Study Of ZnO-based Magnetic Semiconductor With High Transition Element Concentration

Recently, magnetic semiconductors (MS) have attracted considerable attention because of their potential applications in spintronics devices. Magnetic semiconductors are usually synthesized by doping transitional metal elements such as V, Cr, Mn, Fe, Co, and Ni, intoâ…¢-â…¤,â…¡-â…¥, andâ…£group compounds such as InAs, GaAs, ZnO, TiO₂, SnO₂, ZnTe, etc. In_1-xMn_xAs and Ga_1-xMn_xAs are pioneering examples of such magnetic semiconductors with ferromagnetism, but the Curie temperature is too low (The obtained highest Curie temperature in Ga_1-xMn_xAs is 170 K, still far below the room temperature).ZnO-based MS are paid much attention since the theoretical prediction of room temperature ferromagnetism on ZnO- and GaN-based MS. But reported experimental results of ZnO-based magnetic semiconductors by different research groups are quite different and even contradictory, and hence the origin of the ferromagnetism is still an open question from the experimental point of view. Besides some intrinsic origins for ferromagnetism, such as carrier-mediated interaction, super-exchange interaction, ferromagnetism from secondary phases was also supposed due to the low solubility of transition metal elements in the host lattice. In the magnetic semiconductor system, due to the interactions between the s, p electrons of the carriers and the localized d electrons of the doped transitional metal elements, the ferromagnetism can be established, which is regarded as the carrier-mediated ferromagnetism. But the detailed mechanisms of the ferromagnetism of high Curie temperature are still not well understood theoretically. In this thesis, I mainly introduce our recent work about the properties of Co and Fe doped ZnO magnetic semiconductors with room temperature ferromagnetism.1. ZnO-based magnetic semiconductor films with high concentration of the transitional metal elements were prepared by alternately sputtering very thin transitional metal (TM) and ZnO layers. It is well known that the transition metals usually have very low solubility in oxides, such as ZnO, TiO₂, SnO₂,In₂O₃ and so on. Therefore, it is not possible for high concentration doping of TM elements into these oxides in the thermal equilibrium state to form a single phase, such as Zn_1-xCo_xO, without precipitation phases like Co metal clusters. We alternately deposited very thin TM and ZnO layers, such as only 1-3 atomic monolayers, on the water cooled glass substrates. Since the interface roughness and the interdiffusion length are comparable, TM and ZnO may incorporate into each other due to interdiffusion to form Zn_1-xTM_xO phase. X-ray diffraction and transmission electron microscopy didn't find any TM metal clusters in the as-deposited Co-ZnO and Fe-ZnO magnetic semiconductor films.2. Co-ZnO as-deposited samples have been proved to be intrinsic magnetic semiconductor with high Curie temperature and high magnetization. The magnetic measurements showed that the samples are ferromagnetic at room temperature and the Curie temperature is above the room temperature. The saturation magnetizations of a typical sample [ZnO 0.5nm/Co 0.5nm]₆₀ are very high at both 5 K (550 emu/cm³(1.41μ_B/Co))and 290 K (400 emu/cm³(1.03μ_B/Co)), respectively. X-ray magnetic circular dichroism results indicated that the oxidation valence state of Co in high spin state in the host ZnO has contribution to the magnetism, and the impurity phases are excluded by structural measurements. Therefore the origin of magnetisim of Co-ZnO as-deposited samples is suggested to be intrinsic.3.The spin-dependent electrical transport and magnetoresistance in ZnO-based magnetic semiconductors have been studied experimentally andtheoretically. In Co-ZnO magnetic semiconductors a linear relation of lnρversusT^1/2, which shows different slopes and intersections at different magnetic fields, is observed experimentally in the low temperature range. In Fe-ZnO magnetic semiconductors, a universal form of the resistance versus temperature, i.e.,lnρ^∞T_H /T + (T_ES/T)^{-1 2}, is observed experimentally at different magnetic fields. The spin-dependent variable hopping model has been proposed by taking into account the electron-electron Coulomb interaction, the spin-spin exchange interaction and hard gap energy in the same frame, which can well described the observed magnetic transport properties in both Co-ZnO and Fe-ZnO magnetic semiconductor systems. Moreover, large negative magnetoresistance is observed at room temperature in ZnO-based magnetic semiconductors and the negative magnetoresistances are about 11% and 8% at 293K and 34% and 27% at 5K in Co-ZnO and Fe-ZnO magnetic semiconductors, respectively.4.The ferromagnetism of ZnO-based magnetic semiconductors is explained by the modified scenario of the F-center mediated ferromagnetism. The oxygen vacancies are easily fromed in ZnO material. The oxygen vacanciy acts as a shallow doner in Co doped ZnO and traps an electron. Cansidering the interaction of the magnetic cations with the trapped hydrogenic electron in the impurity band, the trapped electrons tend to form bound magnetic polarons, coupling the 3d moments of the ions within their orbits. The interaction between the bound magnetic polarons by means of the shared impurity cations may be ferromagnetic at large concentrations of magnetic impurities. The strong interaction between the weakly localized s,p electrons of the oxygen vacancies and the strong localized d electrons of the Co may enhance the Curie temperature and saturation magnetization.5.The polar Kerr rotation and ellipticity spectra of the as-deposited and annealed Co-ZnO magnetic semiconductors were studied. The Kerr rotation spectra versus the photon energy can be greatly modulated by adjusting the Co concentrationor annealing the samples. Moreover, the observed maximal Kerr rotation, 0.72°in anannealed sample is higher than those of pure Co films, Pt/Co multilayers and Pt_xCo_1-x alloys. The enhanced Kerr rotation in the annealed samples can be explained by the fact that the annealed samples became a nanocomposite system consisting of Co clusters and Co-ZnO magnetic semiconductor.6. Ferromagnetic resonance is used to study Fe-ZnO magnetic semiconductors. The angular dependence of ferromagnetic resonance field for the samples with different compositions was studied. As the Fe content increases, the saturation magnetization increases, and the resonance field in normal resonance mode increases. The linewidth of the ferromagnetic resonance reduces as the Fe content increases, indicating that the inhomogeneity in chemical composition reduces with increasing Fe content. Therefore, suitably increasing Fe content is favorable to obtain magnetic semiconductor films with high saturation magnetization at room temperature.