The role of disorder and defects in the ferromagnetic resonance of spinel ferrite thin films

Spin current generation in heterostructures requires low magnetic damping spin-current sources. To this end, we seek to minimize disorder and defects in spin-current source materials and improve interface quality. In this work, we find that epitaxial spinel ferrite MgAl_0.5Fe_1.5O₄ (MAFO) films exhibit low damping and coercivity in the 10-15nm thickness range with a Gilbert damping parameter α≈0.001 and coercive field H_c<5 Oe. Transmission electron microscopy results indicate these films grow coherently strained on MgAl₂O₄ substrates with minimal defects. Gilbert damping rapidly increases outside of this thickness range, with α≈0.004 for 5nm films and α≈0.03 for 40nm films. Dislocations form at the film/substrate interface in 40nm films, indicating that the film relaxes, causing increased damping and coercivity (H_c>50 Oe). A thickness-dependent magnetization study of MAFO films from 1-45nm thick indicates the presence of a 1.6nm magnetically-dead layer, less than half that found in yttrium iron garnet films. This layer is confirmed to exist at the film/substrate interface by depth profiling via polarized beam neutron reflectivity. The nature of this dead layer is crucial to device integration, and we will discuss interfacing this oxide with platinum and copper/platinum overlayers.