Coherent Spin Pumping Using Easy-plane Antiferromagnets*

Antiferromagnetic (AFM) spin pumping has been shaping a surging frontier of ultrafast spin generating and detection in the terahertz frequency range. Recently, sub-Terahertz spin pumping has been confirmed experimentally in uniaxial AFM materials such as Cr₂O₃ and MnF₂, which agrees with the theoretical model. However, biaxial (easy-plane) AFM materials, which are not only more abundant in nature but also easier to be accessed in frequency than their uniaxial counterparts, are considered to be bad candidates for spin pumping because the Néel vector is linearly polarized due to the easy-plane anisotropy, placing a major restriction on the choice of materials for practical applications. We theoretically challenge this seemingly established conclusion by showing that easy-plane AFM materials can pump DC spin currents with the help of either the Dzyaloshinskii–Moriya interaction (DMI) or an applied magnetic field without inducing the spin-flop transition. The former case, which has been realized in the experiment, is exemplified by α-Fe­₂O₃ in which the canted magnetization exhibits an elliptical precession around its equilibrium direction thanks to the DMI, resulting in a finite DC spin pumping driven by a linearly polarized microwave. The latter case is exemplified by collinear NiO, where a previously unknown chirality-flip can be induced by an applied magnetic field along the in-plane easy-axis direction well below the spin-flop transition, because of which the spin pumping contributions from different sublattices constructively added up and the overall DC signal turns out to be as strong as that in uniaxial AFM materials. Our finding significantly broadens the materials pool for antiferromagnetic spintronics.