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.