Electrically detected spin currents generated by sub-terahertz microwaves

Spin current generation, transport, detection, and conversion are essential ingredients of spintronics. Similar to coherently generated spin currents in ferromagnets, spin currents in antiferromagnets (AFM) can also be generated by a coherent drive and electrically detected via the inverse spin Hall effect (ISHE) using heavy metals. In uniaxial AFM, spin currents are carried by the two eigen-modes, i.e., left- and right-handed (LH and RH) magnon modes, with the frequencies typically in the terahertz regime. Cr2O3, a uniaxial AFM, has the Neel temperature of 308 K. Its easy axis is directed along the c-axis of the corundum lattice. At zero magnetic field, the resonance frequency is 165 GHz for both modes. In the presence of a magnetic field along the c-axis, the RH (LH) mode frequency linearly increases (decreases) with the field. With the excitation frequency of 240 GHz, the AFM resonance occurs only for the RH mode at ~ 2.7 T. In our AFM spin pumping experiments (1), we have successfully demonstrated spin current generation in both Cr2O3/Pt and Cr2O3/Ta heterostructure devices at this AFM resonance condition. The ISHE voltage in either device reverses the sign at the resonance field in the opposite direction. In addition, the polarity of the ISHE voltages detected by Pt and Ta, two heavy metals with opposite spin Hall angles, is opposite to each other. These facts confirm the pure spin current nature generated at the AFM resonance. We have also observed another resonance peak at the spin-flop transition (~ 6 T) and yet another resonance feature for the net induced magnetic moment at a higher field (~ 10.5 T). At the AFM resonance, we observed a sign change in the ISHE voltage at ~45 K as the temperature is varied. Discussions of the unexpected temperature dependence will be presented in the context of the coherent and incoherent magnon thermalization process.
(1) J.X. Li et al., Nature (2020).