  Currently, thermally excited magnons are being intensively investigated owing
to their potential in computing devices and thermoelectric conversion
technologies. We report the detection of 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 (NV) centers in
diamond. Thermal magnon current was observed as modified Rabi oscillation
frequencies of NV spins hosted in a beam-shaped bulk diamond that resonantly
coupled with coherent magnon propagating over a long distance. Additionally,
using a nanodiamond, alteration in NV spin relaxation rates depending on the
applied temperature gradient were observed under a non-resonant NV excitation
condition. The demonstration of probing thermal magnon current mediated by
coherent magnon via NV spin states serves as a basis for creating a device
platform hybridizing spin caloritronics and spin qubits.
