstitial copper in the Si influence the thermodynamics and kinetics process of the formation of copper precipitation significantly.

Description: We present a molecular dynamics simulation of chemical vapor deposition of graphene. Single layer graphene growth on a Ni (100) facet was studied at different substrate temperatures, C flow rates, and C flow energies. Results show that a single layer graphene film grows through a combined deposition mechanism on a Ni substrate, rather than by surface segregation. These simulations suggest that high quality graphene deposition is theoretically possible on Ni (100) facet under high flux energy.

Description: We evaluate the effect of the recombination associated with interlayer transitions in ungated and gated double-graphene-layer (GL) structures on the injection of electrons and holes. Using the proposed model, we derive analytical expressions for the spatial distributions of the electron and hole Fermi energies and the energy gap between the Dirac points in GLs as well as their dependences on the bias and gate voltages. The current-voltage characteristics are calculated as well. The model is based on hydrodynamic equations for the electron and hole transports in GLs under the self-consistent electric field. It is shown that in undoped double-GL structures with weak scattering of electrons and holes on disorder, the Fermi energies and the energy gap are virtually constant across the main portions of GLs, although their values strongly depend on the voltages and recombination parameters. In contrast, the electron and hole scattering on disorder lead to substantial nonuniformities. The resonant inter-GL tunneling enables N-shaped current-voltage characteristics provided that GLs are sufficiently short. The width of the current maxima is much larger than the broadening of the tunneling resonance. In the double-GL structures with relatively long GLs, the N-shaped characteristics transform into the Z-shaped characteristics. The obtained results are in line with the experimental observations [Britnell et al. , Nat. Commun. 4 , 1794–1799 (2013)] and might be useful for design and optimization of different devices based on double-GL structures, including field-effect transistors and terahertz lasers.

Description: The performance limits of monolayer transition metal dichalcogenide (TMDC) field-effect transistors (FETs) with isotropic biaxial strain were examined with the “top-of-the-barrier” ballistic MOSFET model. Using a first-principle theory, we calculated the band structures and density of states of strained monolayer MoS 2 and WS 2 , and used the results in model calculations. Introducing strain moves the positions of the conduction band minimum and valence band maximum in k-space with resultant variation in the effective mass and population of carriers. Introducing 2% tensile strain into n-type MoS 2 FETs decreases the electron effective mass and, at the same time, increases energy separation between the lower and the higher valleys in the conduction band, resulting in 26% improvement of the ON current up to 1260 A/m. Whereas compressive strain results in complicated effects, −2% strain also improves the ON current by 15%. These results suggest that introducing artificial strain is promising to improve TMDC FET performance.

Description: The two-dimensional electron gas (2DEG) observed at the surface of oxide thin films and at the interface between two oxides has been widely discussed, but the mechanism responsible for this behavior is still not well understood. In this work, we study the properties of the SrTiO 3 (001) surface and show that defects are necessary in order to explain this 2DEG. We study the properties of oxygen vacancies at the TiO 2 and SrO terminated surface, and conclude they can explain the metallic behavior experimentally observed. There is a strong tendency for these vacancies to be localized at the surface, where the formation energy is less than 2.92 eV.

Description: We report on the identification of the optimum plasma conditions for a laser-produced plasma source for efficient coupling with multilayer mirrors at 6. x nm for beyond extreme ultraviolet lithography. A small shift to lower energies of the peak emission for Nd:YAG laser-produced gadolinium plasmas was observed with increasing laser power density. Charge-defined emission spectra were observed in electron beam ion trap (EBIT) studies and the charge states responsible identified by use of the flexible atomic code (FAC). The EBIT spectra displayed a larger systematic shift of the peak wavelength of intense emission at 6. x nm to longer wavelengths with increasing ionic charge. This combination of spectra enabled the key ion stage to be confirmed as Gd 18+ , over a range of laser power densities, with contributions from Gd 17+ and Gd 19+ responsible for the slight shift to longer wavelengths in the laser-plasma spectra. The FAC calculation also identified the origin of observed out-of-band emission and the charge states responsible.

Description: The bulk modulus of many amorphous materials, such as metallic glasses, behaves nearly in agreement with the assumption of affine deformation, namely that the atoms are displaced just by the amount prescribed by the applied strain. In contrast, the shear modulus behaves as for nonaffine deformations, with additional displacements due to the structural disorder which induce a marked material softening to shear. The consequence is an anomalously large ratio of the bulk modulus to the shear modulus for disordered materials characterized by dense atomic packing, but not for random networks with point atoms. We explain this phenomenon with a microscopic derivation of the elastic moduli of amorphous solids accounting for the interplay of nonaffinity and short-range particle correlations due to excluded volume. Short-range order is responsible for a reduction of the nonaffinity which is much stronger under compression, where the geometric coupling between nonaffinity and the deformation field is strong, whilst under shear this coupling is weak. Predictions of the Poisson ratio based on this model allow us to rationalize the trends as a function of coordination and atomic packing observed with many amorphous materials.

Description: This paper reports a systematic investigation on the structural and magnetic properties of Fe 2 Cr 1− x Co x Si Heusler alloys with various compositions of x by co-sputtering Fe 2 CrSi and Fe 2 CoSi targets and their applications in magnetic tunnel junctions (MTJs). Fe 2 Cr 1− x Co x Si films of high crystalline quality have been epitaxially grown on MgO substrate using Cr as a buffer layer. The L2 1 phase can be obtained at x  = 0.3 and 0.5, while B2 phase for the rest compositions. A tunnel magnetoresistance (TMR) ratio of 19.3% at room temperature is achieved for MTJs using Fe 2 Cr 0.3 Co 0.7 Si as the bottom electrode with 350 °C post-annealing. This suggests that the Fermi level in Fe 2 Cr 1− x Co x Si has been successfully tuned close to the center of band gap of minority spin with x  = 0.7 and therefore better thermal stability and higher spin polarization are achieved in Fe 2 Cr 0.3 Co 0.7 Si. The post-annealing effect for MTJs is also studied in details. The removal of the oxidized Fe 2 Cr 0.3 Co 0.7 Si at the interface with MgO barrier is found to be the key to improve the TMR ratio. When the thickness of the inserted Mg layer increases from 0.3 to 0.4 nm, the TMR ratio is greatly enhanced from 19.3% to 28%.

Description: An optimal placement of the segmented electrode for increasing the lifetime of the Aton-type Hall thruster, i.e., reducing the plume divergence, is demonstrated using a 2D3V fully kinetic Particle-in-Cell method. Segmented electrodes, embedded near the ionization region of non-segmented case and biased above anode potential, lead to an increased separation between the ionization and acceleration regions and the formation of an efficient acceleration electric field configuration as potential lens. Due to this electrode placement, the sheath near the ceramic walls of the acceleration region is collapsed and an excellent ion beam focusing is demonstrated. The potential contour pockets around the electrodes and the sheath collapse phenomenon are also discussed.

Description: Nominally undoped n-type GaN layers grown by metalorganic chemical vapor deposition on silicon substrates were investigated using Thermal Admittance Spectroscopy and Optical Admittance Spectroscopy (OAS). A defect level was observed at E c – 0.051 eV, and it is correlated with the nitrogen vacancy (N V ) which is a donor in GaN. Illuminating the samples with a monochromatic light with wavelengths ranging from 200 nm to 450 nm, the OAS spectrum was measured at different temperatures and with different excitation light intensities. A dominant peak was observed in the OAS spectrum at λ = 365 nm (3.40 eV); this is attributed to transitions from the valence band to the donor level. Our results show that the saturation level, Gm, of the photoconductance is a function of both light intensity and temperature. The photoconductance decay, after the illumination has been terminated, is non-exponential but it is fully described by the stretched exponential function. The value of β ranges from 0.78 to 0.86. The analysis suggests that the observed photoconductance decay is due to thermal emission of photo-excited carriers from the donor level.

Description: In this contribution, we demonstrate the application temperature dependent capacitance-frequency measurements ( C-f ) to n-i-p hydrogenated amorphous silicon (a-Si:H) solar cells that are forward-biased. By using a forward bias, the C-f measurement can detect the density of defect states in a particular energy range of the interface region. For this contribution, we have carried out this measurement method on n-i-p a-Si:H solar cells of which the intrinsic layer has been exposed to a H 2 -plasma before p-type layer deposition. After this treatment, the open-circuit voltage and fill factor increased significantly, as well as the blue response of the solar cells as is concluded from external quantum efficiency. For single junction, n-i-p a-Si:H solar cells initial efficiency increased from 6.34% to 8.41%. This performance enhancement is believed to be mainly due to a reduction of the defect density in the i-p interface region after the H 2 -plasma treatment. These results are confirmed by the C-f measurements. After H 2 -plasma treatment, the defect density in the intrinsic layer near the i-p interface region is lower and peaks at an energy level deeper in the band gap. These C-f measurements therefore enable us to monitor changes in the defect density in the interface region as a result of a hydrogen plasma. The lower defect density at the i-p interface as detected by the C-f measurements is supported by dark current-voltage measurements, which indicate a lower carrier recombination rate.

Description: The physics of electrically detected magnetic resonance (EDMR) quadrature spectra is investigated. An equivalent circuit model is proposed in order to retrieve crucial information in a variety of different situations. This model allows the discrimination and determination of spectroscopic parameters associated to distinct resonant spin lines responsible for the total signal. The model considers not just the electrical response of the sample but also features of the measuring circuit and their influence on the resulting spectral lines. As a consequence, from our model, it is possible to separate different regimes, which depend basically on the modulation frequency and the RC constant of the circuit. In what is called the high frequency regime, it is shown that the sign of the signal can be determined. Recent EDMR spectra from Alq 3 based organic light emitting diodes, as well as from a-Si:H reported in the literature, were successfully fitted by the model. Accurate values of g -factor and linewidth of the resonant lines were obtained.

Description: The radiation-induced conductivity of some polymers was described mainly in literature by a competition between ionization, trapping/detrapping, and recombination processes or by radiation assisted ageing mechanisms. Our aim is to revise the effect of the aforementioned mechanisms on the complex evolution of Teflon ® FEP under space representative ionizing radiation. Through the definition of a new experimental protocol, revealing the effect of radiation dose and relaxation time, we have been able to demonstrate that the trapping/recombination model devised in this study agrees correctly with the observed experimental phenomenology at qualitative level and allows describing very well the evolution of radiation induced conductivity with irradiation time (or received radiation dose). According to this model, the complex behavior observed on Teflon ® FEP may be basically ascribed to the competition between electron/hole pairs generation and recombination: electrons are deeply trapped and act as recombination centers for free holes. Relaxation effects have been characterized through successive irradiations steps and have been again well described with the defined model at qualitative level: recombination centers created by the irradiation induce long term alteration on the electric properties, especially the effective bulk conductivity. One-month relaxation does not allow a complete recovery of the material initial charging behavior.

Description: The paper presents a new approach to the nature of heat effects and shear modulus softening in metallic glasses. The approach is based on the assumption that the glass contains quenched-in “defects”—elastic dipoles. Using the nonlinear elastic representation of the internal energy of glass with quenched-in elastic dipoles, we derive a simple analytical law, which connects the heat flow and temperature derivative of the shear modulus. Specially performed experiments confirmed the validity of this law. The exothermal and endothermal heat processes in glass reveal through the relaxation of the shear modulus confirming it as a key parameter for the understanding the relaxation processes in glasses.

Description: We investigate the influence of the current waveform on the efficiency and the emission spectra of white, high power InGaN light emitting diodes. We consider rectangular and trapezoidal current pulses, adjusted to provide the same number of charge carriers in the space charge region. Our measurements confirm the theoretical expectation that flattening of the pulse flank increases the power efficiency. This effect is stronger according to the current amplitude. The emission blue peak at trapezoidal pulses is slightly red-shifted compared to that one at rectangular pulses. This indicates a stronger effect of the quantum confined Stark effect for trapezoidal pulse driving.

Description: The influences of GaI 3 , InI, and TlI on the evaporation characteristics of CeI 3 have been studied over the temperature range 900 K to 1400 K using x-ray induced fluorescence. The total vapor densities, summed over all atomic and molecular species, of Ce, I, In, and Tl were obtained. Measurements of Ce were limited to temperatures above 1033 K, the melting temperature of CeI 3 . This is the highest temperature range for which measurements of the vapor pressure of CeI 3 have been made. The vapor pressure of the CeI 3 monomer above the pure CeI 3 salt for temperatures exceeding its melting point can be approximated by log 10 p / Pa = 11.24 ( ± 0.03 ) − 10,690 ( ± 40 )   ( T / K ) − 1 where the numbers in parentheses are standard uncertainties. InI and TlI were shown to modestly enhance the presence of Ce in the vapor phase, up to a factor of 5. GaI 3 produced no enhancement in this temperature range. Numerical simulations of the thermochemical equilibrium suggest the importance of both liquid-phase and vapor-phase complexes. Significant improvement to the method of absolute calibration is discussed.

Description: A low frequency and high amplitude rectangular voltage V has been applied during different increased duration to Twisted Surface Stabilized Ferroelectric Liquid crystal (TwFLC) samples in which the alignment layers of the two substrates were rubbed along two different directions between 0 ° and 90 ° . The optical bistability properties have been evaluated using the specific Mueller Matrix formalism that allows a simultaneous access, through a single-shot measurement, to different polarimetric coefficients. In this new approach, the ellipticity ϵ R and the azimuthal α R polarimetric parameters, extracted from the birefringence Mueller Matrix M R will be considered in priority. Several significant parameters, such as the horizontal offset Δ V , the degree of asymmetry DA , the characteristic area S of the hysteresis loop, are used to characterize the degradation observed into the hysteretic behaviour of the samples, for different values of ψ, at different duration T of exposure to V , before reaching the so-called stripes regime, giving a new experimental point of view concerning the evolution of the dynamic properties of the samples studied. The α R ( V ) and the ϵ R ( V ) hysteresis loops are specifically examined. Static mapping related to ϵ R ( T ) is given too. Among the different possible physical origins of the observed degradation, the in-plane anchoring energy contribution will be particularly examined, and a theoretical model is proposed that also gives access to different physical parameters, through a new approach.

Description: A 2-D vector hybrid hysteresis model for a soft magnetic composite (SMC) material is established, which is combined with classical Preisach model and Stoner-Wohlfarth (S-W) model. The rotational magnetic properties of SMC materials were studied using the vector model, and the computed results were compared with the experimental measurement. It is shown that the vector hybrid model can effectively simulate the rotational magnetic properties under low magnetization fields.

Description: We report on a novel approach to establish switchable pinning of magnetic domain walls in a nanowire with perpendicular magnetic anisotropy by a single in-plane magnetized single-domain nanomagnet positioned on top of the wire. Devices were prepared by depositing a permalloy nanomagnet on top of a nanowire formed from a Co/Ni multilayer with their long axes parallel, separated by a nonmagnetic layer. We show by electrical measurements that the domain wall pinning strength depends critically on the state of the bistable nanomagnet and can differ by more than 10 mT. We also performed micromagnetic calculations that show that the difference in pinning strength is caused by the interaction of the forced Néel wall with the nanomagnet's magnetostatic field.

Description: FeCoCd thin films with 500 nm thickness are directly prepared through electrodeposition in the sulphate bath in which glycine and citric acid were added as complex agents. The composition, structure, and magnetic of FeCoCd films were investigated as a function of Cd 2+ concentration, cathode current density, and deposition temperature. A wonderful soft magnetic FeCoCd film was prepared and its coercivity of easy axis and hard axis are 5 Oe and 4 Oe, respectively. The natural resonance frequency is about 3.0 GHz, which imply that the FeCoCd film is potential candidate for high frequency applications.

Description: Post-annealing of Fe 3 O 4 films in a CO/CO 2 atmosphere results in a significant improvement in magnetic and magnetotransport properties with values close to the single crystal bulk of M s  ∼ 480 emu/cm 3 and negative magnetoresistance of 0.05% in a field of 1 T. By using atomic resolution Z-contrast transmission electron microscopy, we show that improved magnetic properties in the annealed films are due to improved structural ordering as a result of the annealing process.

Description: Spontaneous magnetization and photovoltaic (PV) effects have been measured in (Bi 1- x Ba x )FeO 3-δ ceramics for x  = 0.05, 0.10, and 0.15. The substitution of Ba 2+ ion in the A site of the perovskite unit cell can effectively enhance the ferromagnetic magnetization. The heterostructure of indium tin oxide (ITO) film/(Bi 1- x Ba x )FeO 3-δ ceramic/Au film exhibits significant PV effects under illumination of λ  = 405 nm. The PV responses decrease with increasing Ba concentration. The maximum power-conversion efficiency in the ITO/(Bi 0.95 Ba 0.5 )FeO 2.95 /Au can reach 0.006%. A theoretical model based on optically excited current in the depletion region between ITO film and (Bi 1- x Ba x )FeO 3-δ ceramics is used to describe the I-V characteristic, open-circuit voltage ( V oc ), and short-circuit current density ( J sc ) as a function of light intensity.

Description: To improve the mechanical properties of high temperature superconductors due to the intrinsic brittleness and defects is a central problem for the application of the superconductors. This paper presents an analysis of mechanical behavior for the finite cylindrical type-II superconductors containing an ellipsoidal cavity in an applied magnetic field. After the Bean's critical state model and the method of minimum magnetic energy variation are employed, the dependences of the magnetic properties and magnetoelastic behaviors on the sizes of the cavity and superconducting sample are obtained. The results display that the induced current and magnetic field distributions in such sample are quite different from the case of a cylinder or ring, and a vertical flipping behavior of the magnetostriction curves is found in the magnetization process when the radial wall thickness changes through a critical value. Meanwhile, the overall maxima of the stress components during field descent are enhanced by a factor of 2 or more relative to those in a finite cylinder without any cavity.

Description: Mn 1.4 Ga films with high perpendicular magnetic anisotropy and high crystalline quality were grown on MgO substrates with Cr buffer layer using molecular beam epitaxy. The crystalline structure and the surface morphology of the films have been systematically investigated as functions of in-situ annealing temperature ( T a ) and film thickness. It is found that the magnetic properties can be largely tuned by adjusting T a . As T a increases, both saturation magnetization ( M s ) and uniaxial perpendicular magnetic anisotropy constant ( K u ) increase to the maximum values of 612 emu/cc and 18 Merg/cc at 300 °C, respectively, and then decrease. The morphology also changes with T a , showing a minimum roughness of 2.2 Å at T a  = 450 °C. On the other hand, as the thickness increases, M s and K u increase while coercivity decreases, which indicates there is a magnetic dead layer with a thickness of about 1.5 nm at the interfaces. The detailed examination on the surface morphology of the films with various thicknesses shows a complicated film growth process, which can be understood from the relaxation mechanism of the interfacial strain.

Description: Fe 65 Co 35 alloys are technologically relevant, especially in magnetic storage and composite permanent magnets, due to the fact that they present higher saturation magnetization per volume than any other material. Out of the various approaches undertaken for its production, mechanical ball milling remains the most common and efficient method, especially considering the large industrial scale of the applications. With the development of cost-efficient processing in mind, the influence of performing the synthesis of the FeCo alloys in air instead of the standard argon atmosphere is studied. The structural and magnetic characterization, along with the study of the oxygen content of the samples, proves that synthesizing FeCo alloys in air produce materials with nearly identical magnetic performance as their argon-milled counterpart, with the oxidation extent of the materials consisting almost exclusively of the oxide passivating layer located at the surface. In addition, no aging effect was observed in the saturation magnetization up to 6 months. It is concluded that the use of argon atmospheres, desiccators and/or glove boxes may be entirely removed from the process without affecting the magnetic properties.

Description: Crystals of the layered chalcogenide semiconductor gallium selenide (GaSe) were studied with fluorescence lifetime imaging microscopy in the frequency domain, the excitation source being a cw frequency-doubled Nd:YAG laser modulated between 25 and 50 MHz. The non-zero photoluminescence (PL) lifetime leads to a change of the relative modulation amplitude ( m ) and a phase lag ( ϕ ) of the luminescence with respect to the excitation. The data were analyzed with the polar-plot (or phasor) approach by plotting m   sin ϕ versus m cos ϕ . Data points of different spots on the sample show strong inhomogeneities and form looping structures in the polar plot. Moreover, they extend distinctly outside the characteristic semi-circle. The latter point is due to the nearly quadratic variation of the PL signal with excitation intensity, whereas the looping structures indicate the presence of energy transfer processes between (at least) two different emitting states. The analysis of the data shows that the same exciton state(s) are involved in both absorption and PL emission of GaSe.

Description: The Mn K edge X-ray absorption near edge structures (XANES) of Pr 0.67 Sr 0.33 MnO 3 film (100 nm) on (001) LaAlO 3 substrate was measured at different temperatures to probe the MnO 6 octahedron distortion and corresponding electronic structure. The absorption of high temperature paramagnetic-insulator phase differed from that of the low temperature ferromagnetic-metal phase. The temperature-dependent absorption intensity of Mn K edge XANES was correlated with the relaxation of distorted MnO 6 octahedron, which changed the crystal field acting on the Mn site and the related electronic structure and properties. At low temperature, the splitting of Mn majority e g orbitals decreased and the density of states above the Fermi level increased in the relaxed MnO 6 octahedron, as reflected by a wider separation between two sub-peaks in the pre-edge XANES spectra.

Description: A frequency-tunable current sensor consisting of Terfenol-D/PZT/Terfenol-D magnetoelectric (ME) laminate and Fe 73.5 Cu 1 Nb 3 Si 13.5 B 9 nanocrystalline alloy has been developed. Almost all ME current-sensing devices have higher outputs at resonance conditions, but this advantage is useful only for narrow bandwidth. For the purpose of broadband current sensing, a frequency up-conversion mechanism is introduced by means of nonlinearity of the field-dependence magnetostriction λ ( H ). Current sensitivity enhancement is realized by modulating the low-frequency dynamic magnetostrictive strain to its resonance conditions. This solution provides the possibility to achieve resonance-enhanced sensitivity at the power-line frequency of 50 Hz, and the capability to immune the noise floor. Experimental results show that the modulated sensitivity is increased from 48.6 mV/A to 178.4 mV/A at 50 Hz, and a small current step change of 3 mA can be clearly distinguished by amplitude or phase of the output signals. These results provide possibilities to accurately detect weak currents in the noise ambient at low frequencies.

Description: The dependence of light trapping effects in In 0.3 Ga 0.7 As/GaAs quantum-well solar cells on wavelength and incident angle is experimentally characterized and analyzed. Separation of active device layers from their epitaxial growth substrate enables integration of thin-film semiconductor device layers with nanostructured metal/dielectric rear contacts to increase optical absorption via coupling to both Fabry-Perot resonances and guided lateral propagation modes in the semiconductor. The roles of Fabry-Perot resonances and coupling to guided modes are analyzed via photocurrent response measurements and numerical modeling for light incident at angles of 0° (normal incidence) to 30° off normal. Light trapping enables external quantum efficiency at long wavelengths as high as 2.9% per quantum well to be achieved experimentally, substantially exceeding the ∼1% per quantum well level typically observed. Increased long wavelength quantum efficiency is shown in experimental measurements to persist with increasing angle of incidence and is explained as a consequence of the large number of guided modes available in the device structure.

Description: Using an analogy between dielectric breakdown and fracture of solids, this paper develops a phase field model for the electric damage initiation and propagation in dielectric solids during breakdown. Instead of explicitly tracing the growth of a conductive channel, the model introduces a continuous phase field to characterize the degree of damage, and the conductive channel is represented by a localized region of fully damaged material. Similar as in the classic theory of fracture mechanics, an energetic criterion is taken: The conductive channel will grow only if the electrostatic energy released per unit length of the channel is greater than that dissipated through damage. Such an approach circumvents the detailed analysis on the complex microscopic processes near the tip of a conductive channel and provides a means of quantitatively predicting breakdown phenomena in materials, composites, and devices. This model is implemented into a finite-element code, and several numerical examples are solved. With randomly distributed defects, the model recovers the inverse power relation between breakdown strength and sample thickness. Finally, the effect of the layered structure in a breakdown-resistant laminate is demonstrated through a numerical example.

Description: We have developed an epitaxial growth technique for Fe-based nanocrystals (NCs) on Si substrates with high selectivity of their crystal structure. Ge NCs with controlled shape and strain were initially epitaxially grown on Si substrates covered with an ultrathin SiO 2 film. Using these well-controlled Ge NCs as nucleation sites, Fe-based NCs could be formed with crystal-structure selectivity. In Fe deposition on the Ge NCs at room temperature, bcc-Fe NCs were formed, where epitaxial growth was influenced by the Ge NC shapes related to surface coverage. For Fe deposition at 250–300 °C, Fe-Ge alloying occurred without intermixing with Si. The epitaxially grown crystal structures were determined by the strain state of the Ge NCs: Fe 1.7 Ge NCs with a B8 2 structure for spherical strain-relaxed Ge NCs with a lattice constant close to that of bulk Ge, and ε -FeGe NCs with a B20 structure for flattened strained Ge NCs with a lattice constant close to that of bulk Si. All the NCs had sharp interfaces, where interfacial alloying in the Fe-Si-Ge system was well controlled. This growth technique can be used as a general technique enabling epitaxial growth of well-controlled transition metal-based films and nanostructures.

Description: To realize a granular film composed of L 1 0 -FePt grains with high uniaxial magnetic anisotropy energy, K u , and segregants for heat-assisted magnetic recording, the FePt-TiO 2 /FePt-C stacked film was investigated. The FePt-TiO 2 /FePt-C stacked film has well-isolated granular structure with average grain size of 6.7 nm because the FePt-TiO 2 film follows the FePt-C template film in microstructural growth. However, the K u value is quite low for total thickness of 9 nm: 5 × 10 6 erg/cm 3 . Exploration of the thickness dependence of L 1 0 -FePt(001) peaks in XRD spectra and cross-sectional TEM images suggest that degradation of the L 1 0 ordering appears near the middle of the FePt-TiO 2 layer. The EDX-STEM mapping reveals that Ti atoms exist within the FePt grains in addition to the grain boundary. This indicates the possibility that TiO 2 tends to be incorporated into the FePt grains and that it prevents L 1 0 -ordering of the FePt grains along the normal-to-plane direction.

Description: By analyzing the ferromagnetic resonance linewidth, we show that the Gilbert damping constant in soft magnetic Fe thin films can be enhanced by ∼6 times with Gd doping of up to 20%. At the same time, the magnetic easy axis remains in the film plane while the coercivity is strongly reduced after Gd inclusion. X-ray magnetic circular dichroism measurements reveal a strong increase in the orbital-to-spin moment ratio of Fe with increasing Gd concentration, in full agreement with the increase in the Gilbert damping obtained for these thin films. Combined with x-ray diffraction and vibrating sample magnetometry, the results demonstrate that the FeGd thin films with dilute Gd doping of up to 20% are promising candidates for spin-transfer-torque applications in soft magnetic devices, in which an enhanced damping is required.

Description: The effect of atomic substitutions on the magnetization, exchange, and magnetocrystalline anisotropy energy of L1 0 -ordered FeNi (tetrataenite) is computationally investigated. The compound naturally occurs in meteorites but has attracted renewed attention as a potential material for permanent magnets, and elemental additives will likely be necessary to facilitate the phase formation. Our density functional theory calculations use the Vienna ab-initio simulation package, applied to 4-atom unit cells of Fe 2 X Ni and 32-atom supercells ( X  = Al, P, S, Ti, V, Cr, Mn, Fe, Co). While it is found that most additives deteriorate the magnetic properties, there are exceptions: excess substitutional Fe and Co additions improve the magnetization, whereas Cr, S, and interstitial B additions improve the magnetocrystalline anisotropy.

Description: An experimental study on microwave plasma at atmospheric pressure was conducted by employing optical emission spectroscopy. Based on a microwave plasma generation device developed for nanoparticle synthesis, we studied the influence of input microwave power and gas flow rate on the optical emission behaviors and electron temperature of plasma using Ar, He, and N 2 as working gas, respectively. The physics behind these behaviors was discussed. The results are useful in characterizing microwave plasma at atmospheric pressure and can be used for improving nanoparticle synthesis system for commercial use in the future.

Description: In this paper, a novel 2-DOF permanent magnet planar eddy current damper is proposed, of which the stator is made of a copper plate and the mover is composed of two orthogonal 1-D permanent magnet arrays with a double sided structure. The main objective of the planar eddy current damper is to provide two orthogonal damping forces for dynamic systems like the 2-DOF high precision positioning system. Firstly, the basic structure and the operating principle of the planar damper are introduced. Secondly, the analytical model of the planar damper is established where the magnetic flux density distribution of the permanent magnet arrays is obtained by using the equivalent magnetic charge method and the image method. Then, the analytical expressions of the damping force and damping coefficient are derived. Lastly, to verify the analytical model, the finite element method (FEM) is adopted for calculating the flux density and a planar damper prototype is manufactured and thoroughly tested. The results from FEM and experiments are in good agreement with the ones from the analytical expressions indicating that the analytical model is reasonable and correct.

Description: In this paper, we demonstrate InAs/GaSb hetero-junction (hetJ) and GaSb homo-junction (homJ) p-channel tunneling field effect transistors (pTFET) employing a low temperature atomic layer deposited high-κ gate dielectric. HetJ pTFET exhibited drive current of 35 μ A/ μ m in comparison to homJ pTFET, which exhibited drive current of 0.3 μ A/ μ m at V DS  = −0.5 V under DC biasing conditions. Additionally, with pulsing of 1 μ s gate voltage, hetJ pTFET exhibited enhanced drive current of 85 μ A/ μ m at V DS  = −0.5 V, which is the highest reported in the category of III-V pTFET. Detailed device characterization was performed through analysis of the capacitance-voltage characteristics, pulsed current-voltage characteristics, and x-ray diffraction studies.

Description: We studied quantitative relationship between the intrinsic anomalous Hall conductivity ( σ x y ) and the uniaxial magnetic anisotropy constant ( K u ) of bct-Fe 50 Co 50 using first-principles calculation because these quantities originate from spin-orbit interaction. We found that the obtained σ x y and K u with changing the axial ratio c/a ( 1 ≤ c / a ≤ 2 ) exhibit similar behavior mainly arising from the common band mixing of the minority-spin d xy and d x 2 − y 2 states near the Fermi level which is sensitive to c/a.

Description: This study relates to the heteroepitaxy of InP on patterned Si substrates using the defect trapping technique. We carefully investigated the growth mechanism in shallow trench isolation trenches to optimize the nucleation layer. By comparing different recess engineering options: rounded-Ge versus V-grooved, we could show a strong enhancement of the crystalline quality and growth uniformity of the InP semiconductor. The demonstration of III-V heteroepitaxy at scaled dimensions opens the possibility for new applications integrated on Silicon.

Description: Fully relativistic calculations within the local spin density approximation and the generalized gradient approximation were performed to determine the local spin and orbital magnetic moments, as well as the magnetocrystalline anisotropy energy of Y 3 Ni 13 B 2 , Y 3 Co 13 B 2 , and Y 3 Ni 10 Co 3 B 2 compounds. A weak in-plane magnetic anisotropy is determined for Y 3 Ni 13 B 2 , under the assumption of a crystallographic-like magnetic unit cell and collinear magnetic moments. The calculations predict considerable c-axis anisotropy for Y 3 Co 13 B 2 and Y 3 Ni 10 Co 3 B 2 , but smaller than that of YCo 5 . The values of the magnetocrystalline anisotropy energy correlate well with both the magnitude of the orbital magnetic moment and the orbital magnetic moment anisotropy. The mixing between Co or Ni 3d states and B 2p states, observable at the bottom of the valence band of the 3d metal having a boron atom nearest neighbor, decreases the 3d spin and especially, the 3d orbital magnetic moments. Y 3 Ni 13 B 2 and Y 3 Ni 10 Co 3 B 2 were also investigated by powder neutron diffraction experiments, at temperatures between 1.8 and 249 K. The Co and Ni site averaged magnetic moments calculated in the mixed compound are in fair agreement with the values obtained by the refinement of the magnetic contribution to the diffraction pattern.

Description: To provide an ideal solution for a specific problem of gastric cancer detection in which low-scattering regions simultaneously existed with both the non- and high-scattering regions, a novel hybrid radiosity-SP 3 equation based reconstruction algorithm for bioluminescence tomography was proposed in this paper. In the algorithm, the third-order simplified spherical harmonics approximation (SP 3 ) was combined with the radiosity equation to describe the bioluminescent light propagation in tissues, which provided acceptable accuracy for the turbid medium with both low- and non-scattering regions. The performance of the algorithm was evaluated with digital mouse based simulations and a gastric cancer-bearing mouse based in situ experiment. Primary results demonstrated the feasibility and superiority of the proposed algorithm for the turbid medium with low- and non-scattering regions.

Description: Entangled solid state systems have gained a great deal of attention due to their fruitful applications in modern quantum technologies. Herein, detection of entanglement content from experimental magnetic susceptibility and specific heat data is reported for NH 4 CuPO 4 ·H 2 O in its solid state crystalline form. NH 4 CuPO 4 ·H 2 O is a prototype of Heisenberg spin 1/2 dimer system. Temperature dependent magnetic susceptibility and specific data are fitted to an isolated dimer model and the exchange coupling constant is determined. Field dependent magnetization isotherms taken at different temperatures are plotted in a three dimensional plot. Subsequently, entanglement is detected both from susceptibility and specific heat through two different entanglement measures; entanglement witness and entanglement of formation. The temperature evolution of entanglement is studied and the critical temperature is determined up to which entanglement exists. Temperature dependent nature of entanglement extracted from susceptibility and specific heat shows good consistency with each other. Moreover, the field dependent entanglement is also investigated.

Description: Slow nanocrystallization driving dynamics can be affected by the combination of two factors: sample residual stresses and sample geometry. This effect is evidenced at the initial stages of nanocrystallization of amorphous CoFeSiBCuNb magnetic microwires. Transmission electron microscopy observations indicate how crystallization at temperatures between 730 and 780 K results in a graded microstructure where the crystallization at the surface skin of the microwire, which remains almost amorphous, differs from that of the middle, where elongated grains are observed, and inner regions. However, samples annealed at higher temperatures present a homogeneous microstructure. The effect of gradient microstructure on magnetic properties has been also analyzed and a loss of bistable magnetic behaviour at low temperatures, from that obtained in the amorphous and fully nanocrystallized sample, has been observed and ascribed to changes in sign of magnetostriction for measuring temperatures below 100 K.

Description: Despite the outstanding scintillation performance characteristics of cerium tribromide (CeBr 3 ) and cerium-activated lanthanum tribromide, their commercial availability and application are limited due to the difficulties of growing large, crack-free single crystals from these fragile materials. This investigation employed aliovalent doping to increase crystal strength while maintaining the optical properties of the crystal. One divalent dopant (Ca 2+ ) was used as a dopant to strengthen CeBr 3 without negatively impacting scintillation performance. Ingots containing nominal concentrations of 1.9% of the Ca 2+ dopant were grown, i.e., 1.9% of the CeBr 3 molecules were replaced by CaBr 2 molecules, to match our target replacement of 1 out of 54 cerium atoms be replaced by a calcium atom. Precisely the mixture was composed of 2.26 g of CaBr 2 added to 222.14 g of CeBr 3 . Preliminary scintillation measurements are presented for this aliovalently doped scintillator. Ca 2+ -doped CeBr 3 exhibited little or no change in the peak fluorescence emission for 371 nm optical excitation for CeBr 3 . The structural, electronic, and optical properties of CeBr 3 crystals were studied using the density functional theory within the generalized gradient approximation. Calculated lattice parameters are in agreement with the experimental data. The energy band structures and density of states were obtained. The optical properties of CeBr 3 , including the dielectric function, were calculated.

Description: This paper presents the torque analysis and measurements of a permanent magnet (PM) type eddy current brake (ECB) with a Halbach magnet array based on analytical magnetic field calculations. On the basis of a magnetic vector potential and using a two-dimensional (2D) polar coordinate system, the analytical solution for magnetic flux density, including the eddy current reaction is evaluated. Based on these solutions, the magnetic torque is also determined analytically. A 2D finite element analysis is employed to validate the method used. Practical issues in the analytical study of the PM type ECBs, such as the maximum braking torque, the required rotor speed, and the segment-dependent, are fully discussed. Finally, the braking torque as a function of the rotor speed is measured to verify the results of the analytical study.

Description: In this work, gluconic acid (GA), a low molecular weight, inexpensive and environmentally friendly solvent, was systematically investigated to determine its viability in enhancing the orientation of ferrite particles. Submicron-scale barium hexaferrite (BaM) powders were thoroughly dispersed via sonication for 30 min in various concentrations of GA (0, 2, 2.5, 5, 10, and 25 vol. %) in deionized water. An increase of ∼18% in squareness ( SQ ) and ∼69% in energy product (( BH ) max ) was observed with increase in GA concentration from 0 to 5 vol. %. However, further increase in GA concentration led to a decrease in SQ and ( BH ) max confirming that the effect of GA stems from an improved viscosity of the dispersant, which balances the freely rotating and stationary particles under dynamic compaction within a magnetic field.

Description: In this report, we present the magnetocaloric effect of Gd 1−x Y x alloys (0.0 ≤ × ≤ 0.4) prepared by arc-melting from high purity Gd and Y precursors in inert atmosphere. The formation of Gd 1−x Y x solid solutions was verified by means of X-ray diffraction analysis across the compositional series; also, residual secondary phases Gd and Y were present. Magnetic characterization performed by Vibrating Sample Magnetometry at a maximum applied field of 3.0 T showed a drastic reduction of the magnetization saturation (from 233 emu/g for x = 0.0 to 183 emu/g for x = 0.4), due to a dilution effect of the Y alloying, together with a marked Curie temperature decrease from 296 K to 196 K between x = 0.0 and x = 0.4. The second-order character of the magnetic transition was established by Arrot plots for all the cases. On the other hand, the magnetic entropy variation, determined from numerical integration of Maxwell relation displayed excellent values above 5.30 J/kg K for alloys with x 

Description: We observe that the zircon-type R CrO 4 ( R  = Ho, Gd, Lu) compounds exhibit complicated magnetic properties and large magnetic entropy change due to the strong competition between ferromagnetic and antiferromagnetic interactions. For a field change of 7 T, the maximum values of entropy change and refrigerant capacity reach 28 J kg −1  K −1 and 740 J kg −1 , respectively, for GdCrO 4 whereas the corresponding values for HoCrO 4 are 29 J kg −1  K −1 and 550 J kg −1 . For GdCrO 4 compound, the magnetic entropy change is quite large even at low temperatures well below the ferromagnetic transition.

Description: In-depth profile changes induced by Ga + ion irradiation in Pt(5 nm)/Co(3.3 nm)/Pt(20 nm)/Mo(20 nm) sandwiches MBE grown on Al 2 O 3 substrates are deduced from complex magneto-optic polar Kerr effect (PMOKE) measurements at photon energies between 1 and 5 eV. The Ga + irradiation stimulates a redistribution of Pt and Co and leads to broadening of alloyed regions at Pt-Co and Co-Pt interfaces, which is evaluated using PMOKE spectra. The effect of four Ga + fluences D between zero and 6 × 10 15 Ga + /cm 2 was studied. The observed PMOKE azimuth rotation peak centered at 4.5 eV reaches the maximum of 0.42° at D  = 1 × 10 15 Ga + /cm 2 and becomes thus enhanced by a factor of 3.2 with respect to that in the non-irradiated sample. At D  = 6 × 10 15 Ga + /cm 2 the peak amplitude falls to 0.05°. To find the in-depth profile of Co concentration s in the sandwiches as a function of D , the PMOKE azimuth rotation and ellipticity spectra are compared with a multilayer model, where ideal flat interfaces are replaced by sequences of Co s Pt 1− s layers. The dependence on D is compared with that evaluated by simulations of the structural effects of ion irradiation. At the highest D , the irradiation produces an almost complete erosion of the top Pt and Co accompanied by mixing at the Pt-Mo interface.

Description: Of all the colossal magnetoresistant manganites, La 0.7 Sr 0.3 MnO 3 (LSMO) exhibits magnetic and electronic state transitions above room temperature, and therefore holds immense technological potential in spintronic devices and hybrid heterojunctions. As the first step towards this goal, it needs to be integrated with silicon via a well-defined process that provides morphology and phase control, along with reproducibility. This work demonstrates the development of pulsed laser deposition (PLD) process parameter regimes for dense and columnar morphology LSMO films directly on Si. These regimes are postulated on the foundations of a pressure-distance scaling law and their limits are defined post experimental validation. The laser spot size is seen to play an important role in tandem with the pressure-distance scaling law to provide morphology control during LSMO deposition on lattice-mismatched Si substrate. Additionally, phase stability of the deposited films in these regimes is evaluated through magnetometry measurements and the Curie temperatures obtained are 349 K (for dense morphology) and 355 K (for columnar morphology)—the highest reported for LSMO films on Si so far. X-ray diffraction studies on phase evolution with variation in laser energy density and substrate temperature reveals the emergence of texture. Quantitative limits for all the key PLD process parameters are demonstrated in order enable morphological and structural engineering of LSMO films deposited directly on Si. These results are expected to boost the realization of top-down and bottom-up LSMO device architectures on the Si platform for a variety of applications.

Description: Upper critical field of polycrystalline δ -Mo 1-x Zr x N (0 ≤ x ≤ 0.3) thin films by sol-gel was investigated. It showed that the upper critical field was continuously improved with Zr doping content, and the improvement of ∼10 T in upper critical field was mainly attributed to the combined effects of obvious enhancements in normal-state resistivity with slight changes in T c , obvious decrease in crystallite/grain size and enhanced microstrains. Flux jump was observed in low-level doped thin films due to enhanced critical current density by Zr doping. Finally, the vortex phase diagram of δ -Mo 0.95 Zr 0.05 N thin films was presented, which will provide guidance for investigation about the vortex mechanisms of δ -Mo 1-x Zr x N thin films.

Description: The conventional ultra-narrow bandpass filter structure has only a very limited width of high-reflectance range. This study, by introducing disorder into one-dimensional (1D) photonic crystal, attempts to enlarge the width of high-reflectance range while keeping the ultra-narrow bandpass. Enlargement by 46.8% was obtained after theoretical design. Since this structure contains some degree of disorder already, it has a strong tolerance of the variation of layer thicknesses. Unlike studies using conventional periodic structures, theoretical statistical results in this study demonstrate that high quality remains even after allowing for ±5% variation of layer thicknesses. This indicates that only a very low thickness control precision is required in the future and the production difficulty is immensely lowered. To put the construction to test, a structure has been developed and demonstrated by a magnetron reactive sputtering coating system, which agrees with the theoretical result very well. By introducing disorder into the periodic 1D photonic crystal structure, the high-reflectance range is significantly extended by 37%, with an ultra-narrow pass band of 0.8 nm and intensity of 82%.

Description: We have studied the magnetic and spin-glass (SG) properties of La 0.7 Ca 0.3 MnO 3 single-crystalline nanoparticles, which were prepared by the mechanical milling method with different milling times ( t m ). Analyzing the susceptibility data in the paramagnetic region indicates both ferromagnetic (FM) and anti-FM interactions coexisting in nanoparticles. Additionally, there is a peak associated with the freezing temperature ( T f ) appearing on the real part curve of the ac susceptibility, χ′ ( T ). The T f value increases with increasing frequency as expected for SG systems. The SG behavior was also checked by using the criterion parameter c  = Δ T f / T f Δ(log 10 f ), and the power law τ  =  τ 0 ( T / T g − 1) − z ν . The obtained values of c  ≈ 5 × 10 −2 , τ 0 ≈ 10 −5 s and zν ≈ 2–3 are consistent with those expected for SG-like systems, suggesting an existence of a SG phase transition at T g below T f , which decreases with decreasing ⟨ D ⟩. Basing on ln( f ) versus T f data, and the Néel-Arrhenius model [ln( f ) = ln( f 0 ) - E a / k B T ] and Vogel–Fulcher law [ln( f ) = ln( f 0 ) - E a / k B ( T - T 0 )], the Larmor frequency ( f 0 ), activation energy ( E a ) and effective temperature ( T 0 ) for the samples with different ⟨ D ⟩ were determined. Obtained results indicate the existence a strong interaction between nanoparticles.

Description: We report on numerical analysis on self-oscillation of standing spin wave excited in a nanostructured active ring resonator, consists of a ferromagnetic nanowire with perpendicular anisotropy. The confined resonant modes are along the nanowire length. A positive feedback with proportional-integral-derivative gain control was adopted in the active ring. Stable excitation of the 1st order standing spin wave has been demonstrated with micromagnetic simulations, taking into account the thermal effect with a random field model. The stationary standing spin wave with a pre-determined set variable of precession amplitude was attained within 20 ns by optimizing the proportional-integral-derivative gain control parameters. The result indicates that a monochromatic oscillation frequency f osc is extracted from the initial thermal fluctuation state and selectively amplified with the positive feedback loop. The obtained f osc value of 5.22 GHz practically agrees with the theoretical prediction from dispersion relation of the magneto static forward volume wave. It was also confirmed that the f osc change due to the temperature rise can be compensated with an external perpendicular bias field H b . The observed quick compensation time with an order of nano second suggests the fast operation speed in the practical device application.

Description: The evolution of phase and microstructure has been investigated for the spark plasma sintered anisotropic nanocrystalline SmCo 6.1 Si 0.9 magnets. With the increasing of sintering temperature from room temperature to 680 °C, the phase evolves from 1:7H (TbCu 7 -type) single phase to 1:7H dominant phase plus 2:17 R (Th 2 Zn 17 -type) minor phase, and then to 2:17 R dominant phase with phase-transformation twins structure plus minor 1:5 H (CaCu 5 -type) precipitated phase with particulate morphology. The evolution of phase and microstructure in anisotropic nanocrystalline SmCo 6.1 Si 0.9 alloys, as well as its effect on the coercivity is discussed and compared with that in nanostructured 2:17 type SmCo alloys.

Description: This study demonstrates the effectiveness of heat treatment in optimizing the magnetic properties of cobalt ferrite, compared to other methods such as cation substitution. It also shows how the magnetic properties of the heat treated cobalt ferrite vary under different temperature conditions. Saturation magnetization increased more due to heat treatment than due to Zn-substitution; a cation substitution that is known to result in high saturation magnetization in ferrites. A remarkable observation is that the increase in the saturation magnetization due to heat treatment was not at the expense of Curie temperature as was often reported for cation substituted materials. The observed variations in the magnetic properties were explained on the basis of cation redistribution arising as a result of the heat treatment.