High bias TMR in ferrimagnetic MTJs based on Mn3Ga

Abstract

In a recent work [1], we investigated the spin transfer toque (STT) in epitaxial magnetic tunnel junctions (MTJs) based on ferrimagnetic tetragonal Mn3Ga [2] electrodes and MgO barrier (Fig.1a,b) from first principles ballistic non-collinear spin transport (NEGF + SDFT[LSDA]), using the Smeagol code [3]. We found a long-range spatial oscillation of the in-plane STT (defined as in Ref. [4]) decaying on a length scale of a few nm. We discussed how this STT oscillation can be anticipated from the bulk electronic structure of Mn3Ga and the spin-filtering properties of the MgO barrier, and it is robust against variations in the stack geometry. This is expected to result in a net in-plane torque which oscillates as a function of the Mn3Ga thickness, but stabilizes for sufficiently thick layers (>40 MLs) at larger values than the net STT in conventional Fe/MgO/Fe MTJs [1]. We also discussed a novel all-ferrimagnetic MTJ in which the Fe electrode is substituted with Mn3Ga and found similar STT properties, particularly enhanced in the case of narrow MgO barrier (of 3 MLs), due to resonant interface states.Here we focus on the TMR effect in these MTJ stacks as a function of the applied bias voltage and find a maximum value of over 300%, asymmetry and multiple sign changes in the range ± 2V (Fig.1 c) for the Fe-based junction, in a noteworthy correspondence with experimental observations for similar ferrimagnetic junctions [5]. In the case of the fully symmetric novel ferrimagnetic MTJ, we find a symmetric TMR, peaking at 0 V. This is particularly enhanced for the thin barrier (3 ML) case due to the aforementioned interface resonances. We will discuss effects of geometry optimisation and possible Hubbard U corrections to the LSDA xc-functional on the transport properties, at computed bulk parameters of Mn3Ga closer to the experimental lattice constant and magnetic moments, as well as the role of different interface terminations. In view of the stronger TMR at finite bias for the Fe-based MTJs, our calculations suggest that further efforts are justified on the Fermi-level and interface engineering of ferrimagnetic electrodes, by means of both composition and substrate-induced strain, for demanding spintronic applications.