Interfacial Tuning of Anisotropic Gilbert Damping.

Tuning of the anisotropic Gilbert damping Δα has been realized in ultrathin single-crystalline Fe films grown on GaAs (001). A nonmonotonic dependence of Δα on film thickness t is observed upon varying t about 10 ML (∼1.4 nm). Δα increases for 16 ML>t>8.5 ML, and then decreases for 8.5 ML>t>6.5 ML accompanied by a sign reversal of Δα for t=6.5 ML. The sign reversal of Δα is captured by first-principle calculations, which show that the anisotropic density of states changes sign upon decreasing t. Moreover, t^{-1} dependence of the anisotropic damping indicates the emergence of an anisotropic effective spin mixing conductance according to the theory of spin pumping. The results establish new opportunities for controlling the Gilbert damping and for fundamental studies of magnetization dynamics in reduced dimension.

On the possibility of using X-ray Compton scattering to study magnetoelectrical properties of crystals.

This paper discusses the possibility of using Compton scattering--an inelastic X-ray scattering process that yields a projection of the electron momentum density--to probe magnetoelectrical properties. It is shown that an antisymmetric component of the momentum density is a unique fingerprint of such time- and parity-odd physics. It is argued that polar ferromagnets are ideal candidates to demonstrate this phenomenon and the first experimental results are shown, on a single-domain crystal of GaFeO3. The measured antisymmetric Compton profile is very small (≃ 10(-5) of the symmetric part) and of the same order of magnitude as the statistical errors. Relativistic first-principles simulations of the antisymmetric Compton profile are presented and it is shown that, while the effect is indeed predicted by theory, and scales with the size of the valence spin-orbit interaction, its magnitude is significantly overestimated. The paper outlines some important constraints on the properties of the antisymmetric Compton profile arising from the underlying crystallographic symmetry of the sample.

Electronic structure and magnetic properties of Mn1-xCrxSb alloys.

The electronic structure and magnetic properties of Mn1-xCrxSb alloys were investigated for the full concentration range. The stability of the concentration-dependent magnetic structure of the alloys were analysed on the basis of spin-spiral calculations as well as using the Monte Carlo (MC) simulations based on the Heisenberg model with the exchange coupling parameters calculated from first-principles. A leading contribution to the canted magnetic structure in Mn1-xCrxSb is the competition of the direct Cr-Cr and Mn-Mn exchange interactions having opposite signs. Furthermore, a strong impact of long-distance RKKY-type interactions is demonstrated. MC simulations at finite temperature were used to obtain the magnetic phase diagram for Mn1-xCrxSb alloys, which is in reasonable agreement with the experimental data.

Correlation effects in fcc-Fe(x)Ni(1-x) alloys investigated by means of the KKR-CPA.

The electronic structure and magnetic properties of the disordered alloy system fcc-FexNi1-x (fcc: face centered cubic) have been investigated by means of the KKR-CPA (Korringa-Kohn-Rostoker coherent potential approximation) band structure method. To investigate the impact of correlation effects, the calculations have been performed on the basis of the LSDA (local spin density approximation), the LSDA + U as well as the LSDA + DMFT (dynamical mean field theory). It turned out that the inclusion of correlation effects hardly changed the spin magnetic moments and the related hyperfine fields. The spin-orbit induced orbital magnetic moments and hyperfine fields, on the other hand, show a pronounced and element-specific enhancement. These findings are in full accordance with the results of a recent experimental study.

Magnetic behaviour of Er1-xZrxFe2 intermetallic compounds.

In this work, the structural and magnetic properties of Er1-xZrxFe2 (0.1 ≤ x ≤ 0.4) were investigated. These compounds crystallize in the cubic MgCu2 (C15) structure, the lattice parameters decreasing with Zr content. Electronic structure calculations were performed, showing a good agreement between theory and experiment. All of the samples are ferrimagnetically ordered, presenting compensation points in the M(T) plots. The compensation point values decrease, while the Curie temperatures increase with Zr content. The experimental Fe moments at 5 K decrease with Zr content from 1.70 µB/atom for x = 0.1 to 1.55 µB/atom for x = 0.4. A non-collinear orientation of the magnetic moments was evidenced in these compounds. The magnetocaloric effect was also studied. A modest magnetocaloric effect was found for all of the samples, spreading across a wide temperature range. A maximum |ΔSM| value of 1.19 J kg(-1) K(-1) was found for the sample with x = 0.1 for an applied field change of 0-4 T. Large RCP(ΔS) values were obtained for all of the samples, mainly due to the wide δTFWHM values of the ΔSM(T) curves.

Atomic-scale engineering of magnetic anisotropy of nanostructures through interfaces and interlines.

The central goals of nanoscale magnetic materials science are the self-assembly of the smallest structure exhibiting ferromagnetic hysteresis at room temperature, and the assembly of these structures into the highest density patterns. The focus has been on chemically ordered alloys combining magnetic 3d elements with polarizable 5d elements having high spin-orbit coupling and thus yielding the desired large magneto-crystalline anisotropy. The chemical synthesis of nanoparticles of these alloys yields disordered phases requiring annealing to transform them to the high-anisotropy L1(0) structure. Despite considerable efforts, so far only part of the nanoparticles can be transformed without coalescence. Here we present an alternative approach to homogeneous alloys, namely the creation of nanostructures with atomically sharp bimetallic interfaces and interlines. They exhibit unexpectedly high magnetization reversal energy with values and directions of the easy magnetization axes strongly depending on chemistry and texture. We find significant deviations from the expected behaviour for commonly used element combinations. Ab-initio calculations reproduce these results and unravel their origin.

Fe spin reorientation across the metamagnetic transition in strained FeRh thin films.

A spin reorientation accompanying the temperature-induced antiferromagnetic (AFM) to ferromagnetic (FM) phase transition is reported in strained epitaxial FeRh thin films. (57)Fe conversion electron Mössbauer spectrometry showed that the Fe moments have different orientations in FeRh grown on thick single-crystalline MgO and in FeRh grown on ion-beam-assist-deposited (IBAD) MgO. It was also observed, in both samples, that the Fe moments switch orientations at the AFM to FM phase transition. Perpendicular anisotropy was evidenced in the AFM phase of the film grown on IBAD MgO and in the FM phase of that grown on regular MgO. Density-functional theory calculations enabled this spin-reorientation transition to be accurately reproduced for both FeRh films across the AFM-FM phase transition and show that these results are due to differences in strain.

Magnetic properties of CrSb compounds with zinc-blende and wurtzite structures.

The electronic structure and magnetic properties of Cr-Sb compounds with zinc-blende and wurtzite structure have been studied by means of the Korringa-Kohn-Rostoker (KKR) band structure method. The occurrence of a half-metallic behavior has been investigated for the bulk systems as a function of lattice parameter, as well as for thin films deposited on different substrates. In the latter case the influence of the surface and interface on the electronic structure is discussed in addition. To study magnetic order in the bulk and within the films, exchange coupling parameters have been calculated from first principles. They have been used for subsequent Monte Carlo simulations, based on a classical Heisenberg Hamiltonian, to obtain the Curie temperature.

Structural and magnetic properties of CrSb compounds: NiAs structure.

The structural and magnetic properties of CrSb compounds with NiAs structure have been studied by means of the Korringa-Kohn-Rostoker (KKR) band structure method. An analysis of the structural and magnetic stability has been performed on the basis of total energy calculations for various magnetic states. The magnetic properties at finite temperature have been investigated by means of Monte Carlo simulations on the basis of a classical Heisenberg Hamiltonian and the exchange coupling parameters calculated from first principles. This approach allowed us to determine the critical temperature in good agreement with experiment.

Ab initio calculation of the Gilbert damping parameter via the linear response formalism.

A Kubo-Greenwood-like equation for the Gilbert damping parameter α is presented that is based on the linear response formalism. Its implementation using the fully relativistic Korringa-Kohn-Rostoker band structure method in combination with coherent potential approximation alloy theory allows it to be applied to a wide range of situations. This is demonstrated with results obtained for the bcc alloy system Fe(1-x)Co(x) as well as for a series of alloys of Permalloy with 5d transition metals. To account for the thermal displacements of atoms as a scattering mechanism, an alloy-analogy model is introduced. The corresponding calculations for Ni correctly describe the rapid change of α when small amounts of substitutional Cu are introduced.

Finite-temperature magnetism of CrTe and CrSe.

An investigation on the electronic and magnetic properties of NiAs-type CrTe and CrSe has been performed for ferromagnetic, antiferromagnetic and non-collinear spin configurations, using the spin-polarized relativistic KKR (Korringa-Kohn-Rostoker) band structure method. Calculated exchange coupling parameters, as well as the total energy calculated as a function of the tilt angle of magnetic moments, indicate the presence of a non-collinear spin structure in CrTe and CrSe. The existence of a non-collinear spin structure is also shown by Monte Carlo (MC) simulations used for studies on the temperature dependent magnetization. The results are compared with available results in the literature and are in satisfactory agreement with the experimental results.

Magnetism of FePt surface alloys.

The complex correlation of structure and magnetism in highly coercive monoatomic FePt surface alloys is studied using scanning tunneling microscopy, x-ray magnetic circular dichroism, and ab initio theory. Depending on the specific lateral atomic coordination of Fe either hard magnetic properties comparable to that of bulk FePt or complex noncollinear magnetism due to Dzyaloshinski-Moriya interactions are observed. Our calculations confirm the subtle dependence of the magnetic anisotropy and spin alignment on the local coordination and suggest that 3D stacking of Fe and Pt layers in bulk L1_{0} magnets is not essential to achieve high-anisotropy values.

Probing the spin and orbital high-field susceptibility of magnetic solids on the basis of the XMCD sum rules.

It is demonstrated that the magnetic circular dichroism in x-ray absorption (XMCD) can be used to probe the spin and orbital high-field susceptibilities of spontaneously magnetic solids on the basis of the so-called XMCD sum rules. A corresponding theoretical description is presented that is formulated in terms of the fully relativistic multiple scattering Green's function formalism. Examples for the field-induced changes in XMCD spectra are given together with an application of the sum rules to these spectra that demonstrates their relation to the high-field susceptibility.