B. Ginatempo

Temperature dependent magnetic anisotropy in metallic magnets from an ab initio electronic structure theory: L1(0)-ordered FePt.

Using a first-principles, relativistic electronic structure theory of finite temperature metallic magnetism, we investigate the variation of magnetic anisotropy K with magnetization M in metallic ferromagnets. We apply the theory to the high uniaxial K material, L1(0)-ordered FePt, and find its magnetic easy axis perpendicular to the Fe/Pt layers for all M and K to be proportional to M2 for a broad range of values of M. For small M, near the Curie temperature, the calculations pick out the easy axis for the onset of magnetic order. Our ab initio results for this important magnetic material agree well with recent experimental measurements, whereas the single-ion anisotropy model fails to give the correct qualitative behavior.

Magnetocrystalline anisotropy and compositional order in Fe0.5Pt0.5: Calculations from an ab initio electronic model

The magnetic properties of FePt samples, potential materials for high density recording media, depend sharply on the degree of chemical ordering produced during their preparation. This has prompted our investigation of the chemical order in both paramagnetic (P) and ferromagnetic (F) phases of a Fe0.5Pt0.5 solid solution and its effect upon the magnetocrystalline anisotropy (MAE) of the latter phase. Our “first-principles” theory uses density functional electronic structure calculations and a mean field treatment of both compositional and magnetic “local moment” fluctuations. We find both phases to be unstable to L10 (CuAu)-type order below temperatures of 1975 K (P) and 1565 K (F) which compares well with that found experimentally (≈1600 K) and our estimate of the Curie temperature as 575 K is also in fair agreement (710 K). For L10 order the calculated MAE is uniaxial with a simple form, υL10c(0,0,1)2 sin2 θ, where the coefficient υL10=9.7×108 erg/cm3, c(0,0,1) is the degree of order (between 0 and 0.5)...

Effects of short-range order on the electronic structure of disordered metallic systems

For many years the Korringa-Kohn-Rostoker coherent-potential approximation (KKR-CPA) has been widely used to describe the electronic structure of disordered systems based upon a first-principles description of the crystal potential. However, as a single-site theory the KKR-CPA is unable to account for important environmental effects such as short-range order (SRO) in alloys and spin fluctuations in magnets, among others. Using the recently devised KKR-NLCPA (where NL stands for nonlocal), we show how to remedy this by presenting explicit calculations for the effects of SRO on the electronic structure of the bcc

An ab-initio theoretical investigation of the soft-magnetic properties of permalloys

{\mathrm{Cu}}_{50}{\mathrm{Zn}}_{50}

Fermi surface origin of non-stoichiometric ordering in CuPd alloys

solid solution.

OSCILLATORY EXCHANGE COUPLING ACROSS CR(1-X)VX ALLOY SPACERS

Abstract We study Ni 80 Fe 20 -based permalloys with the relativistic spin-polarized Korringa–Kohn–Rostoker electronic structure method. Treating the compositional disorder with the coherent potential approximation, we investigate how the magnetocrystalline anisotropy, K , and magnetostriction, λ , of Ni-rich Ni–Fe alloys vary with the addition of small amounts of non-magnetic transition metals, Cu and Mo. From our calculations we follow the trends in K and λ and find the compositions of Ni–Fe–Cu and Ni–Fe–Mo where both are near zero. These high permeability compositions of Ni–Fe–Cu and Ni–Fe–Mo match well with those discovered experimentally. We monitor the connection of the magnetic anisotropy with the number of minority spin electrons N ↓ . By raising N ↓ via artificially increasing the band-filling of Ni 80 Fe 20 , we are able to reproduce the key features that underpin the magnetic softening we find in the ternary alloys. The effect of band-filling on the dependence of magnetocrystalline anisotropy on atomic short-range order in Ni 80 Fe 20 is also studied. Our calculations, based on a static concentration wave theory, indicate that the susceptibility of the high permeability of the Ni–Fe–Cu and Ni–Fe–Mo alloys to their annealing conditions is also strongly dependent on the alloys’ compositions. An ideal soft magnet appears from these calculations.

ZrZn 2 : Geometrical enhancement of the local density of states and quantum design of magnetic instabilities

We discuss the peculiarities of the CuPd phase diagram in relation with the Fermi surface of the random alloys, on the basis of first-principle calculations. In particular we study the Fermi surface nesting vectors as functions of the concentration and along a tetragonal Bain path. It turns out that the nesting vectors that can be associated with the experimentally observed ordering transitions towards the B2 phase and L12 phases are commensurate with the lattice only at non-stoichiometric concentrations. The competition between this frustration effect and the electrostatic gain obtained by chemical ordering determines the critical concentrations for the transitions. We argue that the inclusion of these Fermi surface frustration effects is necessary to obtain agreement between theory and experiment on the determination of the phase diagram.

Magnetic anisotropies of Ni–Pt and Co–Pt alloys

We have identified the pieces of the Fermi surface responsible for the long period oscillations of magnetic coupling across Cr and Cr(1 x)Vx alloy spacers in metallic multilayers. Analysing experiments and results of KKR-CPA calculations we find that the periods are determined by the extremal wave vectors of the hole pockets centered on the N-point in the Brillouin zone.

Electronic structure of ordered and disordered Fe3Pt

tions, we study the conditions that, for ZrZn 2, determine the proximity to this magnetic instability. More specifically, we discuss the role played by the geometrical arrangement of the lattice, the hybridization effects, and the presence of disorder, as well as the application of external pressure. These circumstances influence the width of the relevant Zr d bands whose narrowing, due to the reduction of the effective number of neighbors or to an increase of the cell volume, causes an enhancement of the density of states at the Fermi level. Finally, we highlight some general features that may aid the design of other materials close to magnetic instabilities.

Correlation of magnetocrystalline anisotropy of Fe0.5Pd0.5 alloy with chemical order

We present the magnetocrystalline anisotropy of disordered Ni1−xPtx and disordered fcc Co1−xPtx alloys using the spin-polarized relativistic Korringa–Kohn–Rostoker coherent-potential approximation in which the spin-orbit coupling and magnetism are treated on an equal footing. In both the pure fcc ferromagnetic elements (Ni as well as Co), the magnetic easy axis is along the [111] direction of the crystal. Addition of Pt to Ni changes the magnetic easy axis to the [001] direction, in agreement with the experimental observations, although the magnitudes of the calculated magnetocrystalline anisotropy energies for different compositions are somewhat different from the corresponding experimental values. In contrast, addition of Pt to Co does not alter the magnetic easy axis, only the magnitude of the magnetocrystalline anisotropy energy is affected. The origin of this contrasting behavior may lie in the larger size of the magnetic moment of Co (as compared to Ni).