Coherent Spin Pumping Originated from Sub-Terahertz Néel Vector Dynamics in Easy Plane α-Fe_{2}O_{3}/Pt

We present a thorough study of spin-to-charge current interconversion in bulk and thin films of (0001) α-Fe_{2}O_{3}/Pt heterostructures by means of all-optical polarization-controlled microwave excitation at subterahertz frequencies. Our results demonstrate that coherent spin pumping is generated through excitations of both the quasiferromagnetic and quasiantiferromagnetic (q-AFM) modes of antiferromagnetic resonance, provided that the corresponding selection rules are met for the relative orientation between the microwave magnetic field h[over →]_{ac} and the magnetic moment m[over →]_{0} of the hematite. In particular, our results unambiguously show that while a microwave field with h[over →]_{ac}⊥m[over →]_{0} pumps a net spin angular momentum from the quasiferromagnetic mode, spin pumping from the q-AFM mode is only enabled when h[over →]_{ac}||m[over →]_{0}, as expected from the selection rules imposed by the Néel vector dynamics. Our findings significantly advance current understanding of spin pumping in easy-plane antiferromagnets not only by claiming a substantial contribution from the Néel-vector dynamics of the q-AFM mode but also by experimentally determining the relative strength of the intra- and cross-sublattice components of the spin-mixing conductance, challenging recent reports where the absence of spin pumping from the q-AFM mode in hematite was interpreted as a complete cancellation between these two components. We also provide an explanation for the previously reported observations and show how the q-AFM spin pumping actually vanishes for thin films, which we speculate being either due to an increased level of inhomogeneities or to insufficient film thickness for the q-AFM mode to fully realize.