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INTRODUCTION Spinel structure thin film ferrites have been recognized to have significant technological potential in high frequency applications. The promising properties of ferrite thin films have led to many scientific studies' focusing on the materials issues, such as epitaxy, microstructure and strain, that are not present in the bulk, and their effects on electronic and magnetic behaviors. In the studies of epitaxial Fe 30 4 grown on MgO substrates, Margulies et al."' have found that antiphase boundaries (APB), which are a result of the nucleation and growth mechanism, give rise to enhanced intrasublattice superexchange coupling and account for the anomalous magnetic properties of the magnetite films. Similar results were also observed in

CoFe 20 4 films grown on MgO substrates by Dorsey et al.3 and NiFe 20 4 films grown on SrTiO 3 by Venzke et al. 4 . In our studies of CoFe20 4 (CFO) thin films, we present our results on MgO substrates and compare them to those on a spinel structure substrate MgAl 20 4 where we are able to eliminate the formation of antiphase boundaries. Structure characterization of the antiphase boundary free films by transmission electron microscopy demonstrates a high quality of the film, with a dislocation density less than 1.6x1010/cm2. Considering the large crystalline anisotropy and magnetostriction of cobalt ferrite, we expect microstructural effects such as cation distribution and stress on magnetic properties to be enhanced significantly. Cobalt ferrite has an inverse spinel structure, where the 0 atoms make up an FCC lattice, an eighth of the tetrahedral (A) sites are2+occupied by Fe 31 ions and half of the octahedral (B) sites are randomly occupied by Co and Fe3 , ions. Bulk work has shown that heat treatment can vary the cation distribution among the tetrahedral and octahedral sites of the spinel ferrites5 -, thus modifying their magnetic properties. In particular, cobalt ferrite, with a degree of inversion ranging from 80% to 95%, exhibit more site sensitivity than others9" 0 , given the large anisotropic effect of Co2+ ions. Studies of ferrite thin films upon post deposition annealing provide mixed results. While significant improvement in magnetic properties and crystallinity was found in NiFe20 4 films grown on SrTiO 34, the anomalous magnetic behaviors remain in the Fe 30 4 films grown on MgO1' 2 . In our case, while post deposition annealing gave rise to an unexpected decrease of saturation magnetization in films grown on MgO substrates, 201 Mat. Res. Soc. Symp. Proc. Vol. 603 0 2000 Materials Research Society

dramatic change in magnetic anisotropy was observed in films grown on CCO buffered MAO substrates. In our previous studies", the fabrication and characterization of epitaxial cobalt ferrite thin films were described and the magnetic behaviors were well explained by magnetoelastic theory. Here, we report our recent results on films grown under tension and films after annealing. The anomalous magnetic properties observed in these films will be explained in terms of