Preparation and orientation mechanism analysis of (BiTm)<sub>3</sub>(GaFe)<sub>5</sub>O<sub>12</sub> magneto-optical single crystal film with out-of-plane orientation

Liquid-phase epitaxy (LPE) is one of the best techniques for the preparation of single crystal garnet films. However, the specific Faraday rotation angle of Yttrium iron garnet (YIG) is small, and its easy magnetization axis is parallel to the film surface. The YIG requires a large external saturation field, which cannot meet the development needs of magneto-optical devices. It is found that Bi-substituted YIG(Bi:YIG) film has a larger specific Faraday angle. By adjusting the easy magnetization axis of Bi: YIG perpendicular to the film surface, the saturation magnetization of Bi: YIG can be reduced, so that it can work under a small external magnetic field. This meets the development needs of miniaturization and energy saving of magneto-optical device. The saturation magnetization of garnet film can be effectively reduced by substituting Ga³⁺ for YIG crystal, mainly for Fe³⁺ at the 24d position of its tetrahedron. And the lattice constants of Gd₃Ga₅O₁₂ (GGG) and YIG are 1.2383 nm and 1.2376 nm, respectively. However, the radius of Bi³⁺ (10.8 nm) is larger than that of Y³⁺ (9.0 nm), the lattice mismatch of garnet film increases with the incorporation of Bi³⁺. In order to neutralize the lattice expansion caused by Bi³⁺, Tm³⁺ (8.69 nm) with a radius smaller than that of Y³⁺ (9.0 nm) is selected. Based on the theoretical analysis of the magnetocrystalline anisotropy of garnet film, (BiTm)₃(GaFe)₅O₁₂ mono-crystalline films with different growth temperatures and different thickness values are grown by LPE on GGG (111) substrates. The experimental results show that when the thickness of epitaxial film is greater than 1 μm, the influence of shape anisotropy on magnetocrystalline anisotropy can be ignored. With the increase of growth temperature, the substitution number of Bi³⁺ ions decreases gradually, the lattice constant of epitaxial film decreases gradually, and the lattice mismatch first decreases and then increases. Then, the state of compressive stress gradually changes into that of tensile stress. Compared with growth-induced anisotropy, the stress-induced anisotropy is dominant in the change of magnetocrystalline anisotropy. The Verdet constant of (BiTm)₃(GaFe)₅O₁₂ film is 11.8 × 10⁴ rad/Tm@1064 nm. The results show that the prepared (BiTm)₃(GaFe)₅O₁₂ mono-crystalline films have great development potential in magneto-optical devices.