Spin wave propagation in sputter-deposited YIG nanometer films

Yttrium iron garnet (Y 3 Fe 5 O 12 , YIG) is well-known material with an extremely small magnetic damping, $\alpha = \sim 10 ^{-5}$ in bulk, which is two orders of magnitude smaller than that in ferromagnetic metals. The growing demands for YIG-based spintronics have led to the development of YIG thin films with a nanometer thickness range [1, 2]. Recently, magnonics has been attracted considerable interests for the transmission, storage and processing of the information using propagating spin waves [3]. To miniaturize the magnonic devices, it is necessary to reduce the thickness of YIG films for a shorter wavelength. For high quality YIG nanometer films, it has been reported that the YIG thin films by pulsed laser deposition (PLD) show the relatively low damping constant of $2.3 \times 10 ^{-4}$ for 20 nm thickness [1]. From the view point of a broad utility and industry, the sputtering growth is better than PLD. In this study we investigate the spin wave propagation in sputter-deposited YIG nanometer films, and characterize the YIG thickness dependence of the several parameters, such as magnetic damping constant, spin wave group velocity and nonreciprocity. The YIG thin films were grown on 0.5-mm-thick single crystal gallium gadolinium garget (GGG) substrates with (111) orientation by RF magnetron sputtering. During the deposition, the substrate was kept at room temperature, the argon pressure and sputtering power was 0.06 Pa and 150 W, respectively. The films were annealed at 900°C for 8 h in the air. We varied YIG film thickness from 20 nm to 50 nm. Using electron-beam lithography and Ar ion-milling technique, the films were patterned into a circular shape with $10- \mu \mathrm {m} -$diameter for ferromagnetic resonance (FMR) measurement and a rectangular shape with $50- \mu \mathrm {m} -$width for the spin wave measurement. After patterning the YIG films, an insulating layer of 30 nm SiO 2 was deposited on the entire surface. Finally, microwave antennas were deposited for the FMR and spin wave measurement. First, we evaluate the magnetic damping constant $\alpha $ from FMR measurement using a vector network analyzer. Figure 1 (a) shows the FMR linewidth as a function of resonance frequency. $\alpha $ was extracted from the slope of the linear fits to the data. As shown in Fig. 1(b), the $\alpha $ value decreases with increasing the thickness and we obtained lowest value of $\alpha = 1.3 \times 10 ^{-3}$ in 50-nm-thick YIG, which is slightly larger than the reported values for sputter-deposited YIG thin films [2]. While we need further optimization of the sputtering condition and/or annealing process to reduce $\alpha $, it should be noted that the obtained $\alpha $ in this study is significantly smaller than that of ferromagnetic metallic films with similar thickness. Second, the propagating spin wave spectroscopy was performed under the in-plane magnetic field to excite the magnetostatic surface spin wave (MSSW) mode. Figure 2(a) shows the group velocity of spin wave estimated from the oscillation period in transmission spectra. It was about 1.1 km/s in 50-nm-thick YIG waveguide under 14 mT. We found that the spin wave group velocity decreases with increasing the magnetic field and decreasing the film thickness. The group velocity $v_{g}$ can be calculated from the spin wave dispersion $\omega (k)$ as defined $v_{g} = d \omega ( k)/ dk$, which nicely reproduces the experimental results. By comparing the signal intensity, the nonreciprocity defined as $A_{12}/ A_{21}$ was also estimated, where $A_{12}$ and $A_{21}$ denote the signal intensity of $S_{12}$ and $S_{21}$, respectively. The unity value indicates the reciprocal characteristics. In our experiment, the nonreciprocity is mainly caused by x- and z-components of microwave magnetic field [4]. As shown in Fig. 2(b), the nonreciprocity increases with increasing magnetic field and we obtained the largest nonreciprocity of 0.1 at 85 mT. This This nonreciprocity is much more significant than that in ferromagnetic metals [4], which is attributed to the smaller saturation magnetization of YIG than that in ferromagnetic metals.