Oscillatory spin pumping in Nb/NiFe/Nb junctions*

The quantum size-induced oscilating physical phenomena has attracted a lot of interest in condensed matter physics and materials science. In this talk, I will discuss the oscillatory spin pumping in the Nb(100nm)/NiFe/Nb(100nm) trilayer structure. In the normal state of Nb, via systematical varying the thickness of NiFe from 5nm to 20nm, an oscillating Gilbert damping is observed. This oscillating behavior can be attributed to the quantum-interference effect of angular momentum transfer between the local precessing magnetic moment and conduction electrons in thin NiFe that was theoretically predicted by Mills [1]. As the NiFe layer is thin and coupled to nonmagnetic Nb layers, quantum-interference effect of the spin-polarized electrons shows up, which gives rise to the oscillating spin-transfer torque to the NiFe. This oscillating feature is observed to be largely enhanced below the superconducting critical temperatures in the Nb/NiFe/Nb Junctions.
[1]D. L. Mills, Phys. Rev. B 68, 014419 (2003).