Probing Thermal Magnon Current Mediated by Coherent Magnon via Nitrogen-Vacancy Centers in Diamond

Currently, thermally excited magnons are being intensively investigated, owing to their potential in computing devices and thermoelectric conversion technologies. We report the detection of a thermal magnon current propagating in a magnetic insulator yttrium iron garnet under a temperature gradient using a quantum sensor: electron spins associated with nitrogen-vacancy (N-V) centers in diamond. A thermal magnon current is observed as modified Rabi-oscillation frequencies of N-V spins hosted in a beam-shaped bulk diamond that is resonantly coupled with coherent magnon propagating over a long distance. Additionally, using a nanodiamond, alteration in N-V spin-relaxation rates, depending on the applied temperature gradient, are observed under nonresonant N-V excitation conditions. The demonstration of probing a thermal magnon current mediated by coherent magnons via N-V spin states serves as a basis for creating a device platform that hybridizes spin caloritronics and spin qubits.