Magnon spin-orbit torques in two-dimensional van der Waals magnets based heterostructures

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To develop two-dimensional van der Waals crystal based magnon spin-orbit torque devices, the first crucial step is to find an efficient spin polarization source that can be used to activate magnon spin current. By designing a metal-magnet/topological semimetal heterostructure, we find that the bulk spin Hall effect of the semimetal is sizable, leading to a high spin-orbit torque generation efficiency comparable to that in heavy metal based heterostructures. Strikingly, we show evidence that the energy barrier exists even in metal-magnet/semimetal heterojunctions, which governs spin transport manifested as a large observed field-like spin-orbit torque and thus a large imaginary part of the spin mixing conductance contrary to conventional wisdom, providing a new spin transport mechanism.