Resonant amplification of spin waves with analogue black-hole horizons

Spin-wave (SW) amplification utilizing the spin-transfer torques induced by spin-polarized current is numerically investigated. The current drives the magnetic domain wall and creates an effective moving media for SWs. When the polarized current density is sufficiently large the speed of magnetic domain-wall gets faster than the velocity of SW. Combined with a spatially inhomogeneous current density profile prepared with a bottleneck magnetic structure, a peculiar SW cavity is formed. As the cavity length is changed, the transmission and reflection signal of SWs show a resonant amplification feature. We present numerical results for transient amplification of SW pulses in the time domain. Dispersion relation, arising from magnetostatic interactions, which describes the SW propagation in the low wave-number region is included. The SW amplification effects due to competition of damping and nonadiabatic spin-transfer effect in the Zhang-Li spin-transfer torque is discussed. These results benefit both the theoretical understanding and experimental realization of SW lasers utilizing the analogue black-hole horizons.