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.

Magneto-optical properties of metallic ferromagnetic materials

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

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

Magnetization studies of binary and ternary alloys based onFe3Si

X-ray, magnetization, and spin-echo NMR techniques have been used to measure the changes in the atomic ordering along with the effect of these changes on the local moment and the internal field distributions in ${\mathrm{Fe}}_{3\ensuremath{-}x}{\mathrm{V}}_{x}\mathrm{Si}$ alloys for $0\ensuremath{\le}x\ensuremath{\le}1.00$. We have shown that V continues to enter predominantly the $B$ sites in the ${\mathrm{Fe}}_{3}$Si matrix up to $x\ensuremath{\approx}1.0$, which is the limit of V solubility. The ${\mathrm{Fe}}_{3\ensuremath{-}x}{\mathrm{V}}_{x}\mathrm{Si}$ alloys are ferromagnetic up to $x\ensuremath{\approx}0.9$ with the saturation magnetization decreasing linearly with $x$. V substitution into the $B$ sites reduces the moments of the first-near-neighbor (1nn) $\mathrm{Fe}(A,C)$ atoms linearly from their initial value of $1.35{\ensuremath{\mu}}_{B}$ while the moments of $\mathrm{Fe}(B)$ atoms remain constant at $2.20{\ensuremath{\mu}}_{B}$. A model giving a detailed mechanism for the origin of the satellite structure observed in the NMR spectra is proposed for ${\mathrm{Fe}}_{3\ensuremath{-}x}{\mathrm{V}}_{x}\mathrm{Si}$ as well as for ${\mathrm{Fe}}_{3\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Si}$, a previously measured system. According to this model, the changes in the transition-metal internal fields at $B$ sites are produced as a result of changes in the $4s$ spin polarization induced by the perturbations of the 1nn $\mathrm{Fe}(A,C)$ moments. The values calculated from this model for the on-site hyperfine fields of $\mathrm{Fe}(B)$ and $\mathrm{Mn}(B)$ are -205 and -192 kOe, respectively. The on-site contribution to the V hyperfine field is estimated to be about 3 kOe with this model, in reasonable agreement with the very low moment value found for V. This detailed work suggests that the $\mathrm{Fe}(A,C)$ 1nn shell screens the $B$ sites from any direct interaction with a more distant neighbor either magnetically or chemically. Using all of the NMR and magnetization data together with the model, a canting of the Mn moments in ${\mathrm{Fe}}_{3\ensuremath{-}x}{\mathrm{M}}_{x}\mathrm{Si}$ for $0.75lxl1.6$ has been predicted. In addition, the disorder contributions to the NMR spectra have been identified.

Relating structural, magnetization, and hyperfine field studies to a local environment model in Fe 3 − x V x Si and Fe 3 − x Mn x Si

Properties of Heusler-type materials Fe2TSi and FeCo2Si☆

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.

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

A detailed study of changes in the internal fields at Mn, Fe, and Si nuclei in the solid solutions of ${\mathrm{Fe}}_{3\ensuremath{-}x}{\mathrm{Mn}}_{x}\mathrm{Si}$, $0\ensuremath{\le}x\ensuremath{\le}1.6$, as a function of composition has been made by spin-echo NMR at 1.28 K and in zero external magnetic field. Resonances due to Mn in three crystallographically different sites are observed. For $xl0.75$, Mn enters predominantly the site with eight Fe first near neighbors and has an internal field of 259 kG shifted about 5.4 kG to lower values for each added Mn third near neighbor. For $xg0.75$, Mn atoms also enter the other transition-metal site which has four Fe and Mn, and four Si first near neighbors and has an internal field of about 28 kG at $x=0.75$. This field shifts to lower values as $x$ increases. Mn enters the Si sites due to a small residual disorder of about 3% between Si and the Fe site with eight Fe first near neighbors. The internal field of Mn in these sites is about 127 kG at $x=0.08$ and shifts slowly to higher values. The Fe and Si fields are compared with those in ${\mathrm{Fe}}_{3}$Si alloys. The values of the internal fields and their variation with concentration at all sites are consistent with the magnetic moment and magnetization data of Yoon et al. Contributions to the internal fields from various neighbors are analyzed.

K. Raj

Papers

Magnetization studies of binary and ternary alloys based onFe3Si

Relating structural, magnetization, and hyperfine field studies to a local environment model in Fe 3 − x V x Si and Fe 3 − x Mn x Si

Properties of Heusler-type materials Fe2TSi and FeCo2Si☆

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

Correlation of the internal fields, magnetic moments, and site preferences in Fe 3-x Mn x Si alloys