YSGAG: The Ideal Substrate for YIG in Quantum Magnonics

TL;DR

The paper tackles cryogenic damping in YIG thin films caused by paramagnetic GGG substrates, which hinders quantum magnonics. It introduces YSGAG as a diamagnetic, lattice-matched substrate and performs broadband ferromagnetic resonance from room temperature to about 10 mK on YIG/YSGAG and YIG/GGG samples, extracting $α_{eff}$ and linewidths. The results show YIG/YSGAG maintains low damping across the full temperature range (e.g., $α_{eff} ≈ 4.29×10^{-5}$ at RT) with no low-temperature upturn, while YIG/GGG exhibits strong damping growth due to substrate magnetization; YSGAG’s tiny susceptibility further suppresses parasitic effects. This establishes YSGAG as an ideal substrate for YIG in quantum magnonics and paves the way for millikelvin magnon lifetimes suitable for spin-wave–based quantum technologies.

Abstract

Quantum magnonics leverages the quantum properties of magnons to advance nanoscale quantum information technologies. Ferrimagnetic yttrium iron garnet (YIG), known for exceptionally long magnon lifetimes, is a cornerstone material typically grown as thin films on gadolinium gallium garnet (GGG) for lattice matching. However, paramagnetic GGG introduces detrimental damping at low temperatures due to substrate magnetization, undermining quantum applications. Here, we study magnetic damping in a 150$\,$nm-thick YIG film on a yttrium scandium gallium aluminum garnet (YSGAG) substrate, a newly developed diamagnetic alternative to GGG. Using ferromagnetic resonance spectroscopy down to 30$\,$mK, we compare YIG/YSGAG with a conventional YIG/GGG reference system. We demonstrate that the YIG/YSGAG system maintains low damping from 300$\,$K to 30$\,$mK, with $α= 4.29\times10^{-5}$ at room temperature, comparable to the best YIG/GGG films and bulk YIG, with no low-temperature upturn. The diamagnetic substrate eliminates the dissipation mechanisms that dominate on magnetized GGG, preserving low magnetic damping across the full temperature range. Consequently, YSGAG serves as an ideal substrate for YIG films in quantum magnonics and is paving the way for the development of spin-wave-based quantum technologies.

Figures (3)

• Figure 1: (a) Crystallographic structure of the diamagnetic YSGAG substrate. The cubic lattice structure of YSGAG is similar to that of YIG, but with iron ions replaced at the octahedral sites by scandium, gallium, and aluminum, while tetrahedral sites are occupied by gallium and aluminum. The samples were grown in the $\langle111\rangle$ direction, corresponding to one of the body diagonals of the cubic lattice. (b) Effective magnetization $M_{\mathrm{eff}}$ of YIG films vs temperature $T$, on an x-axis logarithmic scale for the YIG/YSGAG (black) and YIG/GGG (red) samples. For the YIG/GGG sample, the corrected $M_{\mathrm{eff}}$ could only be obtained for measurements in the PPMS down to 1.8 K, as precise GGG magnetization data is required to correct for the GGG stray field in the Kittel fit. The inset shows the gyromagnetic ratio $\gamma$ as a function of temperature $T$, plotted on an x-axis logarithmic scale. (c) and (d) Magnetic susceptibility $\chi$ of the YSGAG substrate and GGG substrate accordingly vs the temperature $T$, plotted on an x-axis logarithmic scale.
• Figure 2: (a) Normalized example FMR spectra $\Delta S_{12}$ as a function of frequency $f$ for YIG/GGG and YIG/YSGAG in an in-plane oriented external field $B_0$ of 1 T and temperatures of 300 K and 1.8 K. (b) (upper) FMR FWHM linewidth $\Delta B$ and corresponding Gilbert fit in orange linearly as a function of the FMR resonance frequency $f_{\mathrm{FMR}}$ at 300 K. The closed circles and straight line represent measurements and fit for the YIG/GGG reference system, while the open diamonds and dashed line correspond to the YIG/YSGAG sample. (lower) FMR FWHM linewidth $\Delta B$ logarithmically as a function of the FMR resonance frequency $f_{\mathrm{FMR}}$ for three different temperatures.
• Figure 3: (a) Frequency-band-averaged FMR FWHM linewidth $\langle\Delta B\rangle$ vs temperature $T$, plotted on double logarithmic scale for five different frequency bands. The closed circles represent measurements for the YIG/GGG reference system, while the open diamonds correspond to the YIG/YSGAG sample. (b) FMR FWHM linewidth $\Delta B$ as a function of temperature $T$ for an FMR frequency $f_{\mathrm{FMR}} \approx 8\,\mathrm{GHz}$, displayed on a double logarithmic scale. The graph compares measurements obtained in this work with selected results of sputtered and LPE grown YIG films from relevant literature, enabling a direct evaluation of temperature-dependent damping behavior across different samples, their substrates and studies Cole2023Guo2023Legrand2024.