  Next-generation spintronic applications require material properties that can
be hardly met by one material candidate. Here we demonstrate that by combining
insulating and metallic magnets, enhanced spin-charge conversion and
energy-efficient thermal spin currents can be realized. We develop a nanowire
device consisting of an yttrium iron garnet and permalloy bi-layer. An
interfacial temperature gradient drives the nanowire magnetization into
auto-oscillations at gigahertz frequencies. Interfacial spin coupling and
magnetoresistance of the permalloy layer translate spin dynamics into sizable
microwave signals. The results show prospect for energy-efficient spintronic
devices and present an experimental realization of magnon condensation in a
heterogeneous magnetic system.

Mesoscale and Nanoscale Physics;Other Condensed Matter