Mechanism of a pure spin transfer through an antiferromagnetic insulator

Here, we propose a simple theoretical explanation of the spin transfer effect as a diffusion of low-frequency spin waves excited in yttrium iron garnet (YIG) through the thickness of the antiferromagnetic (AFM) layer. We assume, that the spin waves excited in YIG act as a source an alternating magnetic field at the FM-AFM boundary. We describe the spin dynamics in the AFM using a standard “sigma model” approach where the vector of antiferromagnetism is a principal dynamical variable. In our approach, for each direction of the anisotropy field, the equations of the spin dynamics in the AFM can be reduced to an equation of a vibrating string under the influence of an external force at the end of the string (at the FM-AFM interface). Thus, the alternating magnetic field at the FM-AFM interface could cause the forced excitation of two low-frequency spin wave modes, none of which is the eigenmode of the AFM layer.