Using the full-potential screened Korringa-Kohn-Rostoker method we study the full-Heusler alloys based on Co, Fe, Rh, and Ru. We show that many of these compounds show a half-metallic behavior; however, in contrast to the half-Heusler alloys the energy gap in the minority band is extremely small due to states localized only at the Co (Fe, Rh, or Ru) sites which are not present in the half-Heusler compounds. The full-Heusler alloys show a Slater-Pauling behavior and the total spin magnetic moment per unit cell ${(M}_{t})$ scales with the total number of valence electrons ${(Z}_{t})$ following the rule ${M}_{t}{=Z}_{t}\ensuremath{-}24.$ We explain why the spin-down band contains exactly 12 electrons using arguments based on group theory and show that this rule holds also for compounds with less than 24 valence electrons. Finally we discuss the deviations from this rule and the differences compared to the half-Heusler alloys.

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Slater-Pauling behavior and origin of the half-metallicity of the full-Heusler alloys

Simple rules for the understanding of Heusler compounds

A voltage-induced symmetry change in a ferromagnetic material can change its magnetization or magnetic anisotropy, but these effects are too weak to be used in memory devices. Researchers have now shown that a relatively small electric field can cause a large change in the magnetic anisotropy of a few atomic layers of iron. The results could lead to low-power logic devices and non-volatile memory cells.

Large voltage-induced magnetic anisotropy change in a few atomic layers of iron

Ferromagnetic materials exhibit intrinsic `easy' and `hard' directions of the magnetization. This magnetic anisotropy is, from both a technological and fundamental viewpoint one of the most important properties of magnetic materials. The magnetic anisotropy in metallic magnetic multilayers forms the subject of this review article. As individual layers in a multilayer stack become thinner, the role of interfaces and surfaces may dominate that of the bulk: this is the case in many magnetic multilayers, where a perpendicular interface contribution to the magnetic anisotropy is capable of rotating the easy magnetization direction from in the film plane to perpendicular to the film plane. In this review, we show that the (in-plane) volume and (perpendicular) interface contribution to the magnetic anisotropy have been separated into terms related to mechanical stresses, crystallographic structure and the planar shape of the films. In addition, the effect of roughness, often inherent to the deposition techniques used, has been addressed theoretically. Several techniques to prepare multilayers and to characterize their growth as well as methods to determine the magnetic anisotropy are discussed. A comprehensive survey of experimental studies on the perpendicular magnetic anisotropy in metallic multilayers containing Fe, Co or Ni is presented and commented on. Two major subjects of this review are the extrinsic effects of strain, roughness and interdiffusion and the intrinsic effect of the crystallographic orientation on the magnetic anisotropy. Both effects are investigated with the help of some dedicated experimental studies. The results of the orientational dependence studies are compared with ab initio calculations. Finally, the perpendicular surface anisotropy and the in-plane step anisotropy are discussed.

https://iopscience.iop.org/article/10.1088/0034-4885/59/11/002/pdf

Magnetic anisotropy in metallic multilayers

Theoretical models of Schottky-barrier height formation are reviewed. A particular emphasis is placed on the examination of how these models agree with general physical principles. New concepts on the relationship between interface dipole and chemical bond formation are analyzed, and shown to offer a coherent explanation of a wide range of experimental data.

Recent advances in Schottky barrier concepts

Electronic structures of Heusler alloys Co 2 MnZ (Z=sp elements) were calculated to search half-metallic compounds. In this paper, it will be shown that stoichiometric compounds Co 2 MnZ are half-metallic when Z is Si and Ge. Considering the effect of Z atom on the band structure, we can predict that there are half-metallic compounds in nonstoichiometric compounds Co 2 MnZ 1- x Z x ' and that the position of the Fermi level is adjustable in the energy gap of the minority-spin state by changing the composition x .

Search for Half-Metallic Compounds in Co2MnZ (Z=IIIb, IVb, Vb Element)

To search a half-metallic (HM) film, we calculated the energy band structures of the films of Co 2 MnZ (Z  Si, Ge) which were theoretically predicted to be HM in the bulk. The band structures of the (0 0 1) and (1 1 1) films show that whether the film becomes HM or not is conditional on its thickness and its surface and that the (1 1 1) film with the Z atom surface is appropriate as a FM film.

Theoretical search for half-metalliic films of Co2MnZ (Z Si, Ge)

The band structures of Cr in the paramagnetic- and the perfect antiferromagnetic state are evaluated by the Green's function method. In the former the crystal potential is set up by the self-consistent procedure proposed by Wakoh, while the alternant orbital theory of Slater is applied to the latter. The spin-dependent part of the potential is determined by the Slater exchange with an adjustable parameter which is determined so as to give an observable value of the magnetic moment. The Fermi surfaces of both the states are calculated. The induced magnetic moment, the electronic specific heat and the magnetic form factor are also evaluated. The agreement between theory and experiment seems to be satisfactory. The stability of the perfect antiferromagnetic state against the paramagnetic state is discussed on the basis of the effective Hamiltonian. Finally, the stability of the spin-density-wave state in Cr is ascribed to the special band structure of Cr.

Band Theory of Antiferromagnetic Chromium

It is known experimentally that Heusler alloys, Ni 2 MnGa and Co 2 NbSn undergo the cubic-to-tetragonal lattice transformation, in a ferromagnetic state and a paramagnetic one, respectively. We calculated electronic structures of Co 2 NbSn and Ni 2 MnGa for both cubic and tetragonal structures by KKR method. Comparing the density-of-state of the cubic and tetragonal structures for each alloy, it is expected that the band Jahn-Teller effect cause the lattice transformation in both alloys.

Electronic Structure and Lattice Transformation in Ni2MnGa and Co2NbSn

Band calculation results show that Heusler compounds Fe 2 MnZ (Z=Al,Si,P) have a band gap near the Fermi energy for one spin direction. Except for Fe 2 MnSi, the compounds does not have a half-metallic character because there is a slight density of states at the Fermi energy in the minority-spin state. An analysis of density of states indicates that there exists sufficiently strong d - d hybridization between metal atoms and weak p - d hybridization between Si and metal atoms. Due to this weakness, the replacement of Si by Al or P does not cause considerable change in the DOS structure. However, the weak hybridization defines the electronic character of the system.

Setsuro Asano

Papers

Search for Half-Metallic Compounds in Co2MnZ (Z=IIIb, IVb, Vb Element)

Theoretical search for half-metalliic films of Co2MnZ (Z Si, Ge)

Band Theory of Antiferromagnetic Chromium

Electronic Structure and Lattice Transformation in Ni2MnGa and Co2NbSn

A Half-Metallic Band Structure and Fe2MnZ (Z=Al, Si, P)