Start-to-Finish Tuning of Magnetic Anisotropy in Iron Garnet Thin Films

Engineered anisotropy in garnet thin films has enabled progress in domain wall motion and switching experiments. Specific anisotropy landscapes and interface symmetries can expand the applications of iron garnet (IG) materials with low Gilbert damping for magnonics and field-free switching. While magnetic anisotropy arises from a host of contributions in garnet materials, many of these are determined by parameters that are intrinsic to the composition (e.g. magnetostriction constants, magnetocrystalline anisotropy constants, and saturation magnetization) and by the substrate orientation and lattice parameter, and are not easily varied once the thin film is grown.<br/><br/>In contrast, magnetotaxial¹, or growth-induced, anisotropy arises from cation ordering in nonequivalent dodecahedral sites present at the growth surface and can be tuned throughout the stages of film growth and post processing, since this phenomenon is determined by kinetic factors during growth of the film. Magnetotaxial anisotropy is expected for any garnet with multiple different cations on the dodecahedral site. For any mixed-cation garnet, the degree of order, and thereby the magnetotaxial anisotropy, can be tuned in three ways: (1) via the orientation and lattice parameter of the substrate, which changes the sets of nonequivalent sites available to the arriving cations; (2) via the kinetics during growth which controls the tendency of cations to order into the dodecahedral sites; and (3) by annealing the films to encourage cation diffusion and randomization, reducing the order and the magnetotaxial anisotropy.<br/><br/>In this work, we systematically explore this parameter space to control magnetotaxial anisotropy in mixed (Bi, Lu, Y)IG thin films (thickness ~30 nm) made by pulsed laser deposition on single crystal (111)-oriented garnet substrates with a range of lattice parameters. Structure and strain state of the thin films were measured by high-resolution X-ray diffraction and magnetic properties by vibrating sample magnetometry and ferromagnetic resonance, which showed damping in the range of ~ 0.001-0.0001 and saturation magnetizations of approximately 140 kA/m. Cation order leads to the appearance of a forbidden peak or superlattice X-ray diffraction peak that is not present in a garnet film without dodecahedral order, hence the degree of order was estimated from the intensity of the order-induced [1-10] superlattice peak. We show that the degree of growth-induced order is dependent on film-substrate lattice mismatch in BiYIG films, with greater cation ordering reported for more in-plane compressively strained films. We also highlight the importance of growth parameters, namely laser repetition rate of 1-30 Hz, on the kinetics of ordering of Lu and Y in LuYIG. Lastly, we demonstrate the reduction of cation order and anisotropy in LuYIG films by post-growth annealing. This work shows the role of growth-induced cation order for the production and tuning of magnetic anisotropy in garnet materials for applications in spintronic devices.<br/><br/>1. Kaczmarek, A. C. <i>et al.</i> Atomic order of rare earth ions in a complex oxide: a path to magnetotaxial anisotropy. <i>Nat. Commun.</i> <b>15</b>, 5083 (2024).