Enhancement of Damping in YIG films at MilliKelvin Temperatures due to GGG Substrate

Magnonics is the field of science that deals with data carried by spin waves and their quanta, magnons, in magnetically ordered media [1]. The ferrimagnet yttrium iron garnet (YIG) Y₃Fe₅O₁₂ is the material with the lowest known magnetic damping [2] and therefore often used in experiments and RF technologies. The use of YIG films of thicknesses down to tens of nanometers grown on gadolinium gallium garnet (GGG) Gd₃Ga₅O₁₂ substrates [2] enables the development of magnetic devices and circuits at the nanoscale [3]. Recently, the field of quantum magnonics, operating with single magnons and versatile hybrid structures at millikelvin temperatures, attracts significant attention [3]. However, it is known that lowering the temperature increases the magnetic damping of YIG, which is usually associated with the influence of the paramagnetic GGG substrate [4]. 

In this work, we measured a 97 nm-thick YIG film grown on 500 µm-thick GGG substrate and cut into a quadratic chip with an edge length of 5 mm. We have performed stripline ferromagnetic resonance (FMR) spectroscopy up to 40 GHz, using a physical property measurement system (PPMS), operating at the temperatures between 2 K and 300 K and a dilution refrigerator capable of reaching temperatures of 10 mK. The measurements were performed in in-plane orientation of the external magnetic field. To determine the magnetization of GGG in the temperature range from 2 K to 300 K, we have performed vibrating-sample magnetometry (VSM) on a pure GGG slab (M^s_GGG = 805 kA/m). At low temperatures, the GGG can be saturated to a significantly high value of hundreds of kA/m when magnetic fields of several hundred mT are applied. Our FMR studies show that at temperatures below 50 K and externaly applied fields the paramagnetic GGG is sufficiently magnetized and induces a stray field in the YIG layer.

The induced field is highly inhomogeneous and opposes the externally applied field for the in-plane geometry. The lower the temperature and the higher the external fields, and therefore the higher is the stray field induced by the GGG, which naturally creates a shift in frequency of the FMR [5]. The FMR linewidth ∆B, which is a measure of magnetic damping in the system increases for lower temperatures and is more than eightfold at 2 K compared to room temperature. The stray field induced by the GGG is very inhomogeneous over the area of the YIG film [5], and is therefore one contribution of the anomalous increase in linewidth at low temperatures, confirmed by comparing the experimental results with specialized micromagnetic simulations. However, the magnetic system of the YIG can couple with other dissipation channels that occur at low temperatures. For instance: (1) YIG/GGG exchange and (2) dipolar coupling, (3) heavy-metal impurities. A better understanding of these ultralow temperature damping mechanisms will pave the way for quantum magnonics on thin YIG films.