Spin pumping and spin transport in magnetic heterostructures

Synopsis

High quality, ultrathin magnetic films were prepared by means of molecular beam epitaxy (MBE). Magnetization dynamics and anisotropies were studied by means of ferromagnetic resonance (FMR) in GaAs|Fe|Au(001) structures as a function of the Fe layer thickness, allowing the determination of bulk and interface properties. Spin transport was studied in GaAs|Fe|Au|Pd structures, where two interesting results were found: (1) The spin pumping induced damping showed an oscillatory dependence on the Au spacer layer thickness when this layer’s thickness was less than the electron mean free path. This effect is attributed to the formation of quantum well states in the Au layer. (2) The spin pumping induced damping was quickly suppressed with the addition of the Au spacer layer as compared to GaAs|Fe|Pd samples. It is experimentally shown that this reduction is not related to the removal of magnetic proximity effect induced damping at the Fe|Pd interface. It is shown that the Pd layer can neither be treated as an ideal spin sink or as a simple normal metal (diffusive spin scatter) with respect to spin currents and that the reduction in damping is due to a reflection of spin currents at the Au|Pd interface.

Magnetization dynamics were investigated in ferromagnetic insulator Yttrium Iron Garnet (YIG, Y3Fe5O12). Ferromagnetic resonance was used to determine the spin pumping induced damping in YIG and YIG|Au|Fe|Au structures. In the YIG|Au|Fe|Au structures, the YIG acts as a spin pump and the Fe as a spin sink when the YIG layer undergoes ferromagnetic resonance. Comparing the damping in the YIG and YIG|Au|Fe|Au structures allows one to determine the efficiency of spin pumping at the YIG|Au interface given by the spin mixing conductance. It is found that the spin mixing conductance of as grown YIG films is 10% of that typically found at metallic FM|NM interfaces. Surface treatment of the YIG films by Ar+ etching is able to improve the spin pumping efficiency, approaching closely to that obtained by first principle electron band calculations.