The strain induced magnetic anisotropy (MA) of epitaxial Co-ferrite (CFO) thin films grown on MgAl 2 O 4 (001) (MAO) by reactive sputtering was studied. These films underwent large tetragonal compressive strain due to the lattice mismatch between the substrates and films, resulting in tetragonalities of up to À0.04. Scanning transmission electron microscopy observation combined with fast Fourier transform analysis revealed that the lattice distortion monotonically relaxed with the increasing film thickness. Unlike the CFO(001) films on MgO(001) substrates, a magnetically enhanced layer exists at the interface between CFO and MAO. A large negative uniaxial MA energy of À5.9 MJ/m 3 was confirmed for the thinnest film of 12.9 nm at 300 K by magneto-torque measurements. The induced uniaxial anisotropy decreased with the increasing film thickness owing to misfit relaxation. The magneto-elastic (ME) constant, which was determined by a fitting line for films with different thicknesses and therefore different tetragonalities, was 0:15 6 0:01 GJ/m 3. This result is consistent with the value of 0:14 GJ/m 3 for the bulk and suggests that the framework of the phenomenological ME theory for CFO is valid for a wide range of tetragonalities, at least up to À0.04. Our results also indicate that the misfit engineering of CFO thin films has great potential in inducing enhanced uniaxial MA.

Real-time in situ reflection high energy electron diffraction (RHEED) observations of Fe3O4, γ-Fe2O3, and (Co,Fe)3O4 films on MgO(001) substrates grown by a conventional planar magnetron sputtering was studied. The change in periodical intensity of the specular reflection spot in the RHEED images of three different spinel ferrite compounds grown by two different sputtering systems was examined. The oscillation period was found to correspond to the 1/4 unit cell of each spinel ferrite, similar to that observed in molecular beam epitaxy (MBE) and pulsed laser deposition (PLD) experiments. This suggests that the layer-by-layer growth of spinel ferrite (001) films is general in most physical vapor deposition (PVD) processes. The surfaces of the films were as flat as the surface of the substrate, consistent with the observed layer-by-layer growth process. The observed RHEED oscillation indicates that even a conventional sputtering method can be used to control film thickness during atomic layer depositions.

We report the optimum conditions for the epitaxial growth of CoxFe3−xO4 (CFO) (111) thin films with x = 0.75 on an α-Al2O3(0001) substrate via reactive RF magnetron sputtering. The lattice strain evaluated by X-ray diffraction measurements is as small as 0.11% in spite of the large lattice mismatch between CFO(111) and α-Al2O3(0001). The CFO(111) thin films grown under the optimal conditions exhibit magnetic anisotropy with the easy axis of magnetization perpendicular to the films. The induced uniaxial anisotropy energy can be quantitatively explained by the magnetoelastic effect.

The recently discovered spin Hall magnetoresistance (SMR) effect is a useful means to obtain information on the magnetization process at the interface between a nonmagnetic metal and ferromagnetic insulators. We report the SMR measurements at the interface between platinum and cobalt ferrite thin films for samples with two different preferential directions of magnetization (out-of-plane and in-plane). The directional difference of the magnetic easy axis does not seem to influence the value of SMR.

No abstract