Magnetic Regulation Mechanism Of 3d Transition Metal Dilute Magnetic Semiconductors

Combining the magnetic and semiconductor properties,diluted magnetic semiconductors(DMSs)are a kind of spintronics-based materials,which can manipulate the freedom of spins and charges simultaneously.The key issue of this research area is to realize room temperature ferromagnetism in the DMSs.The discovery of weak ferromagnetism in pure HfO₂ thin films brings a new opportunity for the investigation of3d transition metal based-DMSs.As a representative material of 3d transition metal based-DMSs,it is of great theoretical significance and application prospect to study the preparation,ferromagnetism origin and regulation of ZnO.In this dissertation,we investigate the magnetic origin and regulation mechanism of d⁰ type ZnO,d^x type Co²⁺doped ZnO and f^x type Ce³⁺doped ZnO as the starting point.Then we also study the origin and regulation mechanism of room temperature ferromagnetism in antiferromagnetic d^x type NiO and f^x type CeO₂ nanomaterials.Furthermore,based on the oxide-based DMSs,we investigate the anion defects on the ferromagnetism in paramagnetic ferrites,metal phosphates and nitridates nanomaterials,which will strengthen the foundation for the further applications of DMSs in spintronics.This thesis mainly focuses on the following aspects:(1)Synthesis and ferromagnetism of d⁰ type ZnO-based DMSs.We prepared monodispersed and size-controlled ZnO nanoparticles,uniform Co doped ZnO nanoparticles and porous Ce doped ZnO nanoplates through thermal decomposition method.For the monodisperse ZnO nanoparticles,compared with the bulk ZnO with antiferromagnetic properties,the experimental results show that the ZnO nanoparticles display a positive correlation between concentration of oxygen defect and room temperature ferromagnetism.The saturation magnetization value increases from 2.3 to 12.8memu/g.In view of the weak ferromagnetism of ZnO nanoparticles,we doped ZnO with magnetic ions of d^x type Co²⁺and f^x type Ce³⁺cations.Compared with the undoped ZnO nanomaterials,the saturation magnetization values of Co-ZnO nanoparticles and Ce-ZnO nanoplates are improved to be 32.9 and 27.1 memu/g,respectively.The magnetic cations effectively enhanced the magnetic properties of undoped ZnO nanomaterials,and the magnetic properties were consistent with the change of oxygen defect concentration on the surface.First-principles calculations show that the net magnetic moment of ZnO-based systems is derived from the O 2p orbitals induced by oxygen defects and the 3d or 4f orbitals of the d^x and f^x magnetic elements.Combined with the experimental and computational results,we establish the relationship among micro-nano structure,doping concentration/surface oxygen defect and magnetism,which is of great significance for exploring and understanding the origin and regulation of magnetism of 3d transition metal DMSs.(2)Synthesis and ferromagnetism of d^x type NiO nanoclusters and f^x type CeO₂nanomaterials.The d^x type NiO nanoclusters self-assembled by tiny nanocrystals and CeO₂nanomaterials were prepared by solvothermal method and electrospinning technique,respectively.The ferromagnetic regulation mechanism was also studied.The experimental results show that the saturation magnetization values of NiO nanoclusters decrease from144 to 3 memu/g with the decrease of oxygen defects after annealing treatment in air.The saturation magnetization values of CeO₂ nanomaterials increase from 26.5 to 45.8 memu/g with the increase of oxygen defects after annealing in H₂/Ar.These results indicate that oxygen defects play an important role on the enhancement of room temperature ferromagnetism for d^x type and f^x type DMSs with the bulk antiferromagnetic properties.Theoretical calculations show that the presence of oxygen defects can induce long-range ordered ferrimagnetic coupling or ferromagnetic coupling of magnetic ions-oxygen defect-magnetic ions(Ni²⁺-O_V-Ni²⁺ferrimagnetic coupling in the NiO system)or(Ce^+4-?-O_V-Ce^+4-?ferromagnetic coupling in the CeO₂ system).The net magnetic moment of the system is mainly derived from the local spin O 2p orbital induced by oxygen defect and the magnetic 3d or 4f orbital in the magnetic ions.Therefore,oxygen defects are of great significance in the regulation of ferromagnetism in d^x type and f^x type DMSs with bulk antiferromagnetism.(3)After studying the magnetic regulation mechanism of oxygen defects on oxide-based DMSs,we further investigated the effects of oxygen defects on the magnetism of non-stoichiometric Zn_xFe_3-xO₄ nanoparticles.Monodisperse non-stoichiometric Zn_xFe_3-xO₄ nanoparticles were fabricated by thermal decomposition method.As the particle size of non-stoichiometric Zn_xFe_3-xO₄ nanoparticles increases,atoms ratio of Zn/Fe and the concentration of oxygen vacancies decrease,which render the magnetic transition from superparamagnetism to ferromagnetism.X-ray magnetic circular dichroism spectra reveal that the spin magnetic moments of Fe³⁺are reduced and the orbital magnetic moments are frozen as the atom ratio of Zn/Fe increases,leading to the decrease of saturation magnetization.Air-annealing treatment shows that the saturation magnetization values of all the Zn_xFe_3-xO₄ nanoparticles are reduced,suggesting that oxygen vacancies have great influence on the magnetic properties.The air-annealing treatment can reduce the oxygen defects,which triggers part of the Fe³⁺-O_V-Fe³⁺ferrimagnetic couplings transfer into Fe³⁺-O_L-Fe³⁺antiferromagnetic couplings.In this work,the non-stoichiometric Zn_xFe_3-xO₄ magnetism is regulated by regulating the stoichiometric ratio and oxygen defect concentration,which provides a new insight to understand and tailor the magnetic performance of spinel ferrites.(4)The effects of anion defects on the magnetic properties of 3d transition metal phosphides and nitrides nanomaterials.NiCoP nanoparticles and CoN nanorods were prepared by thermal decomposition method and two-steps hydrothermal-nitriding method,respectively.By reducing the amount of initial phosphorus source,the phosphorus defects in NiCoP nanoparticles increase,resulting in a significant enhancement of saturation magnetization values from 2.9 to 14.1 memu/g.At the same time,the phosphorus defects in surface of NiCoP nanoparticles and nitrogen defects in surface of CoN nanorods were increased by annealing treatment in H₂/Ar atmosphere,respectively.The saturation magnetization values of NiCoP nanoparticles are increased from 14.1 to 22.7 memu/g,and the saturation magnetization values of CoN nanorods are increased from 22.2 to 30.4memu/g.By fitting the magnetization curves,we found that the increase of anion defects can result in enhanced concentrations of effective magnetic particle in the sample,further leading to the improvement of magnetism.The enhanced magnetism is mainly due to the generated ferromagnetic couplings of“magnetic ion-anion defect-magnetic ion”,which is induced by the increased number of magnetic ions near the anion defect and the bigger degree of magnetic moment tilt caused by the increased anion defects.The enhanced magnetism by anion defects is of great significance in the magnetic regulation of metal oxides,phosphides,nitrides,carbides and sulfides.