Interaction Of Ferromagnetic Films With Polar ZnO Surfaces Studied By Synchrotron Radiation Photoemission

With the increasing demand of high-density storage devices, the behavior of ultrathin Fe,Co films on various oxide surfaces has been studied extensively in recent years. The physical and chemical properties of the interface including the film magnetic, electrical properties and chemical stability have become the main focus of the researches. As the growth of the magnetic thin film on oxide surface, the interfacial interaction in terms of chemical states, interdiffusion and geometric structure will show a corresponding change with the film thickness, which will lead to different interfacial properties. Such a research as the film thickness vs interfacial interaction will be very meaningful in the application of these materials. Meanwhile, the annealing process at the interface will effectively prompt the diffusion of the magnetic particles into the oxide substrate. This method attracted a growing attention in the recent DMS studies.In this dissertation, by using the photoemission technique mainly, the electronic structure evolution of Fe/ZnO(000±1) and Co/ZnO(000±1) interfaces have been investigated systematically during the growth and annealing processes. In addition, techniques such as Atomic Force Microscopy(AFM), Field-Emission Scanning electronic microscopy(FE-SEM) and Superconductor Quantum Interference Devices(SQUID) have also been employed to obtain the surface morphology and magnetic properties of the interfaces. The main results are described as follows:1. Synchrotron radiation angle-resolved photoemission spectroscopy (SRARPES) and photoelectron diffraction (PD) techniques were used to investigate the clean ZnO(000-1) surface. The band structures along the FA, FM directions and the photoelectron diffraction in the [-1100] plane have been given as primary results.2. At room temperature, the XPS and SRPES studies for the Fe deposition on different polar ZnO surfaces have shown that the growth mode of Fe film depends on the polarity of ZnO substrate, the Volmer-Weber (VW) mode on the ZnO(0001) surface and the Stranski-Krastanov (SK) mode on the ZnO(000-1) surface have been observed respectively. On the Fe/ZnO(000-1) interface, the results showed obvious Fe2+formation at the initial Fe deposition stage. During the deposition of Fe on ZnO up to 3nm, three meaningful and critical thicknesses of 0.5ML,1ML,3ML have been obtained which may be related to the charge transfer, chemical reaction, and magnetic property, respectively. Meanwhile, the resonant photoemission results at different Fe thicknesses have shown that the initially oxidized Fe atoms do not cause any significant changes in the conduction band of ZnO indicating the weak interaction with ZnO at RT.3. The annealing process of 3nm-Fe/ZnO(000-1) have been studied by SRPES and AFM. Evident proof shows that the ultrathin Fe film has a disagglomeration process at low temperature annealing around 300℃. And a subsequent aggregation process at 600℃leads to the size growth of iron clusters and the reexposure of the substrate. The interfacial reaction takes place at 600℃, metallic Fe is gradually oxidized into Fe2+with the increase of annealing temperature, and even Fe3+above 800℃. Active oxygen released from the ZnO substrate on annealing the samples above 600℃is responsible for the oxidation reaction at the interface. The magnetic coercive force of the sample has been observed to increase with the annealing temperature. The existing ferromagnetism of the 900℃annealing sample may indicate a ferromagnetic phase formation from the interfacial reaction.4. The electronic structures of Co/ZnO interface have been investigated by SRPES. At RT, with both of the polar ZnO surfaces the Co film showed strong reactivity and obvious Co2+oxidation state can be detected even when the film thickness reaches 0.45nm. The annealing process of 1.1nm-Co/ZnO(000-1) have shown that, the 200℃annealing led to the increase of the Zn3d peak from ZnO substrate but no significant oxidation-reduction reaction between Co and ZnO has been detected below 300℃. Metallic Co is gradually oxidized into Co2+with the increase of annealing temperature above 300℃. From a series of RPES results, Co-ZnO interfacial interaction leads to the appearance of four new peaks with the binding energies of 2.0eV, 11.0eV,3.4eV and 6.8eV, respectively, the 2.0eV and 11.0eV peaks should be derived from Co3d6 and Co3d7L of CoO, and the 3.4eV and 6.8eV peaks should be attributed to the magnetic doping structure Znl-xCoxO. The RPES results at different Co thicknesses have shown that the initially oxidized Co atoms lead to a pre-edge structure in Zn3d PEYS which will derived from Co doping into the ZnO lattice. From the valence band structures at different annealing temperatures, a peak with binding energy of 3.4 eV was greatly enhanced at 600℃, which may suggest the formation of Co-doped phase at this temperature.