Wavenumber-dependent magnetic losses in yttrium iron garnet–gadolinium gallium garnet heterostructures at millikelvin temperatures

Magnons have inspired potential applications in modern quantum technologies and hybrid quantum systems
due to their intrinsic nonlinearity, nanoscale scalability, and a unique set of experimentally accessible parameters
for manipulating their dispersion. Such magnon-based quantum technologies demand long decoherence times,
millikelvin temperatures, and minimal dissipation. Due to its low magnetic damping, the ferrimagnet yttrium iron
garnet (YIG), grown on gadolinium gallium garnet (GGG), is the most promising material for this objective.
To comprehend the magnetic losses of propagating magnons in such YIG-GGG heterostructures at cryogenic
temperatures, we investigate magnon transport in a micrometer-thick YIG sample via propagating spin-wave
spectroscopy measurements for temperatures between 4 K to 26 mK. We demonstrate an increase in the dissi-
pation rate with wavenumber at cryogenic temperatures, caused by dipolar coupling to the partially magnetized
GGG substrate. Additionally, we observe a temperature-dependent decrease in spin-wave transmission, attributed
to rare earth ion relaxations. The critical role of the additional dissipation channels at cryogenic temperatures
is underpinned by the comparison of the experimental results with theoretical calculations and micromagnetic
simulations. Our findings strengthen the understanding of magnon losses at millikelvin temperatures, which is
essential for the future detection of individual propagating magnons.