Spin pumping at terahertz nutation resonances

We investigate spin pumping current injected by the nutation resonances of a ferromagnet (FM) or an antiferromagnet (AFM) into an adjacent metal. Comparing the dc spin pumping current between the normal precession and the nutation resonances, we find that the ratio of spin pumping current at the nutation resonance to the precession resonance is more pronounced in AFMs. We further show that the spin pumping current injected by the nutation resonance is opposite in sign as compared with the normal precession mode. This could offer a useful experimental signature for identifying such nutation resonances. Analyzing the nature of the nutational eigenmodes, we show that the sign change in spin current is rooted in a reversal of the precession sense for the nutation mode(s). Furthermore, the nutational modes in AFMs are found to be dominated by precession of one of the two sublattices only.