ALBERT

Beschreibung: The Mössbauer spectroscopy on iron, antimony, and tin isotopes was applied to study the hyperfine magnetic interactions in the iron antimonide Fe 1.22 Sb of a B 8-type crystal structure. The values of effective magnetic fields in the temperature range 10–150 K were calculated for the iron atoms at structurally nonequivalent positions. It was shown that antimony in Fe 1.22 Sb is not involved into magnetic interactions at T = 77 K in contrast to tin introduced into the compound as a Mössbauer probe.

Beschreibung: The review is devoted to systematic description of results on relaxation of magnetization in magnetically ordered crystals previously obtained by the authors. The ideas of the phenomenological theory of magnetism formulated by Landau and Lifshitz are analyzed. A general method of constructing the dissipation function for both magnetically ordered systems and paramagnets is described. In the case of magnetically ordered systems the dissipation of exchange and relativistic nature is considered. It is found that for constructing the dissipation function it is necessary to take into account not only the symmetry of a crystal, but also the laws of conservation of magnetization. It is shown that in the case of a ferromagnet, the ground state is characterized by a continuous degeneracy parameter; the Landau-Lifshitz relaxation term gives qualitatively incorrect results (abnormally large attenuation of spin waves). According to the proposed method the spectra of spin waves and their attenuation were calculated and analyzed for ferromagnets with uniaxial, tetragonal and cubic symmetry as well as for two-sublattice uniaxial ferrites. It was found that the relaxation of the magnetization vector has a two-step character in ferromagnets and a multistep character in ferrites. In ferrites, the fastest process is the relaxation of length of the antiferromagnetic vector. It is shown that this relaxation is caused by the exchange interaction between sublattices of a ferrite and is enhanced by the exchange interactions within the sublattices. The relaxation of the total magnetization of a ferrite is much slower and, as in the case of a simple ferromagnet, is described by non-uniform exchange interactions and relativistic interactions. The results obtained are in a good agreement with recent experimental data.

Beschreibung: We show that nanomechanical shuttling of single electrons may enable qualitatively new functionality if spin-polarized electrons are injected into a nanoelectromechanical single-electron tunneling (NEM-SET) device. This is due to the combined effects of spin-dependent electron tunneling and Coulomb blockade of tunneling, which are phenomena that occur in certain magnetic NEM-SET devices. Two effects are predicted to occur in such structures. The first is a reentrant shuttle instability, by which we mean the sequential appearance, disappearance and again the appearance of a shuttle instability as the driving voltage is increased (or the mechanical dissipation is diminished). The second effect is an enhanced spin polarization of the nanomechanically assisted current flow.

Beschreibung: The features of ac magnetic susceptibility appearing in thin superconducting films with a high critical current were investigated for the first time. These features were found in some films of YBa 2 Cu 3 O 7–δ in magnetic-field and temperature dependencies of the ac susceptibility in the form of Z-like step for the real component, χ m ′ ( H ) or χ m ′ ( T ), and V-like dip for the imaginary component, χ m ″ ( H ) or χ m ″ ( T ), where the index m indicates measurements at a maximum of the polar diagram χ′′(χ′) (“maximum of losses”). Out of the maximum of losses the effect is also observed and its value depends on the depth of entrance and exit of vortices. It is suggested that the effect is caused by dependencies of the critical current and flux creep on a magnetic field. The magnetic susceptibility for a thin disk was calculated approximately using modified Clem-Sanchez formulas, in which a dependence of the critical current density on the applied field, j c ( H ), was introduced. The calculations based on the real dependence j c ( H ) in the films studied and with a simplest account for the flux creep allowed to obtain the dependencies χ m ′ ( H ) and χ m ″ ( H ), qualitatively similar to experimental, including the Z- and V-features at a large critical current. It is shown that for these samples the Clem-Sanchez relations between the critical current density, on the one hand, and components of the ac magnetic susceptibility and also the amplitude of the ac field at the maximum of losses, on the other hand, are violated.

Beschreibung: We have studied the temperature dependence of the ac magnetic susceptibility χ ( T ) = χ ′ ( T ) + i χ ″ ( T ) of YBCO tapes in a parallel magnetic field in both the Meissner and vortex states. For the vortex state, we have observed two maximums in the χ ″ ( T ) dependence. The position and the magnitude of one of these maxima are described well by the nonlocal critical state model. The second maximum and corresponding kink in the function χ ′ ( T ) observed close to the temperature T c of the superconducting transition are unexpected. The origin of this maximum cannot be explained within the usual notions of the high-temperature superconductivity. We suppose that it is related to some magnetic restructuring in the superconducting layer just above T c . Our results put a question on the correctness of the interpretation of some previous microwave experiments.

Beschreibung: Within the framework of the time-dependent Ginzburg–Landau equations, the dynamics of the order parameter in narrow superconducting channels of different lengths is investigated in the ac voltage-driven regime. The resistive state of the system at low frequencies of the applied voltage is characterized by the formation of time-periodic groups of oscillating phase-slip centers (PSCs). Increasing the frequency reduces the lifetime of these periodic groups. Depending on the length of the channel, the ac voltage either tends to restore the state with a single central PSC in periodic groups or minimizes the number of forming PSCs and induces their ordering in the system. For relatively short channels, a further increase in frequency leads to the suppression of the order parameter without PCS formation. For the systems with the channel length exceeding the specified limit, the formation of PSC starts after a certain time delay which increases rapidly with frequency. The current-voltage characteristics of a relatively short channel at different applied voltage frequencies are calculated. It is found that the current-voltage characteristics exhibit a stepped structure, where the height of the first step is determined by the quadruple value of the Josephson frequency.

Beschreibung: The Bose-Einstein condensation (BEC) of magnons created by a strong pumping in ferromagnetic thin films of yttrium iron garnet used as systems of finite size is considered analytically. Such a peculiarity, typical for this magnetic material, as the presence of a minimum in the spectrum of spin waves at a finite value of the wave vector is taken into account. The definition of high-temperature BEC is introduced and its characteristics are discussed. A role of boundary conditions for spin variables is analyzed, and it is shown that in the case of free spins on the boundary the magnon lattice can form in the system. The factors responsible for its appearance are discussed

Beschreibung: As found from numerous microwave experiments on the unconventional Fe-based superconductors, the temperature dependence of the quasiparticle scattering rate τ − 1 cannot be accurately described within the framework of standard Drude module in the popular approximation of ω τ   ≪   1 , where ω is the signal frequency. To account for the discrepancy, we have extended the classical Drude model for the case of arbitrary values of ω τ , and obtained the expression for τ − 1 as a function of experimentally measurable quantities, namely the real and imaginary parts of the microwave surface impedance. We then show the temperature dependence of τ − 1 in superconducting Ba(Fe 1- x Co x ) 2 As 2 single crystal pnictide derived from the Ka-band surface impedance measurements within the framework of the modified expression. The measurements indicate the extent to which assumption of ω τ   ≪   1 gives results different from those obtained without this restriction, i.e., incorrect results.

Beschreibung: The transformation of the hysteretic field dependence of the magnetization of a ferromagnetic thin layer in contact with a magnetically hard antiferromagnet is considered. It is shown that this interaction leads to a shift of the hysteresis loop from the configuration symmetric with respect to magnetic field (exchange bias). Furthermore, upon increasing the magnitude of the exchange interaction, within a narrow range of the magnitudes, there occurs a qualitative change in the hysteresis loop shape and its subsequent disappearance; hence the field dependence of the magnetization becomes monotonous and single-valued.

Beschreibung: The values and temperature dependences of the dynamic Young's modulus E ( T ) of two metals (Ti, Zr) in crystalline (CR) and nanostructured (NS) states are measured by the composite vibrator method at a frequency of ω = 5·10 5 s −1 over temperatures of 2–20 K. The transition from CR (coarse grained polycrystals) to NS (ultra-fine grained polycrystals) is induced by severe plastic deformation (SPD). The transition of the metals into the NS state is found to be accompanied by a softening of the modulus by a few percent and by the appearance of a distinctive feature in the E NS ( T ) curves: a transition from a logarithmic to a power-law dependence with increasing temperature at a crossover temperature T c  ≈ 6–8 K, a behavior typical of the elastic moduli of glasses. Structural distortions in the NS state with atomic disorder properties (a glass subsystem or “phase”) are interpreted in terms of dislocation concepts because of the accumulation of high densities of dislocations in metals with random configurations of the dislocation lines induced by SPD. In an appendix the dynamic properties of the dislocation lines fragments are compared, and an analogy established, with the dynamics of quasi-local excitations in glasses (so-called two-level tunnel systems and harmonic oscillators), which are invoked in the theory of glasses to describe their low-temperature properties.