Effect of Annealing on the Structural and FMR Properties of Epitaxial YIG Thin Films Grown by RF Magnetron Sputtering

Abstract

Yttrium iron garnet (YIG) or Y 3 Fe 5 O 12 has continued to attract attention over the years due to its low microwave loss, essential for microwave device technology [1]. Apart from microwave application, recently the nanometer (nm) thick YIG films have been used to realize spin transfer torque devices, magnetic logic devices, and frequency tunable metamaterials [2]. This has enhanced the interest in the study of physical properties of YIG thin films. The fabrication of epitaxial YIG films in micrometer thick range on Gadolinium Gallium Garnet (Gd 3 Ga 5 O 12 or GGG) substrates has been commonly done by liquid phase epitaxy (LPE). However, Pulsed laser deposition (PLD) and RF sputtering are known to be better suited techniques to deposit nm thick epitaxial YIG films on GGG and polycrystalline YIG films (on substrates other than GGG) [3–4]. Recently large amount of work has been reported on PLD deposited YIG thin films that are grown on different substrates [3–4]. One curious finding has been that whenever YIG films were grown on GGG substrate, effective saturation magnetization $( {4}\pi M_{eff})$ larger than bulk has been observed when studied using Ferromagnetic resonance (FMR). where ${4}\pi M_{eff} = {4}\pi M_{S}-H_{U}$. Here ${4}\pi M_{S}$ is the bulk saturation magnetization and $H_{U}$ is uniaxial anisotropy field. In our previous study, we performed a systematic structural, magnetic and microwave studies on PLD deposited YIG/GGG films and established that the high ${4}\pi M_{eff}$ value is due to the presence of negative $H_{U}$ arising from the compressive stresses present in the film. The presence of this stress in YIG film was confirmed by multiple {hkl} stress measurements with grazing incidence X-ray diffraction performed at different depth of penetration on the YIG film. The question then arose whether this effect is only seen in YIG/GGG films deposited by PLD technique or can also be observed in thin films grown by other techniques. RF sputtering is a technique widely used for industrial application due to uniform and large area deposition. Hence, to cheek the above hypothesis of stress induced anisotropy, we further extended our study to YIG thin films deposited on GGG substrate by RF sputtering. In the present work, we report the growth and microwave studies at different frequency of ~250 nm thick YIG film deposited on GGG (111) substrates by RF magnetron sputtering. (Edward Z400) at room temperature. The YIG film is deposited at an RF power of 100 W and in an argon atmosphere of 1.6 × 10−2 mbar. The chamber was evacuated to 5 × 10−6 mbar prior to film deposition using a turbo molecular pump. The target to substrate distance was maintained at 4 cm. After the deposition, the thin films were ex-situ annealed in air at 700 °C for different time intervals (2, 4, 6 and 10 hours). X-ray diffraction (XRD) shows that as-grown layer is amorphous and pure YIG phase is formed with preferred (111) orientation after annealing. We observe ~0.4% increased lattice parameter (a) for 2h annealed YIG films (12.429A) over the YIG bulk (12.376 A) and this difference in ‘a’ is reduced with the increase in annealing time (a=12.410 A for 10h annealed film). The microwave properties have been investigated using a field-modulated broadband FMR in the frequency range 2–18 GHz. The magnetic field (H) was applied normal to the film plane. Fig. 1(a) shows the measured FMR fields as a function of frequency for YIG/GGG films annealed for different times. The ${4}\pi M_{\mathrm {eff}}$ for each film has been estimated by fitting experimental data to Kittel formula for perpendicular position [3]. Similar to our previous results on PLD deposited YIG/ GGG films, all the rf sputtered annealed YIG films here show ${4}\pi M_{\mathrm {eff}}$ value larger than the SQUID measured ${4}\pi M_{S}$ value (~1780 G). Hence the higher value of ${4}\pi M_{\mathrm {eff}}$ observed in the rf sputtered case can also be explained by considering the presence of negative $H_{U}$. The ${4}\pi M_{\mathrm {eff}}$ value also found to decrease with the increase in annealing time. This indicates the presence of negative $H_{U}$ decreases with the increase in annealing time due to stress relaxation. These values are, however, much smaller than those observed for PLD films, where we observed $H_{U} = -830$ Oe. Another interesting result obtained from the present study pertains to the FMR line width (ΔH as a function of annealing temperature. Fig. 1(b) represents variation of ΔH as a function of annealing time and is plotted for frequencies 6 GHz and 18 GHz. It is seen that, the 10h annealed film shows a minimum ΔH value, lying between 30 Oe to 54 Oe over a wide frequency range (2–18 GHz). Compared to the 2h films, the FMR ΔH was reduced by ~50% upon annealing for 10h. Although the FMR linewidth and coercively $( H_{C})$ of rf sputtered films $( \mathrm {H}_{C}=18$ Oe, ΔH Oe) are nearly six times larger than that of PLD deposited YIG films $( \mathrm {H}_{C}=3$ Oe, ΔH = 5 Oe), they are still much smaller than some of the results reported in recent years [5–6]. From the present investigations it is evident that the annealing helps in YIG phase formation as well as in reducing the stress, resulting a decrease in the FMR line width. Following a detailed analysis, a correlation between the microstructure and microwave properties has been established.