Observation of phase dependent voltage signals in microwave spin pumping experiments

Abstract

Microwave excitation of magnetization dynamics in a ferromagnetic material (FM)-normal metal (NM) interface induces spin currents into the NM via spin pumping. A symmetric DC voltage is often observed due to the inverse spin Hall Effect (ISHE). Apart from DC-ISHE, spin rectification (SR) effects due to microwave excitations also produce a DC voltage response. The voltage signals are sensitive to the phase difference between the electric and magnetic fields inside a FM. In this work, we investigate the effect of the microwave signal direction on the measurement of DC voltage signals by exciting radio frequency signals along two opposite paths using a vector network analyzer. We observe two distinct DC voltages at constant magnetic fields, over a wide range of microwave frequencies and powers. At resonance fields, these two voltage states are many microvolts apart, opening up a new possibility for using SR and spin pumping for spintronic devices.