Among the Mn-based Heusler ferrimagnets are a number of topical materials for the emerging field of Antiferromagnetic Spintronics, combining low moments, high spin-polarisation and anisotropy. In a recent work (1), we described the long-range spatial oscillation of the spin-transfer torque (STT) in magnetic tunnel junctions (MTJs) based on tetragonal (DO22) Mn3Ga (2), present both in Fe/MgO/Mn3Ga and in Mn3Ga/MgO/Mn3Ga tri-layers. Using first principles ballistic non-collinear spin transport (NEGF+SDFT) (3,4), we related the STT oscillation to the mismatch of the Fermi wavevectors of the majority and minority D1 symmetry band in Mn3Ga in the direction of transport. We studied the high-bias TMR effect in these stacks and found, for the symmetric MTJs, a maximum value of over 100% and a sign change below 1V, in accordance with experimental observations for similar ferrimagnetic MTJs (5). Another Mn-based Heusler Mn3Al has been shown to exhibit half-metallicity and almost ideally fully compensated moment in its cubic DO3 phase (6) and proposed MTJs with GaAs have shown large theoretical TMR ratios (7). Our ab initio geometry optimisation of Mn3Al revealed a stable tetragonal DO22 solution with almost fully compensated moment (<0.002 μB/f.u.) and an in-plane lattice constant commensurate with MgO, which prompted a study of a familiar tri-layer. As geometrically the Mn3Ga and Mn3Al stacks with MgO are very similar, this could offer further insights about their Spintronic capacity. Here we compare the spin-dependent transport properties of two such junctions (3ML MgO spacer, see Fig. 1(a)). We find that Mn3Al is half-metallic (lacking a majority spin channel at EF) in the direction of transport (Fig.1 (d)), which results in an enormous TMR effect (Fig.1 (c)), but also in the suppression of the long-range STT oscillation (Fig.1 (b)) and a faster decay of the STT with the barrier thickness. The STT is significant in magnitude (1) and pushes both interfacial spins in the same direction (clockwise in Fig.1 (a)). In view of the above, aluminium-rich solid solutions of Mn3Al-Mn3Ga could offer enhanced low-bias TMR performance, while preserving tetragonality, at manageable growth-induced strain, in practical device stacks.

References:
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