Non‐Equilibrium Magnon Engineering Enabling Significant Thermal Transport Modulation

Thermal conductivity, a fundamental parameter characterizing thermal transport in solids, is typically determined by electron and phonon transport. Although electrical transport properties are material‐specific, recent advance in understanding phonon transport has led to new insights on controlling the thermal conductivity via phonon engineering techniques. Here, an unconventional way of artificially modulating the heat conduction in solids is demonstrated via engineering of magnon transport. Time‐domain thermoreflectance measurements in ferromagnetic metal/insulator junction systems reveal that the thermal conductivity of the ferromagnetic metals and interfacial thermal conductance vary substantially depending on the spatial distribution of non‐equilibrium spin currents. Systematic measurements of the thermal transport properties with changing the boundary conditions for spin currents unveil that magnons significantly modulate the heat conduction by ≈10 W m −1 K −1 even in ferromagnetic metals at room temperature, upsetting the conventional wisdom that the magnon thermal conductivity is very small in metals except at low temperatures. This magnon‐engineered thermal transport offers a new principle and manner for active thermal management.