Simple rules for the understanding of Heusler compounds

Spintronics is a multidisciplinary field involving physics, chemistry, and engineering, and is a new research area for solid-state scientists. A variety of new materials must be found to satisfy different demands. The search for ferromagnetic semiconductors and stable half-metallic ferromagnets with Curie temperatures higher than room temperature remains a priority for solid-state chemistry. A general understanding of structure-property relationships is a necessary prerequisite for the design of new materials. In this Review, the most important developments in the field of spintronics are described from the point of view of materials science.

Spintronics: a challenge for materials science and solid-state chemistry.

The Curie temperatures for the Heusler compounds Co{sub 2}TiAl, Co{sub 2}VGa, Co{sub 2}VSn, Co{sub 2}CrGa, Co{sub 2}CrAl, Co{sub 2}MnAl, Co{sub 2}MnSn, Co{sub 2}MnSi, and Co{sub 2}FeSi are determined ab initio from the electronic structure obtained with the local-density functional approximation and/or the generalized gradient approximation. Frozen spin spirals are used to model the excited states needed to evaluate the spherical approximation for the Curie temperature. The spherical approximation is found to describe the experimental Curie temperatures very well which, for the compounds selected, extend over the range from 95 to 1100 K; as a function of the valence electron count, they show an approximately linear trend which finds an explanation by our calculations.

Understanding the trend in the Curie temperatures of Co2-based Heusler compounds: Ab initio calculations

This paper reports on the bulk properties of the quaternary Heusler alloy Co2Mn1�xFexSi with the Fe concentration x =0,1/2,1. All samples, which were prepared by arc melting, exhibit L21 long-range order over the complete range of Fe concentration. The structural and magnetic properties of the Co2Mn1�xFexSi Heusler alloys were investigated by means of x-ray diffraction, high- and low-temperature magnetometry, Mossbauer spectroscopy, and differential scanning calorimetry. The electronic structure was explored by means of highenergy photoemission spectroscopy at about 8 keV photon energy. This ensures true bulk sensitivity of the measurements. The magnetization of the Fe-doped Heusler alloys is in agreement with the values of the magnetic moments expected for a Slater-Pauling-like behavior of half-metallic ferromagnets. The experimental findings are discussed on the basis of self-consistent calculations of the electronic and magnetic structure. To achieve good agreement with experiment, the calculations indicate that on-site electron-electron correlation must be taken into account, even at low Fe concentration. The present investigation focuses on searching for the quaternary compound where the half-metallic behavior is stable against outside influences. Overall, the results suggest that the best candidate may be found at an iron concentration of about 50%.

https://arxiv.org/pdf/cond-mat/0606108.pdf

Properties of the quaternary half-metal-type Heusler alloy Co2Mn1-xFexSi

Cobalt-based full-Heusler compounds with composition Co2M'Z (where M' is a transition metal and Z is a main group element) are attracting attention due to their predicted half-metallic behaviour, a greatly desired property for spin-dependent electron transport devices. Knowledge of the basic magnetic properties of these materials, in particular in the form of thin films, is required both to fully exploit these promising materials and to understand their underlying electronic structure and establish structure–property relationships. In this topical review, we present a survey of the magnetic anisotropy, exchange and damping of Co2M'Z compounds. These properties are directly related to spin–spin and spin–orbit interactions.

/pdf/magnetic-anisotropy-exchange-and-damping-in-cobalt-based-5dw51r0qpc.pdf

Magnetic anisotropy, exchange and damping in cobalt-based full-Heusler compounds: an experimental review

Quaternary Heusler alloys Co2Cr1 xFexAl with varying Cr to Fe ratio x were investigated experimentally and theoretically. The electronic structure and spectroscopic properties were calculated using the full relativistic Korringa-Kohn-Rostocker method with coherent potential approximation to account for the random distribution of Cr and Fe atoms as well as random disorder. Magnetic effects are included by the use of spin dependent potentials in the local spin density approximation. Magnetic circular dichroism in X-ray absorption was measured at the L2,3 edges of Co, Fe, and Cr of the pure compounds and the x = 0.4 alloy in order to determine element specific magnetic moments. Calculations and measurements show an increase of the magnetic moments with increasing iron content. Resonant (560eV - 800eV) soft X-ray as well as high resolution - high energy (� 3.5keV) hard X-ray photo emission was used to probe the density of the occupied states in Co2Cr0.6Fe0.4Al.

https://iopscience.iop.org/article/10.1088/0022-3727/39/5/S06/pdf

Keisuke Kobayashi

Papers

Electronic structure and spectroscopy of the quaternary Heusler alloy Co2Cr1−xFexAl