Antiferromagnet-based magnonic spin-transfer torque

In an antiferromagnet (AF) with uniaxial anisotropy, spin-up and spin-down magnons coexist and form an intrinsic degree of freedom resembling electrons. When polarized by an adjacent ferromagnet (F), a magnonic pure spin current can be thermally generated in an AF. We explore thermal magnon transport in an insulating F/AF/F trilayer where propagating magnons inside the AF spacer can transfer angular momenta between the two Fs. We find that a sufficiently large temperature gradient can switch the downstream F via a magnonic spin-transfer torque if it is initially antiparallel with the upstream F. A reciprocal switching is achievable by reversing the temperature gradient. In typical materials, we estimate the threshold to be 1 K/nm at room temperature, which can be reduced by enhancing the interfacial exchange coupling and by increasing the temperature.

Figure 1

Schematics of the proposed switching mechanism based on a F/AF/F trilayer. (a) Interfacial exchange coupling lifts the degeneracy of the magnons with opposite spins; the degenerate is retrieved away from the interface. (b) In the antiparallel configuration, magnon populations on opposite interfaces are imbalanced in opposite ways. A sufficiently large temperature gradient can deliver majority spins from F1 (hot side) to F2 (cold side), leading to a population inversion that switches the downstream magnetization. A steady magnon spin current is established after the switching process is completed.

Figure 2

(a) $F\left(T\right)$ in Eq. (8) and the effective magnon spin polarization $P\left(T\right)$. (b) Angle dependence of the magnonic STT for single- and multidomain AFs. (c) Domains are random in the lateral dimensions but they do not end along $x$. (d) Phase boundary separating the switching and the subthreshold regimes for $N_{\text{F}}=10,\alpha /\pi =0.01,K_{\text{F}}/K_{\text{AF}}=0.1$, and $J_{\text{AF}}/J=100$. Inset: The switching time $t_{\text{SW}}$ (scaled by $t_R=2\pi \hslash /K_{\text{F}})$ as a function of the temperature gradient at $k_{B}T=5\mathrm{\Delta }$.