A Study On The Structure And Properties Of ZnO Diluted Magnetic Semiconductors Tuned By High Magnetic Field

Diluted magnetic semiconductors(DMS), which refer to magnetic ions partially substitute cations of the host semiconductor materials, have attracted considerable attention for potential applications in spin electronics and magnetic devices that allow the manipulation of both charge and spin degrees of freedom. The crucial challenge for practical application of the diluted magnetic semiconductors materials is the achievement of room-temperature ferromagnetic transition. Since the predication of high temperature ferromagnetic DMS, ZnO semiconductor doped with Transition-metal(TM) to achieve ferromagnetism at room temperature becomes one of the most extensively studies of DMSs materials. The research of TM doped ZnO is arguably the most attractive topic in condensed-matter physics, magnetic and spintronic materials, etc.This thesis focus on the TM doped ZnO diluted magnetic semiconductors with the aids of several analysis technics of XRD, SEM, TEM, Raman, PL, UV-Vis, XPS, VSM and PPMS to study the origin of ferromagnetism of TM doped ZnO DMS. High pulsed magnetic field was introduced into the process of hydrothermal method. Furthermore, the meachnism of high magnetic field applied during hydrothermal process was elucidated systematically. Three main research parts are included in this thesis.Firstly, 1%Cr-1%Ni codoped ZnO samples were synthesized with various experimental parameters. The growth habit of ZnO DMS prepared by hydrothermal method was investigated by tuning some chemical and physical parameters during experimental process. The concentration of zinc and hydroxide source and sequence of adding solution just affect slightly the particle size and almost do not have effect on the magnetic property. The p H value of solution decides the morphology of crystals according to the style of charge in the precursor. Among the physical parameters, reaction temperature and pressure of autoclave mainly mediate the stage of nucleation. And reaction time and reaction mediums mainly change the growth stage of crystals.Secondly, the influence of codoping strategy was elucidated. The ferromagnetic property is improved by the Cr-Ni codoping strategy in ZnO. With the increasing dopants, ferromagnetism reduces due to the antiferromagnetic super-exchange between the nearest-neighbour magnetic cations. 1%Fe-1%Mn codoped ZnO shows room temperature ferromagnetism. However, both 1%Fe-1%Fe and 1%Mn-1%Mn monodoped ZnO exhibit paramagnetic behaviors at room temperature. The synergistic effect of codoping strategy was definite.Finally, The effect of high magnetic field on the microstructure and magnetic property of 1%Cr-1%Ni, 1%Cr-1%Mn and 2%Ni-2%Al codoped ZnO DMS was studied systematically to understand the origin of ferromagnetism of TM doped ZnO DMS. The influence of high pulsed magnetic field on the crystal morphology is not obvious and this effect varys with the type of doping elements. The effect of high pulsed magnetic field on the magnetic property of TM doped ZnO varied according to the transition metal. First, the pulsed magnetic field improves the concentration of oxygen vacancies in the Cr or Ni doped ZnO matrix and enhances the room temperature ferromagnetism. Second, zinc vacancies become the dominant defect with the help of high pulsed magnetic field in Mn doped ZnO DMS and deduce room temperature ferromagnetism. The former agrees well with the bound magnetic polaron theory and results of first-principle calculation as given by J.M.D Coey and Sato et al. The latter is in agreement with the computational results of Dietl et al. In the Cr or Ni doped ZnO, oxygen vacancies are very easily to form in matrix. However, the Mn doping facilitates the appearance of zinc vacancies. The high pulsed magnetic field enlarges the polarity-dependence.Comparing with the related publications, this research not only successfully prepared various TM-doped ZnO DMS with intrinsic room temperature ferromagnetic and ensures the improvement of codoping strategy for the ferromagnetism, but also demonstrates that the effect of high magnetic field changes with the different doping transition elements.