Ultrathin bismuth-yttrium iron garnet films with tunable and compensated magnetic anisotropy

TL;DR

This work tackles the challenge of achieving tunable and compensated magnetic anisotropy in ultrathin BiYIG films while maintaining low magnetic damping. It employs high-temperature off-axis rf sputtering to grow epitaxial BiYIG on four (111)-oriented garnet substrates, enabling controlled strain and stoichiometry; comprehensive structural and chemical characterizations confirm coherent, fully strained growth with minimal interdiffusion. By varying substrate lattice mismatch, off-axis deposition angle, and oxygen flow, the authors demonstrate precise control of the effective anisotropy, including regimes of complete compensation at specific thicknesses, and establish a linear coupling between strain and $M_{ ext{eff}}$ with a slope around $410$ mT per 1% strain. Dynamic measurements reveal exceptionally narrow FMR linewidths down to 1 mT at 10 GHz for 10 nm films and low damping ($ ext{α} oughly 4 imes10^{-4}$) in ultrathin 2–5 nm films, highlighting BiYIG as a promising material for advanced magnonic and spin-orbitronic devices with enhanced magneto-optical properties.

Abstract

We report on the epitaxial growth of nm-thick films of bismuth-substituted yttrium iron garnet (BiYIG) by high-temperature off-axis radio-frequency magnetron sputtering. We demonstrate accurate control of the magnetic properties by tuning of the sputtering parameters and epitaxial strain on various (111)-oriented garnet substrates. BiYIG films with up to -0.80\% lattice mismatch with the substrate remain fully strained up to 60~nm-thick, maintaining a high crystalline quality. Transmission electron microscopy and energy-dispersive X-ray spectroscopy confirm coherent epitaxial growth, the absence of defects, and limited interdiffusion at the BiYIG/substrate interface. Varying the tensile or compressive strain between -0.80\% and +0.56\% in BiYIG allows for accurate compensation of the total magnetic anisotropy through magneto-elastic coupling. The effective magnetic anisotropy of sputtered BiYIG films can be further tuned via the off-axis deposition angle and the oxygen flow during growth, which determine the cation stoichiometry. Under optimized growth conditions, a ferromagnetic resonance (FMR) linewidth of 1~mT at 10~GHz is reliably obtained even for thicknesses as low as 10~nm. We also report small FMR linewidths in ultrathin (2-5~nm) BiYIG films grown on diamagnetic substrate yttrium scandium gallium garnet. These findings highlight the promise of low-damping, strain-engineered nm-thick BiYIG films for implementing advanced functionalities in spin-orbitronic and magnonic devices. Specifically, the magnetic-anisotropy compensation and low damping enable large cone-angle magnetization dynamics immune to magnon-magnon nonlinear scattering.

Figures (10)

• Figure 1: (a) Schematic diagrams of the epitaxial relationship of BiYIG with different garnet substrates with (111) orientation. For the nominal composition Bi$_{\rm 0.8}$Y$_{\rm 2.2}$Fe$_{\rm 5}$O$_{\rm 12}$, the GGG substrates provide compressive strain (+0.56% mismatch with substrate), YSGG is about lattice-matched (-0.08%), while the GYSGG (-0.48%) and GSGG (-0.80%) substrates provide a marked biaxial tensile strain. (b) Schematic representation of the diffusion of atomic species in the deposition plasma for off-axis magnetron sputtering. The angle $\beta$ is the geometric angle between the target center-substrate line and the normal to the target surface. The deposition rate reduces with increasing off-axis angle $\beta$, while the film composition also evolves with $\beta$. Other control parameters are the plasma rf power $P$ and the Ar/O$_{2}$ gas mixture composition.
• Figure 2: (a)-(d) High-resolution XRD for BiYIG with different deposition times, grown on GGG, YSGG, GYSGG, and GSGG substrates, respectively (each curve is offset by $10^5$ counts). Black curves are the diffractograms fitted to the data using an enhanced kinematical diffraction model. (e)-(h) Thickness dependence of the out-of-plane lattice parameter ($a_{\perp}$) and the equivalent unstrained lattice parameter ($a$), extracted from the fits and compared to the substrate lattice parameter ($a_{\rm{sub}}$).
• Figure 3: (a) X-ray reciprocal space map acquired near the (624) peak for a 60 nm-thick BiYIG film on a GSGG substrate. The vertical and horizontal white dashed lines indicate alignment with the substrate (624) peak within the film plane and along the film normal direction. The red dashed line indicates the peak positions expected in case of a fully relaxed growth. (b) High-angle annular dark-field STEM image of an ultrathin (6 nm) BiYIG film on a YSGG substrate. The scale bar is 2 nm. (c) Fast Fourier Transform (FFT) of the HAADF-TEM image with labeled $444$ and $2\bar{2}0$ peaks. (d) Distribution of each element, obtained by spatially resolved EDX across the YSGG/BiYIG interface [see the detailed position in (b)]. (e) Raw data for EELS and (f) deduced distributions of Sc, Fe and O as a function of position along the film's normal. The YSGG/BiYIG interface is located at $z=0$ nm [see the detailed position in (b)].
• Figure 4: (a) Ferromagnetic resonance field $\mu_0 H_{\rm res}$ as a function of the excitation frequency $f$, measured on the 36-nm thick BiYIG films grown on different substrates. The solid curves are fits based on the Kittel formula. (b) Thickness dependence of the effective magnetization $\mu_0 M_{\rm eff}$ estimated from the FMR of the BiYIG films grown on different substrates. (c) Thickness dependence of $\mu_0 M_{\rm eff}$ of BiYIG grown on GYSGG for different off-axis deposition angles $\beta$. (d) $\mu_0 M_{\rm eff}$ in the large thickness limit, as a function of the epitaxial strain. The red line is a linear fit to the data for the four substrates.
• Figure 5: (a) In-plane FMR field $\mu_0 H_{\rm res}$ as a function of excitation frequency measured for a 10-nm thick BiYIG film deposited on YSGG with an off-axis angle of 39$^\circ$. The inset shows the measured derivatives of the complex microwave transmission $\operatorname{Re},\operatorname{Im} \partial S_{21} / \partial B$ as a function of external magnetic field around resonance. (b) Resonance linewidth $\mu_0 \Delta H$ as a function of frequency, measured for either in-plane and out-of-plane FMR. The solid lines are fits to Eq. \ref{['eq:LW']}. (c) Resonance linewidth $\mu_0 \Delta H$ as a function of frequency measured for BiYIG films on YSGG with thickness 2 nm, 3 nm, 4 nm, 5 nm, in-plane FMR. (d) Thickness dependence of Gilbert damping $\alpha$ and inhomogeneous linewidth $\mu_0 \Delta H_0$ of BiYIG on YSGG.