Jacob Hanson-Flores

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.

Here, we describe a custom-designed quasi-optical system continuously operating in the frequency range 220 GHz to 1.1 THz with a temperature range of 5–300 K and magnetic fields up to 9 T capable of polarization rotation in both transmitter and receiver arms at any given frequency within the range through a unique double Martin–Puplett interferometry approach. The system employs focusing lenses to amplify the microwave power at the sample position and recollimate the beam to the transmission branch. The cryostat and split coil magnets are furnished with five optical access ports from all three major directions to the sample sitting on a two-axes rotatable sample holder capable of performing arbitrary rotations with respect to the field direction, enabling broad accessibility to experimental geometries. Initial results from test measurements on antiferromagnetic MnF2 single crystals are included to verify the operation of the system.