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

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.

我们报道了采用高温离轴射频磁控溅射法制备铋掺杂钇铁石榴石（bismuth-substituted yttrium iron garnet, BiYIG）纳米薄膜的外延生长过程。通过调控各类（111）取向石榴石衬底上的溅射参数与外延应变，我们实现了对磁性能的精准调控。与衬底晶格失配最高达-0.80%的BiYIG薄膜在厚度直至60 nm时仍保持完全应变状态，且具备优异的晶体质量。透射电子显微镜与能量色散X射线光谱表征证实，该薄膜呈现相干外延生长模式，无缺陷产生，且BiYIG与衬底的界面处原子互扩散程度极低。通过将BiYIG中的拉伸或压缩应变调控在-0.80%至+0.56%之间，可借助磁弹性耦合精准补偿总磁各向异性。溅射制备的BiYIG薄膜的有效磁各向异性，还可通过离轴沉积角度与生长过程中的氧气流量进一步调控——这两个参数决定了薄膜的阳离子化学计量比。在优化的生长条件下，即便厚度低至10 nm，仍可稳定获得10 GHz频率下1 mT的铁磁共振（ferromagnetic resonance, FMR）线宽。我们还在以抗磁衬底钇钪镓石榴石生长的超薄（2~5 nm）BiYIG薄膜中观测到了极低的FMR线宽。上述研究结果凸显了低阻尼、应变调控型纳米级BiYIG薄膜在自旋轨道电子学与磁子学器件中实现先进功能的应用潜力。具体而言，磁各向异性补偿与低阻尼特性，可使锥角磁化动力学免受磁子-磁子非线性散射的影响。