SpinWave Dynamics in Antiferromagnets under Electric Current
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CurrentInduced SpinWave Doppler Shift in Antiferromagnets
J. Phys. Soc. Jpn. 90, 103705 (2021).
Electric current causes a Doppler effect in spin waves in ferromagnets through a spintransfer torque. We report that antiferromagnets allow two such spintransfer torques and present a microscopic analysis that interpolates ferro and antiferromagnetic transport regimes.
Investigating the effect of electric current on magnetic materials is crucial in spintronics. In ferromagnets, electric currents are known to drive domainwall motion and cause a Doppler shift in the spinwave spectrum. These phenomena, known as the spintransfer 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 spintransfer effect is not intuitively understood, calling for a microscopic analysis.
It was long supposed that there is only one (reactive) spintransfer 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 spintransfer torques: one arising through the coupling to the uniform spin density ( ) and the other through the staggered spin density ( ). The two spintransfer torques make equal contributions of to the spinwave Doppler shift, while only one ( ) acts on domain walls [1]. This feature is in stark contrast to ferromagnets, in which a single spintransfer torque leads to both Doppler shift and domainwall 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 spintransfer torques due to electrons on a twodimensional square lattice by considering not only the nearestneighbor (intersublattice) hopping but also the nextnearestneighbor (intrasublattice) 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 spintransfer torques reduce to the wellknown spintransfer 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)
CurrentInduced SpinWave Doppler Shift in Antiferromagnets
J. Phys. Soc. Jpn. 90, 103705 (2021).
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