Spin Superfludity Enabled by a Geometrically Tunable Nonlocal Spin Hall Magnetoresistance

Spin superfluidity (SSF) inovles the near-dissipationless transport of spin angular momentum across a magnetic material via a cohrently precessing spin spiral [1], making it a promising route towards magnetic analogs of superconducting devices, as well as novel spintronics devices [2]. While conventional easy-plane magnets can exhibit SSF in theory [3], the required device geometries are incompatible with current fabrication methods and/or spin injection via the spin Hall effect (SHE) in an adjacent heavy metal. Considering the ideal case of an insulating magnet, we analyze the nonlocal spin and charge transfer efficiencies of SSF devices with a lateral geometry [4], which has been underexplored due to an absence of compatible materials. Using a hydrodynamic formulation of the Landau–Lifshitz–Gilbert equation in conjunction with magneto-circuit theory, we show that the spin transfer efficiency is maximized when the area of the injector is much larger than the combined area of the detector and transport channel. Moreover, we relate the charge transfer efficiency and the nonlocal resistance at the detector to the conventional spin magnetoresistance (SMR) at the heavy metal/insulator interface and consider its geometrical tunability. Our analysis extends the theory of conventional SMR by identifying these relationships as manifestations of its unrecognized Onsager reciprocal, which is negative: spin-pumping–induced SMR.<br/><br/>[1] E. B. Sonin, Sov. Phys. JETP 47, 1091 (1978); Adv. Phys. 59, 181 (2010). [2] D. Hill, S. K. Kim, and Y. Tserkovnyak, Phys. Rev. Lett. 121, 037202 (2018) [3] S. Takei and Y. Tserkovnyak, Phys. Rev. Lett. 112, 227201 (2014). [4] H. Skarsvåg, C. Holmqvist, and A. Brataas, Phys. Rev. Lett. 115, 237201 (2015).