Junyu Tang

The orbital motion of chargeless bosons, unlike that of electrons, does not generate a magnetic moment and thus cannot directly interact with magnetic fields. To formulate the orbital angular momentum (OAM) of magnons, we first identify its proper conjugate variable by considering the Aharonov-Casher effect, which gives rise to a virtual perturbation to the equilibrium state, allowing us to calculate the magnon OAM as a virtual response to an infinitesimal electric field divergence. At finite temperatures, both self-rotation and topological contributions to the magnon OAM are explicitly derived, analogous to their electronic counterpart but with the correct bosonic statistics. In a two-dimensional honeycomb lattice, we show that the Dzyaloshinskii-Moriya interaction induces a large magnon OAM in both the ferromagnetic and antiferromagnetic phases. Our formalism can be generalized to other chargeless bosons with intrinsic spin.

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.

Interplay between topological electrons and magnetic ordering enables efficient electrical control of magnetism. We extend the Kane-Mele model to include the exchange coupling to a collinear antiferromagnetic (AFM) order, which allows the system to exhibit the quantum anomalous Hall and quantum spin Hall effects in the absence of a net magnetization. These topological phases support a staggered Edelstein effect through which an applied electric field can generate opposite non-equilibrium spins on the two AFM sublattices, realizing the Néel-type spin-orbit torque (NSOT). Contrary to known NSOTs in AFM metals driven by conduction currents, our NSOT arises from pure adiabatic currents devoid of Joule heating, while being a bulk effect not carried by the edge currents. By virtue of the NSOT, the electric field of a microwave can drive the AFM resonance with a remarkably high efficiency, outpacing the magnetic field-induced AFM resonance by orders of magnitude in terms of power absorption.

Unidirectional magnetoresistance (UMR) in a bilayer heterostructure, consisting of a spin-source material and a magnetic layer, refers to a change in the longitudinal resistance on the reversal of magnetization and originates from the interaction of non-equilibrium spin accumulation and magnetization at the interface. Since the spin polarization of an electric-field-induced non-equilibrium spin accumulation in conventional spin-source materials is restricted to be in the film plane, the ensuing UMR can only respond to the in-plane component of magnetization. However, magnets with perpendicular magnetic anisotropy are highly desired for magnetic memory and spin-logic devices, whereas the electrical read-out of perpendicular magnetic anisotropy magnets through UMR is critically missing. Here we report the discovery of an unconventional UMR in the heterostructures of a topological semimetal (WTe₂) and a perpendicular magnetic anisotropy ferromagnetic insulator (Cr₂Ge₂Te₆), which allows to electrically read the up and down magnetic states of the Cr₂Ge₂Te₆ layer through longitudinal resistance measurements.

We resolve the debate over the existence and magnitude of cross-sublattice (CS) contributions to spin pumping and spin-transfer torques in a two-sublattice antiferromagnet connected to a non-magnetic metal. Guided by symmetry considerations, we first relate the controversial CS terms to specific components in the spin conductance matrix. Then, we quantify these components by studying the spin-dependent electron scattering on a fully compensated interface. We ascertain the absence of all CS contributions in the collinear regime. Even in the non-collinear regime, the CS contributions only constitute a higher-order correction to the existing theory.