In Search of Quantum Spin Transfer in Spin-Orbit Bilayers

Spins (s) injected into a ferromagnet generate a spin-transfer torque that can alter the thermal fluctuations of magnetization (M). At low temperatures, with minimal fluctuations, no spin-transfer effects are expected when s and M are collinear. However, recent studies predict “quantum spin transfer” through entanglement between collinear s and M [1]. Here, we search for a DC transport signature of quantum spin transfer in a spin-orbit bilayer, where a current in Pt pumps y-polarized spins into y-magnetized permalloy.

At room temperature, the DC resistance is mostly quadratic with current, as expected from Joule heating. Subtracting the quadratic background, we find a residual resistance monotonic with current and its slope reversing with the field polarity. This corresponds to spin-orbit torque tuning the average y-component of M (captured by anisotropic magnetoresistance) via modulated thermal fluctuations of M. At 7 K, the resistance response appears quadratic, but subtracting the quadratic background reveals a distinct V-shaped trend in residual resistance vs current, invariant with field polarity. This aligns with current-generated non-thermalized phonons at cryogenic temperatures [2], but not with quantum spin transfer, which should vary with field polarity [1]. Our work unveils coexisting transport signatures in spin-orbit nanostructures that may find use in cryogenic devices.

[1] A. Zholud et al., Phys. Rev. Lett., 119, 257201 (2017).

[2] G. Chen et al., Phys. Rev. X, 10, 011064 (2020).