Spin transfer torque in antiferromagnetic spin valves: From clean to disordered regimes

Abstract

Current-driven spin torques in metallic spin valves composed of antiferromagnets are theoretically studied using the nonequilibrium Green's function method implemented on a tight-binding model. We focus our attention on G-type and L-type antiferromagnets in both clean and disordered regimes. In such structures, spin torques can either rotate the magnetic order parameter coherently (coherent torque) or compete with the internal antiferromagnetic exchange (exchange torque). We show that, depending on the symmetry of the spin valve, the coherent and exchange torques can either be in the plane, ∝ n × (q × n) or out of the plane ∝ n × q, where q and n are the directions of the order parameter of the polarizer and the free antiferromagnetic layers, respectively. Although disorder conserves the symmetry of the torques, it strongly reduces the torque magnitude, pointing out the need for momentum conservation to ensure strong spin torque in antiferromagnetic spin valves.

Antiferromagnetic spintronics

“…G-type (checkerboard) and A-type (layered) antiferromagnets were considered and the case of the ferromagnetic spin valve is given for comparison. From Saidaoui, Manchon, and Waintal, 2014. In antiferromagnet/tunnel-barrier/antiferromagnet tunnel junctions, the symmetry of the torques is found to be the same as in metallic spin valves and their bias dependence is similar to that in ferromagnetic tunnel junctions (Saidaoui, Waintal, and Manchon, 2017) (Fig. 18).…”

“…In fact, in contrast to ferromagnetic spin valves, which are well described within incoherent semiclassical models, quantum coherence is crucial to enable the transmission of staggered spin density from one part of the spin valve to the other. Recent tight-binding calculations Saidaoui, Manchon, and Waintal, 2014) have indeed demonstrated that spin dephasing and mere spin-independent disorder in the spin valve dramatically quenches the spin torque efficiency (see Fig. 16).…”

“…Therefore, a spin-flip event is associated with a flip of the sublattice state. Saidaoui, Manchon, and Waintal (2014) showed that the potential for electrically injecting a spin current from an antiferromagnet into a normal metal drastically depends on the type of antiferromagnetic texture. Figure 24 shows three configurations: (a) ferromagnetic, (b) G-type [or checkerboard texture, as in Fig.…”

Antiferromagnetic spintronics

“…Moreover, microscopic calculations showed that in these allantiferromagnetic spin valves, the non-relativistic STTs are subtle, spin-coherent quantum-interference phenomena relying on perfectly epitaxial and commensurate multilayers. 2,22,23 This may explain why the STT in antiferromagnetic spin valves has not yet been identified experimentally.…”

“…22,23 By adding to the structure a second, free antiferromagnet with a commensurate lattice one can infer from the above considerations the symmtries of the STTs acting in the second antiferromagnet. Since p 1 = −p 2 is staggered in this case, the effective field ∼ s i ∼ M i × p i , driving the (anti)damping-like STT, is non-staggered and is therefore inefficient.…”