In uniaxial antiferromagnets (AFM), spin currents are carried by the two eigen-modes of the systems, i.e., left- and right-handed magnon modes. Similar to ferromagnets, spin currents can be generated by spin pumping and spin Seebeck effect (SSE), the former being associated with coherent magnons and the latter with incoherent magnons. I will present our recent experimental studies of two uniaxial AFM materials, FeF2 [1] and Cr2O3 [2]. FeF2 has strong uniaxial anisotropy resulting in a strong spin-flop transition field (~42 T). We have measured the spin Seebeck effect in FeF2 in magnetic fields far below the spin-flop field so that no induced magnetic moment is present at low temperatures. In addition to a low-temperature peak that is attributed to the AFM magnons, we have also observed a critical SSE peak in the vicinity of the AFM transition. In Cr2O3, we have successfully generated spin currents via coherent excitation at the AFM resonance by 240 GHz microwaves, and electrically detected by two heavy metals with opposite spin Hall angles. I will present our experimental results and analysis of the peculiar temperature dependence of the spin pumping responses in terms of coherent and incoherent magnons. 

[1] J. X. Li et al., Spin Seebeck effect from antiferromagnetic magnons and critical spin fluctuations in epitaxial FeF2 films, Phys. Rev. Lett. 122, 217204 (2019).
[2] J. X. Li et al., Spin current from sub-terahertz-generated antiferromagnetic magnons, Nature 578, 70 (2020).