VO2 undergoes a monoclinic-tetragonal structural-phase transition (SPT) with a metal-insulator transition (MIT). SPT with a modulation of about 0.7% in lattice constant can give rise to lattice strain in a neighboring material that combined with VO2. In this thesis, (100) and (101) VO2 films were hetero-epitaxially or epitaxially deposited on the various substrates. Presumably, the structural transition of (100) VO2 induces a change of about +0.62% in lattice constant a and that of about -0.7% in lattice constant c within the film plane. Whereas, it is expected that upon heating the (101) VO2 has lattice distortions of +0.19% and 0.62% for two orthogonal in-plane directions, respectively. The (100) VO2 was grown on C-plane Al2O3 and (100) TiO2, while the (101) VO2 was made on R-plane Al2O3 and (101) TiO2. The C-plane Al2O3 has a hexagonal lattice structure on the surface, which cause a textured grain growth of the tetragonal VO2 within the plane because of an existence of three equivalent directions for in-plane lattice alignment. Such a texture within the plane probably makes reduce an effective lattice distortion of a neighboring magnetic film which is caused by the structural transition of hetero-epitaxial (100) VO2 film on the C-plane Al2O3. On the contrary, the epitaxial VO2 film on (100) TiO2 does can induce effectively lattice strain to a neighboring magnetic film because it is single crystalline. The Co film with an in-plane magnetic anisotropy(IMA) and the [Co/Pt] multilayer with a perpendicular magnetic anisotropy(PMA) were deposited on VO2 films by D.C sputtering. The temperature dependent magnetic properties of Co-based magnetic films coupled with VO2 were studied to investigate the magnetoelastic effect which was caused by the SPT of VO2. In case of PMA samples, it seemed that the SPT of VO2 didn’t affect on the magnetic properties of the neighboring magnetic film along the easy axis direction. The in-plane lattice distortion by the SPT of VO2 cannot make magnetoelastic effect along the perpendicular direction to the plane. Unlike that, in case of IMA samples, the interfacial stress by the lattice constant modulation of VO2 directly affected on the neighboring magnetic film which has an easy-axis within the plane. In particular, the effect of SPT of VO2 was observed not only along easy and hard axes in IMA samples, but also along in-plane hard-axis direction in PMA samples. Around the temperatures of SPT of VO2 upon heating and cooling, the temperature dependence of coercive field exhibited an anomaly like a peak behavior or a curvature change. The magnetic anisotropy energy density values which showed a change between before and after SPT of VO2 were estimated. In addition, a change of coercivity of the neighboring magnetic film near the SPT of VO2 was dependent on the thickness of VO2 because the SPT behavior itself in VO2 depends on the film thickness.
Thesis Advisor: Prof. Joonghoe Dho