Abstract
Abstract Investigation of Fe3O4 and ZnCoO based Magnetic Tunnel Junctions This research focuses on the investigation of Fe3O4 and ZnCoO based magnetic tunnel junctions (MTJs) with crystalline MgO barrier, especially on the fabrication of electrodes of Fe3O4 and ZnCoO and their magnetic and transport properties. Three main topics are discussed in this dissertation. First of all, the growth of epitaxial Fe3O4 films is studied on MgO (100) substrates at room temperature by using reactive ion beam deposition (IBD). The Verwey transition, unique feature of stoichiometric Fe3O4, of 110K was observed on epitaxial (100) Fe3O4 films. To integrate epitaxial Fe3O4 films on Si substrates when the Fe3O4 are the electrode for MTJ, epitaxial (111) Fe3O4 films were grown on Si substrates with introducing Cu conducting underlayer at room temperature. X-ray φ-scans and TEM diffraction pattern revealed unusual 12-fold symmetry of the epitaxial (111) Fe3O4 films on Cu (001) underlayers due to the presence of two sets of epitaxial (111) grains in Fe3O4 films. In addition, a clear Verwey transition of room temperature growth (111) Fe3O4 films with Cu conducting underlayers on Si substrate was also observed at around 116K. The second topic is the fabrication of the dilute magnetic semiconductor – ZnCoO and the investigation of the exchange coupling between the AFM layer and ZnCoO. Epitaxial ZnCo0.07O films on the Cu underlayer were fabricated at room temperature by ion beam deposition on Si substrates, which revealed room-temperature ferromagnetic behavior with coercivity of 70 Oe. To integrate DMSs into spintronic devices, we studied exchange biasing between ZnCoO and NiO. In order to prevent the problems of interfacial reactions between the ZnCoO and NiO layers, the quasi-epitaxial full-oxide exchange-bias system (ZnCoO/NiO) was prepared at room temperature. In the epitaxial ZnCo0.07O /NiO system, exchange fields accompanying vertical magnetization shifts were observed after field cooling. Transitions of exchange fields and magnetization shifts were observed at 50 K, above which the magnetization shift disappeared and the exchange field was significantly reduced. Both the exchange field and the magnetization shift increased with increasing cooling-field strength at temperatures below 50 K, which might be attributed to the existence of “frozen” spins in ZnCoO. The observed linear dependence of the exchange field on the magnetization shift may directly elucidate the role of pinned spins on the exchange fields. In the third topic, the fully oxides MTJ device composed of Fe3O4 and ZnCoO ferromagnetic layer with a crystalline MgO barrier was fabricated at room temperature. The novel structure of full stack MTJ, SiO2// Ta 20nm/ ZnO 2.5nm/ MgO 1.2nm/ Fe3O4 50nm/ MgO 3nm/ ZnCoO 50nm/ Ta 10nm, by using an IBD system. This stack included a conducting electrode, Ta, suitable for lift-off process, a textured MgO barrier with sharp interface and highly (100) textured FM layer. The non-linear I-V curves for the fully oxides MTJs patterned by lift-off process were observed, which indicated that the electrical transport was dominated by tunneling. The MR of the full oxides stack MTJ was 2.68% at 150K. Furthermore, the clear plateau of the R-H curve was observed clearly in the low field region, which represented the typical pseudo-spin valve switching behavior. It was a strong evidence to prove that DMS could provide the spin-polarized electrons.