Search

Electronic structures of bielectrode junctions under external bias voltages were recently solved numerically on an ab initio basis using nonequilibrium Green-function methods in combination with the density-functional theory. Here we examine this technique by applying it to two electrodes within a jellium model separated by a vacuum layer. We investigate the charge distribution, the shape of the tunnel barrier and the current. Our results suggest that this method is valid only in a limited range of external bias voltages.

We have calculated Heisenberg exchange parameters for bcc-Fe, fcc-Co, and fcc-Ni using the scalar-relativistic spin-polarized Green function technique within the tight-binding linear muffin-tin orbital method and by employing the magnetic force theorem to calculate total energy changes associated with a local rotation of magnetization directions. We have also determined spin-wave stiffness constants and found the dispersion curves for metals in question employing the Fourier transform of calculated Heisenberg exchange parameters. Curie temperatures were calculated both in the mean-field approximation and within the Green function random phase approximation. The latter results were found to be in a better agreement with available experimental data., Comment: RevTeX 3.1, 13 pages, 4 figures (PS files); v2: revised version

Spontaneous volume magnetostriction of the intermetallic compound GdAl 2 with a cubic C15 Laves structure is studied from first principles using the local spin-density approximation combined optionally with an open-core treatment of the atomic-like Gd-4f orbitals. Local magnetic moments and equilibrium lattice parameters are evaluated for two spin structures, namely for the ferromagnetic ground state and for a state with randomly oriented Gd moments. The resulting spontaneous volume magnetostriction compares well with experimentally observed values; its negative sign however cannot be ascribed to a variation of the size of the local Gd moment but has its origin in the volume dependence of exchange interactions between the Gd moments. The Curie temperature and its pressure dependence are derived from the calculated pair exchange interactions as well; their values are found in reasonable agreement with experiment supporting thus the above explanation of the anomalous magnetoelastic behavior of the GdAl 2 compound.

A first-principle theory of spontaneous volume magnetostriction is presented. It is based on self-consistent electronic structure calculations for a magnetically ordered ground state and a disordered local moment state. The effect of highly localized 4f orbitals of rare-earth (R) atoms is taken into account by an open-core treatment within the local spin-density approximation. The theory is applied to hexagonal gadolinium and to selected intermetallic compounds with the cubic C15 Laves structure: RCo2 (R=Gd, Dy, Er) and GdAl2. The results are compared to those of experiment and discussed in terms of: (i) magnitudes of the local moments and (ii) volume-dependent exchange interactions.

The Landauer–Buttiker approach formulated in the framework of the spin-polarized surface Green function technique and the tight-binding linear muffin-tin orbital method is used to study the magnetotransport in the presence of the domain walls (DW) from first principles. Our approach allows to treat both the ballistic and the diffusive regime of the DW transport on equal footing. The effect of disorder is included in terms of lateral supercells confined to individual atomic layers within a DW. In this paper we apply the method to fcc(0 0 1)-based DW of elemental ferromagnets.

We investigate the compositional dependence of the total energy of the mixed crystals (Ga,Mn)As co-doped with As, Sn, and Zn. Using the ab initio LMTO-CPA method we find a correlation between the incorporation of acceptors (Mn, Zn) and donors (Sn, antisite As). In particular, the formation energy of As_Ga is reduced by approx. 0.1 eV in the presence of Mn, and vice versa. This leads to the self-compensating behavior of (Ga,Mn)As., 8 pages, 2 figures, presented at the XXXI Int. School of Semiconducting Compounds, Jaszowiec 2002, Poland

The transmission matrix approach was used to evaluate the perpendicular magnetotransport in metallic multilayers on an abinitio level. The spin-polarized, surface Green function technique was employed within the framework of the tight-binding, linear muffin-tin orbital method. The effect of impurities was included in terms of lateral supercells with random arrangements of two types of atoms. This approach treats both the ballistic and the diffusive regimes of magnetotransport on equal footing. The method was also applied to face-centered-cubic-based Co/Cu/Co(001) trilayers.

Mechanical alloying of the Fe powder with 29 at.% Al and 1.5 at.% Mo, corresponding to THERMENOL composition, is investigated. The main attention is devoted to the dynamics of the alloying process, phase composition, and magnetic properties studied using X-ray diffraction (XRD), Mossbauer spectrometry, electron microscopy, and magnetic measurements. The analyses of the XRD patterns after various times of milling yielded stepwise alloying up to the formation of a quasi-binary (Fe-Mo)-Al system after 32 h. In this state, the alloy has reached the finest structure of homogeneously distributed elements well established by the scanning electron microscopy. However, the subsequent milling step has led to a partial decomposition. This is clearly visible by the XRD as well as by the Mossbauer phase analysis and reflected in the macroscopic magnetic characteristics after both milling and thermal treatments. The alloying by Mo induced changes in magnetic properties depending on the milling time: for shorter times, the coercivity has been reduced as compared to Fe-Al alloys of similar compositions, whereas for longer times, an increase in the coercivity due to induced strains and defects has been observed.