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

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Abstract
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 multilayers1 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 predicted2,3,4,5,6,7 and observed experimentally with point contacts8,9 and lithographically patterned samples10,11. 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 maser2 or ‘SWASER’ (spin-wave amplification by stimulated emission of radiation).