Preparation And Magnetism Of ZnO Diluted Magnetic Semiconductors Bulk Samples Doped By Transition Metallic Elements Co & Mn

Diluted magnetic semiconductors (DMSs), i.e., semiconductors doped with magnetic metallic elements, have been the frontier research objects in the field of condensed state physics and material science because of the possibility of assembling charge and spin degrees of freedom in a single substance. As one of the II–VI compound semiconductors, ZnO has drawn much attention because theoretical studies predicted room-temperature ferromagnetism in a semiconducting ZnO recently, normally n type, when doped with suitable magnetic ions. In this thesis, magnetic transition metallic elements Co and Mn were introduced into ZnO by a standard solid-state reaction method, the magnetization behaviors and the origin of ferromagnetism of Zn_1-xCo_xO and Zn_1-x(Mn_1/2Co_1/2)_xO samples were studied. The main content of the thesis is as follows: In chapter one, the study background and application perspectives of DMSs are introduced. Some of basic properties of DMSs and ZnO, such as lattice structure, energy gap, etc. are also reviewed. In chapter two, the preparation process and the possible magnetic interactions of the Zn_1-xCo_xO and Zn_1-x(Mn_1/2Co_1/2)_xO samples has been introduced. In chapter three, the structure and magnetization behaviors of Zn_1-xCo_xO samples have been experimentally investigated. Based on the experiment results, the origin of magnetism and the mechanism of magnetic interaction of Zn_1-xCo_xO are discussed. Main experiment results are listed: (1) The crystal structure of Zn_1-xCo_xO samples prepared by a standard solid-state reaction method is a single phase. (2) The Zn_1-xCo_xO samples prepared at low temperature (<600℃) show a low-temperature ferromagnetic ordering and the PM-FM transition temperature is about 40K. (3) Both sintering temperature and impurity concentration have remarkably effect on the magnetization behaviors of Zn_1-xCo_xOferromagnetic ordering is only observed in the samples with low impurity concentration prepared at a low temperature. (4) Ferromagnetism of Zn_1-xCo_xO samples come from the ferromagnetic coupling between different ferromagnetic clusters formed in the paramagnetic background, which is intrinsic. (5) A possible explanation for the magnetization behaviors of Zn1-xCoxO samples is present according to using magnetic percolation theory. In chapter four, additional impurity Mn is introduced into Zn1-xCoxO samples, and their magnetic properties have been investigated. Experiment shows: (1) The samples (x<15%) are single phase wurtzite structure, and for the sample (x=30%), the main phase remains wurtzite structure, meanwhile different antiferroamgnetic secondary phase is observed. (2) The Zn1-x(Mn1/2Co1/2)xO samples prepared at low temperature (<700oC) show ferromagnetic ordering in low-temperature and the magnetic transition temperature is 45K, however, the samples prepared at high temperature (>700oC) manifest paramagnetism. (3) The magnetization of Zn1-x(Mn1/2Co1/2)xO samples is very difficult to reach saturation. The magnetization of the sample with 10% impurity is just 0.4emu/g in an applied magnetic field of 70000Oe at 10K, which is less than the theory value of saturation magnetization. (4) The study of susceptibility χas a function of temperature T confirms the presence of Griffiths phase, and qualitative explanation is proposed for the behavior of susceptibility based on a phenomenological expression for χ(T). (5) Additional Mn in Zn1-xCoxO samples is help to formation of ferromagnetic ordering.