Damping Enhancement in YIG at Millikelvin Temperatures due to GGG Substrate

TL;DR

This study investigates the increased magnetic damping in YIG films on GGG substrates at millikelvin temperatures, identifying the GGG substrate's magnetic properties as a key damping source and suggesting ways to mitigate it for quantum tech.

Contribution

It reveals the dominant damping mechanism in YIG/GGG systems at millikelvin temperatures and provides theoretical and numerical insights into damping enhancement.

Findings

Tenfold increase in FMR linewidth at millikelvin temperatures.

GDD-driven linewidth enhancement can reach up to 6.7 times.

Deviation from Gilbert damping model above 18 GHz.

Abstract

Quantum magnonics aims to exploit the quantum mechanical properties of magnons for nanoscale quantum information technologies. Ferrimagnetic yttrium iron garnet (YIG), which offers the longest magnon lifetimes, is a key material typically grown on gadolinium gallium garnet (GGG) substrates for structural compatibility. However, the increased magnetic damping in YIG/GGG systems below 50$\,$K poses a challenge for quantum applications. Here, we study the damping in a 97$\,$nm-thick YIG film on a 500$\,\mu$m-thick GGG substrate at temperatures down to 30$\,$mK using ferromagnetic resonance (FMR) spectroscopy. We show that the dominant physical mechanism for the observed tenfold increase in FMR linewidth at millikelvin temperatures is the non-uniform bias magnetic field generated by the partially magnetized paramagnetic GGG substrate. Numerical simulations and analytical theory show that the GGG-driven linewidth enhancement can reach up to 6.7 times. In addition, at low temperatures and frequencies above 18$\,$GHz, the FMR linewidth deviates from the viscous Gilbert-damping model. These results allow the partial elimination of the damping mechanisms attributed to GGG, which is necessary for the advancement of solid-state quantum technologies.