Direct Observation of Spin Current Oscillation in a Ferromagnet

Spin current is a crucial element in spintronics, and its diffusion in materials is typically characterized by monotonic decay. However, when the material hosting the spin current is also a magnet, the spin current is expected to exhibit spatial oscillations, the observation of which remains elusive. Here, we demonstrate the spatial oscillation of a spin current in a nickel film by measuring the thickness-dependent inverse spin Hall effect in $\mathrm{Ni}/\mathrm{YIG}$ bilayers. The inverse spin Hall current in nickel is found to oscillate with its film thickness, in contrast to nonmagnetic materials, and that the oscillation period quantitatively agrees with theoretical predictions based on differences in the Fermi wave vector between majority and minority carriers. Our findings reveal a previously hidden behavior of spin-transport dynamics and identify a new degree of freedom for manipulating spin current, with potential implications for future spintronic devices.