Observation of magnonic spin-orbit torque in epitaxial Co films

Using spin-orbit torque (SOT) to manipulate magnons has played a crucial role in magnon spintronics. However the effect of magnons on SOT remains elusive within the context of electron spintronics. Here, we report the observation of magnon-induced SOT in an epitaxial Co film through precise low-magnetic-field measurements of second harmonic voltages in both longitudinal and transverse directions. Unlike the conventional field-like SOT, which operates independently of the magnetic field, the magnonic SOT exclusively emerges at low magnetic fields and undergoes rapid decay as the magnetic field increases. Our analysis reveals a significant linear correlation between the field-dependent decay of the observed SOT and the magnon-induced unidirectional magnetoresistance effect. This correlation provides compelling evidence that magnons drive the observed SOT.