Generation and detection of phase-coherent current-driven magnons in magnetic multilayers

The magnetic state of a ferromagnet can affect the electrical transport properties of the material; for example, the relative orientation of the magnetic moments in magnetic multilayers(1) underlies the phenomenon of giant magnetoresistance. The inverse effect-in which a large electrical current density can perturb the magnetic state of a multilayer-has been predicted(2-7) and observed experimentally with point contacts(8,9) and lithographically patterned samples(10,11). Some of these observations were taken as indirect evidence for current-induced excitation of spin waves, or 'magnons'. Here we probe directly the high-frequency behaviour and partial phase coherence of such current-induced excitations, by externally irradiating a point contact with microwaves. We determine the magnon spectrum and investigate how the magnon frequency and amplitude vary with the exciting current. Our observations support the feasibility of a spin-wave maser(2) or 'SWASER' (spin-wave amplification by stimulated emission of radiation).