Simple rules for the understanding of Heusler compounds

A review of new developments in theoretical and experimental electronic-structure investigations of half-metallic ferromagnets (HMFs) is presented. Being semiconductors for one spin projection and metals for another, these substances are promising magnetic materials for applications in spintronics (i.e., spin-dependent electronics). Classification of HMFs by the peculiarities of their electronic structure and chemical bonding is discussed. The effects of electron-magnon interaction in HMFs and their manifestations in magnetic, spectral, thermodynamic, and transport properties are considered. Special attention is paid to the appearance of nonquasiparticle states in the energy gap, which provide an instructive example of essentially many-body features in the electronic structure. State-of-the-art electronic calculations for correlated d-systems are discussed, and results for specific HMFs (Heusler alloys, zinc-blende structure compounds, CrO2, and Fe3O4) are reviewed.

/pdf/half-metallic-ferromagnets-from-band-structure-to-many-body-1hrl8mp5lm.pdf

Half-metallic ferromagnets: From band structure to many-body effects

We study the origin of the gap and the role of chemical composition in the half-ferromagnetic Heusler alloys using the full-potential screened Korringa-Kohn-Rostoker method. In the paramagnetic phase the ${C1}_{b}$ compounds, like NiMnSb, present a gap. Systems with 18 valence electrons, ${Z}_{t},$ per unit cell, like CoTiSb, are semiconductors, but when ${Z}_{t}g18,$ antibonding states are also populated, thus the paramagnetic phase becomes unstable and the half-ferromagnetic one is stabilized. The minority occupied bands accommodate a total of nine electrons and the total magnetic moment per unit cell in ${\ensuremath{\mu}}_{B}$ is just the difference between ${Z}_{t}$ and $2\ifmmode\times\else\texttimes\fi{}9.$ While the substitution of the transition metal atoms may preserve the half-ferromagnetic character, substituting the sp atom results in a practically rigid shift of the bands and the loss of half-metallicity. Finally we show that expanding or contracting the lattice parameter by 2% preserves the minority-spin gap.

/pdf/origin-and-properties-of-the-gap-in-the-half-ferromagnetic-1d93np84qf.pdf

Origin and properties of the gap in the half-ferromagnetic Heusler alloys

We investigate the atomic disorder effects on the half-metallicity of the full-Heusler alloy ${\mathrm{Co}}_{2}({\mathrm{Cr}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x})\mathrm{Al}$ from first principles by using the Korringa-Kohn-Rostoker method with the coherent potential approximation. Our results show that disorder between Cr and Al does not significantly reduce the spin polarization of the parent alloy ${\mathrm{Co}}_{2}\mathrm{CrAl},$ while disorder between Co and Cr makes a considerable reduction of the spin polarization. It is observed that the spin polarization of ${\mathrm{Co}}_{2}({\mathrm{Cr}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x})\mathrm{Al}$ decreases with increasing Fe concentration x in both the ordered ${L2}_{1}$ and the disordered $B2$ structures, and that the effects of the disorder on the spin polarization is significant at low Fe concentrations. The results suggest that a highly spin-polarized ferromagnet with high Curie temperature will be obtained if a ${\mathrm{Co}}_{2}({\mathrm{Cr}}_{1\ensuremath{-}x}{\mathrm{Fe}}_{x})\mathrm{Al}$ with the ordered ${L2}_{1}$ structure can be fabricated at low Fe concentrations.

Atomic disorder effects on half-metallicity of the full-Heusler alloys Co 2 ( Cr 1 − x Fe x ) Al : A first-principles study

Heusler alloys, such as ${\mathrm{Co}}_{2}\mathrm{MnSi}$ and ${\mathrm{Co}}_{2}\mathrm{MnGe},$ have been predicted from first--principles to be half metallic and potential candidates for spintronic applications. However, spin polarizations of only 50--60 % were experimentally obtained for these compounds---a decrease attributed to defects in the Mn and Co sublattices. Accurate ab initio full-potential linearized augmented plane wave calculations are performed in order to determine the effects of several types of defects (such as antisites and atomic swaps) on the electronic and magnetic properties of the bulk Heusler compounds. Our findings, in general agreement with experiments, show that Mn antisites have the lowest formation energy and retain the half-metallic character. On the other hand, Co antisites have a slightly higher formation energy and a dramatic effect on the electronic properties: the defect states that locally destroy half metallicity are energetically localized and are screened out in a couple of atomic shells. In this case, the spin polarization at the Fermi level is strongly reduced, and the spin polarization due to the s electrons, responsible for the tunneling current, is in excellent agreement with experiment. Finally, both Mn-Si and Mn-Co atomic swaps have very high formation energies, keeping however the half-metallic character.

Role of structural defects on the half-metallic character of Co 2 MnGe and Co 2 MnSi Heusler alloys

The electronic structure of the half-metallic ferromagnet NiMnSb with three different types of atomic disorder is calculated using the layer Korringa-Kohn-Rostoker method in conjunction with the coherent potential approximation. Results indicate the presence of minority-spin states at the Fermi energy for degrees of disorder as low as a few percent. The resulting spin polarization below 100% is discussed in the light of experimental difficulties confirming the half-metallic property of NiMnSb thin films directly.

First-principles calculation of the effect of atomic disorder on the electronic structure of the half-metallic ferromagnet NiMnSb

Using half-metallic ferromagnets in spin-dependent devices, like spin valves and ferromagnetic tunnel junctions, is expected to increase the device performance. However, using the half-metallic ferromagnet NiMnSb in such devices led to much less than ideal results. One of the possible sources for this behavior is atomic disorder. First-principles calculations of the influence of atomic disorder on the electronic structure of NiMnSb underline the sensitivity of half-metallic properties in NiMnSb to atomic disorder. In this article, we report on the disorder dependence of the total magnetic moment calculated by applying the layer Korringa–Kohn–Rostoker method in conjunction with the coherent potential approximation. We consider the following types of disorder: (1) intermixing of Ni and Mn, (2) partial occupancy of a normally vacant lattice site by Ni and Mn, and (3) partial occupancy of this site by Mn and Sb. In all cases the composition is kept stoichiometric. All three types of disorder decrease the mome...

Disorder dependence of the magnetic moment of the half-metallic ferromagnet NiMnSb from first principles

The magnetic anisotropy of strained epitaxial Co films deposited on strained Cu(111) on Si(110) and miscut Cu(111) are reported. First, a Cu(111) buffer layer with an in-plane strain is formed by epitaxial growth on Si(110). Co films deposited on the buffer layer develop a uniaxial anisotropy axis originating from the strain along the twofold axis of the Si(110) surface. The magnetic anisotropy of this film is compared to that of a Co film deposited on a miscut Cu(111) single crystal with uniaxial step-induced anisotropy in the Co layer. The measured hysteresis loops are compared to hysteresis loops simulated within a Stoner-Wolfarth model with a varying fraction of uniaxial and triaxial anisotropies. The measurements and the model are in qualitative agreement and reveal uniaxial and triaxial anisotropies of the same order of magnitude for both systems.

D. Orgassa

Papers

First-principles calculation of the effect of atomic disorder on the electronic structure of the half-metallic ferromagnet NiMnSb

Disorder dependence of the magnetic moment of the half-metallic ferromagnet NiMnSb from first principles

Magnetic Properties of Co Films on Cu/Si(110) and Cu(111)