Spin-Wave Dynamics in Antiferromagnets under Electric Current

Investigating the effect of electric current on magnetic materials is crucial in spintronics. In ferromagnets, electric currents are known to drive domain-wall motion and cause a Doppler shift in the spin-wave spectrum. These phenomena, known as the spin-transfer effect, can be understood as the exchange of spin angular momentum between magnetization and conduction electrons. In antiferromagnets, neither the conduction electrons nor antiferromagnetic spins carry macroscopic spin angular momentum, and the spin-transfer effect is not intuitively understood, calling for a microscopic analysis.

It was long supposed that there is only one (reactive) spin-transfer torque in antiferromagnets, as in ferromagnets. In this study, starting from a microscopic Hamiltonian with conduction electrons, we show that antiferromagnets have two different types of spin-transfer torques: one arising through the coupling to the uniform spin density ( ) and the other through the staggered spin density ( ). The two spin-transfer torques make equal contributions of to the spin-wave Doppler shift, while only one ( ) acts on domain walls [1]. This feature is in stark contrast to ferromagnets, in which a single spin-transfer torque leads to both Doppler shift and domain-wall motion. The Doppler shift depends on chirality of antiferromagnetic magnons, thus an electric current can be used to differentiate the two modes via the shift in wavelength or frequency.

We microscopically calculated the spin-transfer torques due to electrons on a two-dimensional square lattice by considering not only the nearest-neighbor (inter-sublattice) hopping but also the next-nearest-neighbor (intra-sublattice) hopping; the former induces “antiferromagnetic transport” in the sense that the electrons feel the alternating magnetization, whereas the latter induces “ferromagnetic transport,” rendering the electrons feel a uniform magnetization. One can interpolate the two transport regimes (ferromagnetic and antiferromagnetic) by varying the hopping parameters. In the limit of ferromagnetic transport, the two spin-transfer torques reduce to the well-known spin-transfer torque in a ferromagnet. In the antiferromagnetic transport regime, the two torques collaborate or compete, and the overall Doppler shift depends on microscopic parameters (such as band filling); it is negative (same sign as for ferromagnets) at small band filling and changes sign as the lower band becomes filled towards the antiferromagnetic band gap.

[1] J. J. Nakane and H. Kohno, Phys. Rev. B 103, L180405 (2021).

(Written by J. Nakane on behalf of all authors)