An ability to control spin currents is important for probing many spin related phenomena in the field of spintronics and for designing logic and memory devices with low dissipation. Spin-orbit torque is an important example in which spin current flows across magnetic interface and helps to control magnetization dynamics. In this talk, I will discuss the spin superfluid transport associated with collective modes in magnetic insulators. We observe that in two dimensional systems at finite temperatures spin superfluidity is affected by the presence of topological defects. We further propose to use the Hall response of topological defects, such as merons and antimerons, to spin currents in 2D magnetic insulator with in-plane anisotropy for identification of the Berezinskii-Kosterlitz-Thouless (BKT) transition in a transistor-like geometry. Our numerical results relying on a combination of Monte Carlo and spin dynamics simulations show transition from spin superfluidity to diffusive spin transport, accompanied by the universal jump of the spin stiffness and exponential growth of the transverse vorticity current. We propose a superfluid spin transistor in which the spin and vorticity currents are modulated by tuning the in-plane magnet across BKT transition, e.g., by changing the exchange interaction, magnetic anisotropy, or temperature [1-3].
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