Time-Resolved Measurements of Surface Spin-Wave Pulses at Millikelvin Temperatures

We experimentally investigate the propagation of pulsed magnetostatic surface spin-wave (magnon) signals in an yttrium iron garnet (YIG) waveguide at millikelvin temperatures. Our measurements are performed in a dilution refrigerator at microwave frequencies. The excellent signal-to-noise ratio afforded by the low-temperature environment allows the propagation of the pulses to be observed in detail. The evolution of the envelope shape as the spin wave travels is consistent with calculations based on the known dispersion relation for YIG. We observe a temperature-dependent shift of the ferromagnetic resonance frequency below 4 K, which we suggest is due to the low-temperature properties of the substrate below the film, gallium gadolinium garnet. Our measurement and the accompanying calculations give insight into both low-temperature magnon dynamics in YIG and the feasibility of the use of propagating magnons in solid-state-quantum information processing.