Injection of a pure spins current into a magnetic insulator modifies the chemical potential of magnons and can lead to intriguing phenomena such as long-range spin transport and magnon condensation. We study nanowires fabricated from a bilayer of a metallic ferromagnet and an insulating ferrimagnet. Via interlayer exchange coupling, magnons can be thermally transferred from one layer into another, constituting a pure spin current. The injection generates a non-equilibrium population of incoherent magnons. Nonlinear magnon interaction transfers the angular momentum from incoherent magnon cloud to a low-energy magnon state, thereby reducing its effective damping. Above the critical thermal spin current, magnonic system undergoes a phase transition establishing a coherent low-energy magnonic state. This process corresponds to bosonic condensation of non-equilibrium magnons into a coherent magnon condensate, manifesting through microwave emission from the nanowire. The design of bi-magnetic heterostructures further offers opportunities for realization of macroscopic exchange-mediated spin flows (also known as spin superfluids). Using micromagnetic simulations, we investigate the impact of dipolar interactions and soliton formation in the heterostructures. We find a surprising interaction of the exchange-mediate spin flows with incoherent magnons that stabilizes the long-range spin transport. Parts of this work were supported by Spins and Heat in Nanoscale Electronic Systems (SHINES), an Energy Frontier Research Center funded by the US Department of Energy and by NSF under grant ECCS-1810541. Igor Barsukov graduated with Diploma in Physics from Ruhr-University Bochum (Germany). He commenced his graduate studies at the Academy of Sciences in Prague (Czechia) as a recipient of the Marie-Curie Fellowship and obtained the PhD degree from the University Duisburg-Essen (Germany). As postdoctoral researcher, he moved to UC Irvine. In 2016, he joined the Department of Physics and Astronomy at UC Riverside as Assistant Professor. His lab focuses on experimental condensed matter physics and in particular on spintronics. The research interests include microwave and terahertz spectroscopy, spin transport, nonlinear spin dynamics, and thermodynamics of magnetic systems.
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Dr. Igor Barsukov, University of California