Unique Signatures of Superfluid Angular Spin Current Transport in Easy-Plane Ferromagnets

Easy-plane magnetic films have been theorized to exhibit spin superfluidity, providing a promising route towards magnetic analogs of superconducting devices [1]. However, the typical orientation of the easy plane requires device geometries that are incompatible with conventional spin injection. Moreover, there is skepticism about the implications of the predicted effect, which involves the transport of spin polarization, not spin density [2]. Furthermore, its efficiency is sensitive to damping and the boundary conditions at the spin injector and detector [3, 4]. In this work, we address these challenges by investigating easy-plane ferromagnets that are compatible with the lateral device geometry [4]. Using a hydrodynamic formulation of the Landau–Lifshitz–Gilbert equation in conjunction with magneto-circuit theory, we show that the polarization transport in such devices is indeed mediated by the non-diffusive superflow of spin torque, also known as angular spin current. We identify a corresponding electrical Meissner effect [5] by predicting spin-pumping contributions to the local and nonlocal spin Hall magnetoresistance (SMR). Moreover, we show that the nonlocal SMR can be tuned by the relative dimensions of the device's components. Additionally we show that the primary contributions to the magnonic second sound [6] in these devices correspond to classical longitudinal and relativistic transverse Doppler effects. Finally, we synthesize complementary frameworks—relativistic domain wall trains [7], dispersive exchange flows [3], and conserved spin torque [2]—to provide new insights into the underlying mechanisms of spin superfluidity. Our study legitimizes spin superfluidity in easy-plane magnets and provides a roadmap for unambiguously realizing this phenomenon in practical devices.

1. D. Hill, S. K. Kim, and Y. Tserkovnyak, Phys. Rev. Lett. 121, 037202 (2018)

2. Sun, Qf. and Xie, X. C. Phys. Rev. B 72, 245305 (2005)

3. Iacocca, E. and Hoefer, M. A., Phys. Rev. B 99, 184402 (2019)

4. Evers, M., and Nowak, U., Phys. Rev. B 101, 184415 (2020)

5. Wang, Zb., Sun, Qf. and Xie, X. C., Eur. Phys. J. B 86, 496 (2013)

6. Sonin, E. B., Phys. Rev. B 99, 104423 (2019)

7. Smith, D. A. et al., Phys. Rev. Appl. 16, 054002 (2021)