Systematic control of stress-induced anisotropy in pseudomorphic iron garnet thin films

Iron garnets are one of the most well-studied magnetic materials that enabled magnetic bubble memories and magneto-optical devices employing films with a perpendicular easy axis. However, most studies have been conducted on rather thick films (>1 μm), and it has not been elucidated whether it is possible to align the magnetic easy axis perpendicular to the film plane for much thinner (<100 nm) films by overcoming shape anisotropy. We studied the effects of epitaxial strain and film composition on the magnetic properties of 50-nm-thick garnet thin films grown by pulsed-laser deposition. Y ₃Fe₅O₁₂ was selected as the most prototypical garnet and Sm_3-xTm_xFe₅O₁₂ (x=1, 2, 3) was selected in view of its negatively large magnetostriction constants. We employed (111) planes of single crystalline Gd₃Ga₅O ₁₂ and (CaGd)₃(MgGaZr)₅O₁₂ substrates to tune the epitaxial strain. Thin films with a pseudomorphic structure were fabricated with the in-plane strain (ε_//) ranging from -1.5% to +0.5%, corresponding to the stress-induced anisotropy field (H_A) ranging from -40 kOe to +25 kOe, respectively. The magnetization ratio of the out-of-plane to in-plane component (M_⊥/M _//) systematically varied in accord with H_A, yielding M_⊥/M_// >1 for thin films with H_A values larger than 20 kOe. Among the films grown, Tm₃Fe₅O ₁₂ on Gd₃Ga₅O₁₂ showed the largest ε_// and H_A values of +0.5% and +25 kOe, respectively, to realize an apparently perpendicular easy axis, confirmed by a large M _⊥/M_// value of 7.8. Further, magnetic force microscope images showed a maze pattern typical of a perpendicularly magnetized film. These results reveal a method for tailoring the magnetic anisotropy of garnet ultrathin films by utilizing epitaxial strain. These thin films may be utilized to obtain nanoscale magnetic bubbles for use in novel devices.