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

Abstract

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 (>1μm), and it has not been elucidated whether it is possible to align the magnetic easy axis perpendicular to the film plane for much thinner (<100nm) 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. Y3Fe5O12 was selected as the most prototypical garnet and Sm3−xTmxFe5O12 (x=1, 2, 3) was selected in view of its negatively large magnetostriction constants. We employed (111) planes of single crystalline Gd3Ga5O12 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 (HA) ranging from −40kOe to +25kOe, respectively. The magnetization ratio of the out-of-plane to in-plane component (M⊥/M//) systematically varied in accord with HA, yielding M⊥/M// >1 for thin films with HA values larger than 20kOe. Among the films grown, Tm3Fe5O12 on Gd3Ga5O12 showed the largest ε// and HA values of +0.5% and +25kOe, 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.