Effective exchange fields in spin-torque resonance of magnetic insulators

•We model spin-torque FMR spectra of magnetic insulators.•The imaginary part of the spin mixing conductance may be anisotropic.•The phase shift between ac current and Oersted field should be studied.

We report additional results on the spin-torque ferromagnetic resonance (ST-FMR) of a bilayer system made from a magnetic insulator such as Y3Fe5O12Y3Fe5O12 (YIG) and a heavy normal metal such as Pt in terms of the interface spin-mixing conductance and including spin pumping. We analyze experimental ST-FMR spectra for out-of-plane and in-plane magnetization configurations in terms of an anisotropic imaginary part Gi of the mixing conductance (or interface effective field). The estimated ratio between imaginary and real parts Gi/Gr≲0.3Gi/Gr≲0.3 is sensitive to an (unknown) phase shift between microwave current bias and associated Oersted field.