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.

In this work, results of {\it ab-initio} band structure calculations for $A_2BC$ Heusler compounds that have $A$ and $B$ sites occupied by transition metals and $C$ by a main group element are presented. This class of materials includes some interesting half-metallic and ferromagnetic properties. The calculations have been performed in order to understand the properties of the minority band gap and the peculiar magnetic behavior found in these materials. Among the interesting aspects of the electronic structure of the materials are the contributions from both $A$ and $B$ atoms to states near the Fermi energy and to the total magnetic moment. The magnitude of the total magnetic moment, which depends as well on the kind of $C$ atoms, shows a trend consistent with the Slater-Pauling type behavior in several classes of these compounds. The localized moment in these magnetic compounds resides at the $B$ site. Other than in the classical Cu$_2$-based Heusler compounds, the $A$ atoms in Co$_2$, Fe$_2$, and Mn$_2$ based compounds may contribute pronounced to the total magnetic moment.

Calculated electronic and magnetic properties of the half-metallic, transition metal based Heusler compounds

In this paper, results of ab initio band structure calculations for A2BC Heusler compounds that have A and B sites occupied by transition metals and C by a main group element are presented. This class of materials includes some interesting half-metallic and ferromagnetic properties. The calculations have been performed in order to understand the properties of the minority band gap, the peculiar transport properties and magnetic behaviour found in these materials. Among the interesting aspects of the electronic structure of the materials are the contributions from both A and B atoms to the total magnetic moment. The magnitude of the total magnetic moment shows a trend consistent with the Slater–Pauling type behaviour in several classes of these compounds. The total magnetic moment also depends on the kind of C atoms although they do not directly contribute to it. In Co2 compounds, a change of the C element changes the contribution of the t2g states to the moment at the Co sites. The localized moment in these magnetic compounds resides at the B site. Other than in the classical Cu2-based Heusler compounds, the A atoms in Co2, Fe2 and Mn2-based compounds may contribute significantly to the total magnetic moment. It is shown that the inclusion of electron–electron correlation in the form of LDA + U calculations helps to understand the magnetic properties of those compounds that already exhibit a minority gap in calculations where it is neglected. Besides the large group of Co2 compounds, half-metallic ferromagnetism was here found only in such compounds that contain Mn.

https://iopscience.iop.org/article/10.1088/0022-3727/40/6/S01/pdf

In this work a simple concept was used for a systematic search for materials with high spin polarization It is based on two semiempirical models First, the Slater-Pauling rule was used for estimation of the magnetic moment This model is well supported by electronic structure calculations The second model was found particularly for ${\mathrm{Co}}_{2}$ based Heusler compounds when comparing their magnetic properties It turned out that these compounds exhibit seemingly a linear dependence of the Curie temperature as function of the magnetic moment Stimulated by these models, ${\mathrm{Co}}_{2}\mathrm{FeSi}$ was revisited The compound was investigated in detail concerning its geometrical and magnetic structure by means of x-ray diffraction, x-ray absorption, and M\"ossbauer spectroscopies as well as high and low temperature magnetometry The measurements revealed that it is, currently, the material with the highest magnetic moment $(6{\ensuremath{\mu}}_{B})$ and Curie temperature (1100 K) in the classes of Heusler compounds as well as half-metallic ferromagnets The experimental findings are supported by detailed electronic structure calculations

/pdf/geometric-electronic-and-magnetic-structure-of-co2fesi-curie-54ut7uba4e.pdf

Geometric, electronic, and magnetic structure of Co2FeSi: Curie temperature and magnetic moment measurements and calculations

This work reports on structural and magnetic investigations of the Heusler compound Co2FeSi. X-ray diffraction and Mosbauer spectrometry indicate an ordered L21 structure. Magnetic measurements by means of x-ray magnetic circular dichroism and magnetometry revealed that this compound is, currently, the material with the highest magnetic moment (6μB) and Curie temperature (1100K) in the classes of Heusler compounds as well as half-metallic ferromagnets.

Investigation of Co2FeSi: The Heusler compound with highest Curie temperature and magnetic moment

A detailed NMR, magnetization, x-ray, and neutron-diffraction study is reported for the ${\mathrm{Fe}}_{3\ensuremath{-}x}{\mathrm{Co}}_{x}\mathrm{Si}$ system over the entire range of Co concentration ($0\ensuremath{\le}x\ensuremath{\le}3.00$). As the Co concentration $x$ is increased, the x-ray measurements indicate that a single phase having the fcc $D{O}_{3}$-type structure is maintained (with some variation in lattice constant) up to $x=2.15$. It is found that Co selectively enters the ($A$,$C$) sites for the higher concentrations studied in this work ($x\ensuremath{\le}2.00$) with a high degree of order for $xl1.50$. The variation in lattice constant with Co concentration correlates well with the bulk magnetization and can be described by a simple empirical relation which is an extension of Vegard's law. Neutron-diffraction and magnetization measurements have enabled a determination of the $\mathrm{Fe}(B)$, $\mathrm{Fe}(A,C)$, and $\mathrm{Co}(A,C)$ magnetic-moment dependence on Co concentration for these alloys. In particular, we note that the moment on the substituted Co atoms appears well localized and remains essentially constant (+1.7${\mathrm{\ensuremath{\mu}}}_{\mathit{B}}$) throughout the range $0\ensuremath{\le}x\ensuremath{\le}2.15$. In addition, the variations of the internal hyperfine fields with Co concentration at all sites [$\mathrm{Fe}(B)$, $\mathrm{Fe}(A,C)$, $\mathrm{Co}(A,C)$, and $\mathrm{Si}(D)$] have been studied by spin-echo NMR. In order to explain the magnetic-moment and internal field behavior, a model emphasizing the short range interaction approach is presented. Since Co enters the ($A$,$C$) sites, the short range interaction model involves only the $1nn$ configurations for the $\mathrm{Fe}(B)$ atoms. The model successfully describes the detailed behavior of the magnetic moments and internal fields at all sites and enables a subdivision of the observed internal field into contributions due to the $4s$ spin polarization transferred from neighboring moments and the polarization resulting from the on-site moment. Such an approach has already proved successful for transition metal substitutions into the $B$ sites.

S. J. Pickart

Papers

Relating structural, magnetic-moment, and hyperfine-field behavior to a local-environment model in Fe 3-x Co x Si

Atomic and magnetic ordering in Fe3−xVxSi alloys