Power-Dependent Characteristics of Spin Current Transfer in Metal Bilayer Devices under High-Power Pulse Excitation

The power-dependent transfer characteristics of spin currents generated at the interface of the permalloy/Pt bilayer device have been investigated over a wide power range from a few tens of milliwatt to 396 W. We built a high-power pulse excitation system for spin pumping, which achieves large electromotive force (EMF) values of 10 mV at 396 W excitation through the inverse spin Hall effect (ISHE) and demonstrates that the EMF generation after pulse excitation is very fast. Under strong pulse microwave excitation more than 80 W, the EMF spectrum exhibits an asymmetrical lineshape, which is well reproduced by simulations that take into account the fold-over effect due to the nonlinear ferromagnetic resonance excitation. The maximum output power at an external load through spin pumping and the ISHE is shown to increase in proportion to the square of the input microwave power (P_in) in the power range below 80 W. This power generation proportional to P_in² is unique to spin current-mediated power flow. In the strong excitation regime with the fold-over type EMF spectra, the EMF values of the peak magnetic field position are found to increase less linearly due to spectral broadening. This feature can be used for power generation that increases nonlinearly with respect to the input excitation power, where the nonlinearity is adjusted by varying the magnetic field position.