In the present work we describe the general mechanism of tetragonal distortion in Heusler compounds X2YZ. From 286 compounds studied using density functional theory (DFT) 62% were found to be tetragonal at zero temperature. Such a large share of compounds with tetragonal distortions can be explained by the peak-and-valley character of density of states (DOS) of these compounds in cubic phase (arising from localized d-bands and van Hove singularities) in conjunction with a smooth shift of peaky DOS structure relative to the Fermi energy, EF , when valence electrons are added to the system. A shift of DOS in Y or Z-series leads to alternation of stable and nonstable cubic phases depending on the value of DOS at EF in the cubic phase. Groups of compounds with a large share of tetragonal distortions are identified and explained.

Although high-tunnelling spin polarization has been observed in soft, ferromagnetic, and predicted for hard, ferrimagnetic Heusler materials, there has been no experimental observation to date of high-tunnelling magnetoresistance in the latter. Here we report the preparation of highly textured, polycrystalline Mn3Ge films on amorphous substrates, with very high magnetic anisotropy fields exceeding 7 T, making them technologically relevant. However, the small and negative tunnelling magnetoresistance that we find is attributed to predominant tunnelling from the lower moment Mn–Ge termination layers that are oppositely magnetized to the higher moment Mn–Mn layers. The net spin polarization of the current reflects the different proportions of the two distinct termination layers and their associated tunnelling matrix elements that result from inevitable atomic scale roughness. We show that by engineering the spin polarization of the two termination layers to be of the same sign, even though these layers are oppositely magnetized, high-tunnelling magnetoresistance is possible.

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Heusler alloys are a large family of compounds with complex and tunable magnetic properties, intimately connected to the atomic scale ordering of their constituent elements. We show that using a chemical templating technique of atomically ordered X′Z′ (X′ = Co; Z′ = Al, Ga, Ge, Sn) underlayers, we can achieve near bulk-like magnetic properties in tetragonally distorted Heusler films, even at room temperature. Excellent perpendicular magnetic anisotropy is found in ferrimagnetic X3Z (X = Mn; Z = Ge, Sn, Sb) films, just 1 or 2 unit-cells thick. Racetracks formed from these films sustain current-induced domain wall motion with velocities of more than 120 m s−1, at current densities up to six times lower than conventional ferromagnetic materials. We find evidence for a significant bulk chiral Dzyaloshinskii–Moriya exchange interaction, whose field strength can be systematically tuned by an order of magnitude. Our work is an important step towards practical applications of Heusler compounds for spintronic technologies.

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The spin Hall effect originating from 5d heavy transition‐metal thin films such as Pt, Ta, and W is able to generate efficient spin–orbit torques that can switch adjacent magnetic layers. This mechanism can serve as an alternative to conventional spin‐transfer torque for controlling next‐generation magnetic memories. Among all 5d transition metals, W in its resistive amorphous phase typically shows the largest spin–orbit torque efficiency ≈0.20–0.50. In contrast, its conductive and crystalline α phase possesses a significantly smaller efficiency of ≈0.03 and no spin–orbit torque switching is realized using α‐W thin films as the spin Hall source. Herein, through a comprehensive study of high‐quality W/CoFeB/MgO and the reversed MgO/CoFeB/W magnetic heterostructures, it is shown that although amorphous‐W has a greater spin–orbit torque efficiency, the spin Hall conductivity of α‐W (|σSHα‐W|=3.71×105 normalΩ−1 normalm−1) is ≈3.5 times larger than that of amorphous W (|σSHamorphous‐W|=1.05×105 normalΩ−1 normalm−1). Moreover, spin–orbit torque‐driven magnetization switching using a MgO/CoFeB/α‐W heterostructure is demonstrated. The findings suggest that the conductive and high spin Hall conductivity α‐W is a potential candidate for future low‐power consumption spin–orbit torque memory applications.

Heusler compounds are of interest as electrode materials for use in magnetic tunnel junctions (MTJs) due to their half metallic character, which leads to 100% spin polarization and high tunneling magnetoresistance. Most work to date has focused on the improvements to tunneling magnetoresistance that can stem from the use of Heusler electrodes, while there is much less work investigating the influence of Heusler electrodes on the spin transfer torque properties of MTJs. Here, we investigate the bias dependence of the anti-damping like and field-like spin transfer torque components in both symmetric (Co2MnSi/MgO/Co2MnSi) and asymmetric (Co2MnSi/MgO/CoFe) structure Heusler based MTJs using spin transfer torque ferromagnetic resonance. We find that while the damping like torque is linear with respect to bias for both MTJ structures, the asymmetric MTJ structure has an additional linear component to the ordinarily quadratic field like torque bias dependence and that these results can be accounted for by a free electron tunneling model. Furthermore, our results suggest that the low damping and low saturation magnetization properties of Heusler alloys are more likely to lead significant improvements to spin torque switching efficiency rather than their half metallic character.

Antiferromagnet spintronic devices eliminate or mitigate long-range dipolar fields, thereby promising ultrafast operation. For spin transport electronics, one of the most successful strategies is the creation of metallic synthetic antiferromagnets, which, to date, have largely been formed from transition metals and their alloys. Here, we show that synthetic antiferrimagnetic sandwiches can be formed using exchange coupling spacer layers composed of atomically ordered RuAl layers and ultrathin, perpendicularly magnetized, tetragonal ferrimagnetic Heusler layers. Chemically ordered RuAl layers can both be grown on top of a Heusler layer and allow for the growth of ordered Heusler layers deposited on top of it that are as thin as one unit cell. The RuAl spacer layer gives rise to a thickness-dependent oscillatory interlayer coupling with an oscillation period of ~1.1 nm. The observation of ultrathin ordered synthetic antiferrimagnets substantially expands the family of synthetic antiferromagnets and magnetic compounds for spintronic technologies.