ALBERT

Notes: The ferromagnetic coupling across Pd interlayers has been determined from hysteresis loop measurements on perpendicular-magnetized multilayers of the type M3/8 Ru/M2/xPd/M1. Here M1, M2, and M3 denote magnetic layers consisting of (strongly ferromagnetically coupled) Co/Pd multilayers with perpendicular magnetic anisotropy: M1=[3Co+4×(4Co+6Pd)], M2=[3Co+4×(4Co+6Pd)+7Co] and M3=[20×(6Pd+4Co)+3Co]. (The layer thicknesses are in A(ring).) The determination of the ferromagnetic coupling across the Pd interlayer with variable thickness x was enabled by pinning the second magnetic layer M2. This pinning was achieved by the relatively strong antiferromagnetic coupling (by the Ru layer) to the thickest magnetic layer M3. The hysteresis loops displayed a staircase behavior, with two or three steps (levels), depending on the strength of the ferromagnetic coupling. From the fields at which the steps occurred, the ferromagnetic coupling across Pd, as well as the antiferromagnetic coupling across the 8 A(ring) Ru layer, were determined. A monotonic decrease of the former was observed from 0.17 mJ/m2 at 14 A(ring) Pd to ≈0.02 mJ/m2 at 20 A(ring) Pd.

Notes: The magnetic anisotropy and the saturation magnetization have been investigated in several series of Co/Ni multilayers, with relatively thick Co and Ni layers, grown in ultrahigh vacuum (UHV) and high vacuum (HV) by vapor deposition. The volume and interface contributions to the magnetic anisotropy were determined from magnetization measurements. The Co/Ni interface contribution was found to favor perpendicular anisotropy. A series of 2 A(ring) Co/4 A(ring) Ni multilayers, grown in HV, exhibited an easy axis perpendicular to the layer planes, as evidenced both by polar Kerr rotation and ferromagnetic resonance (FMR) measurements. The first- and second-order anisotropy constants were determined from FMR. Indications were found that the presence of FMR signals in (perpendicular easy) samples with low magnetic sublayer thicknesses depends on the magnitude of the surface anisotropy and on the amplitude of fluctuations in the magnetic layer thicknesses.

Notes: In the diluted magnetic semiconductor Sn1−xMnxTe, magnetic ordering occurs at low temperature when the charge carrier density p exceeds a critical value. By means of single-crystal neutron diffraction on samples with x=0.03, p = 7 × 1020 cm−3 and x=0.06, p = 11 × 1020 cm−3, we have shown that this ordered phase is ferromagnetic. For the 6% Mn crystal indications were found for magnetic fluctuations just below the ordering temperature. The experimental results rule out the existence of a (reentrant) spin-glass phase at these Mn concentrations and charge carrier densities.

Notes: PbySn1−x−yMnxTe in various compositions, including Sn1−xMnxTe, displays an intriguing phase diagram which includes the carrier concentration p as an important parameter. A ferromagnetic state was found to exist only above a critical concentration pc≈3×1020 cm−3 at which a heavy hole band will be filled and starts to contribute to the RKKY interaction between the Mn ions.1 The present contribution deals with the magnetic phase diagram for low Mn-concentrations (〈3%) as a function of the carrier density. The low temperature magnetic properties of PbSn(Mn)Te and Sn(Mn)Te were studied by means of magnetization, susceptibility and specific heat. In analogy with the existing results for other compositions, a ferromagnetic state is observed (Tc≈aitch-theta≈2–3 K), for p exceeding pc. However, for extremely high carrier densities, p(approximately-greater-than)1021 cm−3, the data show a substantial deviation from what is commonly observed. This is characterized by a significant drop of Tc well below aitch-theta and a rather broad maximum in the specific heat. Tentatively this transformation at higher p might be attributed to a ferromagnetic (F) to spin-glass-like transition, which, qualitatively, is in harmony with the competitive balance between ferro- and antiferromagnetic (AF) exchange characteristic for the simple RKKY interaction: an increasing wave number (∼p1/3) will increase the number of antiferromagnetically long-range coupled ions, at the expense of the strong ferromagnetic coupling between Mn2+ at shorter distances. Moreover, the mean distance between the Mn ions for x=0.02 is such that this delicate interplay between F and AF coupling should be observable.

Notes: A unique sample was prepared on a Cu(100) single crystal, consisting of three Co layers separated by two Cu layers in the form of wedges oriented perpendicular to each other: Cu(100)/80 A(ring) Co/Cu wedge A/30 A(ring) Co/Cu wedge B/30 A(ring) Co/7 A(ring) Cu/30 A(ring) Au. Position-sensitive magneto-optical Kerr effect measurements along Cu wedge B, at a fixed position on Cu wedge A corresponding to maximum antiferromagnetic (AF) coupling, enabled us to investigate not only the AF but also the ferromagnetic (F) coupling between the two 30 A(ring) Co layers as a function of the Cu thickness. The measurements confirmed both long and short period coupling oscillations in the AF regime, and revealed the predicted extension of the short period through the F regime.

Notes: A variational estimate of the free energy of a quantum spin chain based on the stochastic behavior of the quantization axes is proposed. It allows a description of the nonlinear excitations and quantum spin waves within the same framework.For an easy-plane ferromagnet with an external field in the plane it reveals that the nonlinear excitations (solitons) are predominantly described by the classical degrees of freedom. Considerable effort has been devoted to the theoretical description1 and experimental verification of the existence of nonlinear excitations. In this field magnetic chain systems have played an important part.2 Chain systems with an easy-plane anisotropy, such as (C6 H11 NH3 )CuBr3 (CHAB) and CsNiF3 , which are subjected to a symmetry breaking field, were found to be soliton bearing.3 Theoretically the description of the nonlinear effect is based on the mapping of the spin dynamics on a sine-Gordon model. Further research4 indicated that the interpretation of the thermodynamical properties of real systems in terms of the sine-Gordon model required however quantum and anisotropy corrections. Since the dipolar interactions between the spins in the chain are considered to be negligible, the direction of the spin chain in configuration space with respect to any preferred direction in spin space does not enter explicitly in the Hamiltonian. This makes it interesting to define a site-dependent z direction in spin space which makes an angle (aitch-thetan ,φn ) with the z direction in configuration space.The model Hamiltonian can then be written in terms of the spin operators defined with respect to the site-dependent quantization axis. The representation with equal quantization axis for all the sites is related to the representation with different quantization axes at each site by a rotation in spin space. The unitary transformation that describes the rotation contains the angles (aitch-thetan ,φn ), which we have treated as random variables. Based on this observation we have proposed a new variational approach for the quantum spin chain. We will outline the main results here. A full account is published elsewhere.5In our approach the free energy of the system can be separated in a quantum part and a classical part. The free energy F of the chain is then approximated by an upper bound F〈Fcl +Fqm , where βFcl =−ln[E(exp−βVcl )] and βFqm =−ln Tr(exp−β〈Hqm 〉). The first part is a configurational classical free energy, depending on the choice of the probability distribution for the quantization axes via the expectation E. The second contribution is the free energy of a modified quantum chain, whose parameters are classical averages and can be obtained from the configurational classical free energy. For easy-plane ferromagnetic chains, the nonlinear excitations are comprised in the classical part, whereas the quantum part contains predominantly linear excitations. For those thermodynamical quantities that are insensitive to the contribution of the quantum excitations, the chain is thermodynamically equivalent with a classical sine-Gordon system. The out-of-plane corrections are incorporated in the quantum part of the free energy. This behavior is corroborated experimentally by measurements of the excess heat capacity of CHAB for different magnetic fields with the same in-plane component.6 The validity of our approach is, however, not restricted to easy-plane ferromagnetic chains. Also for more complex systems with, for example, an orthorhombic or helical symmetry, the introduction of stochastic quantization axes can shed new light on some of the thermodynamical properties of these quantum systems in terms of their classical equivalent.

Notes: Electron-paramagnetic-resonance (EPR) measurements were performed in the temperature range T=1.3–100 K in samples of the semimagnetic Pb1−x−ySnyMnxTe semiconductor with carrier concentrations p=1.6×1019−1.4×1021 cm−3. Magnetization and Hall effect were also measured. Compositions in the range 0.12〈y〈0.72 and x=0.02, 0.04, and 0.06 were studied. We found a strong influence of the carrier concentration on the temperature dependence of the width of EPR line (ΔH). A very rapid increase of ΔH with increasing temperature is observed in the samples with high carrier concentrations: p=5.5, 6.8, and 14×1020 cm−3. The magnitude of this effect is strongly reduced in the sample with the carrier concentration p=2.85×1020 cm−3. In n- and p-type samples with the lowest carrier concentrations (p(approximately-equal-to)2×1019 cm−3), no temperature dependence of ΔH is observed at high temperature. The strong effect of the carrier concentration on ΔH can be understood, in agreement with magnetic measurements, as a result of the two-carrier (light for p〈pc=3×1020 cm−3 and very heavy, available only for p (approximately-greater-than) pc) Korringa relaxation mechanism.

Notes: An optical polarization modulation technique was adapted to provide a simple, fast, and flexible method for studying the kinetics and growth characteristics of thin oxide layers, using Al2O3 as an example. The optical technique allows precise determination of the amount of remaining metallic Al as a function of the initial Al thickness, while scanning a laser spot across the wedge. Optical data suggest that the oxide growth rate for the ultrathin layers may be dependent on the specific microstructure. In situ x-ray photoelectron spectroscopy performed on homogenous samples confirmed the interpretation of the optical results. © 2000 American Institute of Physics.

Notes: We have produced arrays of Co nanowires in anodic porous alumina filters by means of electrodeposition. The structure and magnetization behavior of the wires was investigated with nuclear magnetic resonance (NMR) and magnetization measurements. NMR shows that the wires consist of a mixture of fcc and hcp texture with the (0001) texture of the hcp fraction oriented preferentially perpendicular to the wires. The magnetization direction is determined by a competition of demagnetizing fields and dipole–dipole fields and can be tuned parallel or perpendicular to the wires by changing the length of the wires. © 1999 American Institute of Physics.

Notes: Real-time in situ optical measurements were performed during the oxidation of ultrathin Al layers with a view to study oxidation kinetics and provide process control. The optical technique combines high temporal and spatial resolution with submonolayer sensitivity. In situ x-ray photoelectron spectroscopy measurements on the same samples are in good agreement, confirming the accuracy and sensitivity of the technique. For all thicknesses studied, the initial oxidation is extremely rapid, with quasilogarithmic behavior over the whole time scale. This technique could be utilized to study a wide variety of in situ reactive processes. © 2001 American Institute of Physics.