Electronic Structure And Magnetism Of Oxide-based Ferromagnetic Semiconductors

Spintronics aims to combine both spin and charge degrees of electrons as the carrier of information.The development of spintronics significantly enriches the condensed matter physics.The key material of spintronics is called "magnetic semiconductor" which has received great attentions in the past decades.Transitional metal ions are introduced into the crystalline lattices of the semiconductors,and the ferromagnetism is induced by the ferromagnetic exchange interaction between transitional metal ions.Much effort has been devoted to search for high Curie temperature magnetic semiconductors.However,it's hard to achieve high Curie temperature for transition metal ions doped Si or Ge,which attributes to the low concentration of the dopants.Consequently the transition metal ions are too far to form the long range ferromagnetic interaction.In addition,the coupling between transition metal ions may be antiferromagnetic rather than ferromagnetic.Therefore the Curie temperature is too low and it's hard to make great progress with silicon or germanium based magnetic semiconductors.Compared with silicon and germanium,oxide semiconductor hosts are more attractive since:1)The radii of dopants,such as the transition metal ions,are similar with that of the host cations and therefore it's convenient to achieve the high doping concentration;2)Recent studies show that the oxygen vacancy defects in oxide semiconductors can form the donor impurity state near the band gap,which can mediate the ferromagnetic interactions between the transition metal ions.Therefore much effort has been concentrated to the oxide-based magnetic semiconductors.However,the origin of the intrinsic ferromagnetism is still under debate.The destination of the such dissertation is to find out the influence of defects on the magnetization and the origin of ferromagnetism of magnetic oxide semiconductors.To theoretically study the magnetic semiconductors,two approaches are usually employed:â…°)model Hamiitonian orâ…±)first-principles calculation based on density functional theory.The later one has been selected in such dissertation.We have calculated the electronic structures of oxide-based magnetic semiconductors using first-principles calculation software. Based on the calculated band structures,we discuss magnetic exchange interaction between transition metal ions and the origin of ferromagnetism. Based on the density functional theory and computer technology,many first-principle calculation software packages,such as Vasp,Castep,Siesta,PWscf(Quantum-Espresso), emerged in the last few decades.In addition,plane-wave basis set and pseudo-potentials are often emplemented in these packages.The basic steps of investigating magnetic semiconductors by first-principle calculation software are as follows:First we build a supercell consisting of 30 - 80 atoms to simulate the host material,in which some cations would be substituted by transition metal ions.In addition,we can remove some host ions to introduce defects if necessary.Then we calculate the electron and spin densities of different systems,and analyse the origin of ferromagnetism based on the band structures and magnetic moments of ions.All the results of this project have been calculated by the Quantum-Espresso package,which is based on plane-wave basis set and pseudo-potentials. Espresso is a non-commercial software with opening forum for developers and users.LDA or GGA exchange-correlation potentials in PZ or PBE form has been used.It should be indicated that most of the first-principles codes suffers the underestimation of the band gap and the overestimation of 3d states energy.As a result,LDA+U method should be implemented,which has been integrated in the Espresso package.In addition,the Hubbard U can be calculated based on the linear response approach rather than an adjustive parameter in the other codes.First,the electronic structure of Co-doped ZnO magnetic semiconductor was investigated by LDA and LDA+U methods.The antiferromagnetic order between nearest-neighbor magnetic ions via the middle O ion was predicted when the intrinsic defects such as O vacancies and Zn interstitials were not taken into account.In sharp contrast with the half-metallic characteristic predicted by most previous theoretical calculations,the CoZnO system has semiconductor band structures,which is in good agreement with the results of photoemission spectroscopy.The absence of state near the Fermi level also accords with the poor conductivity of the on-site samples observed experimentally.The removal of O atom from ZnO forms the defect states around the gap which consist of the dangling Zn:4s electrons.Similar with the defect free CoZnO,the neutral V_o can't induce the ferromagnetism.For the V_o²⁺,the outward displacement of the dopants leads the exchange interaction too weak.Only the V_o¹⁺,which provides the fully polarized defect states,can mediate the strong ferromagnetic interaction.However,V_o¹⁺in both ZnO and CoZnO is unstable compared with neutral V_o and V_o²⁺.Therefore only V_o can't induce the intrinsic ferromagnetism without other kinds of defects.We assume that there must be other acceptor defects to compensate partial electrons of the neutral V_o to form the stable polarized defect states.Two acceptors in ZnO,including nitrogen substituting O and zinc vacancy,have been studied to verify the above assumption.N acceptors enhance the antiferromagnetic coupling on the contrary while Co-V_o-Co-O-Vzn complex is theoretically predicted to be in stable ferromagnetic spin ordering.The isolated V_Znor Zn_i also can't induce the ferromagnetism. We conclude that both V_o and V_Znplay the key role of inducing the ferromagnetism in CoZnO.Then we investigated the electronic structure and magnetic properties of Fe-doped In₂O₃ magnetic semiconductors.The presence of Vo can lead to strong ferromagnetic coupling between the nearest neighboring Fe cations.Spin density and band-projected charge distribution in the vicinity of the oxygen vacancies reveal that the ferromagnetic exchange is mediated by the donor impurity state,which mainly consists of Fe:3d and Fe:4s electrons trapped in oxygen vacancies.Such results provide direct evidence for the F-center mediated exchange interaction in oxide-based magnetic semiconductors.Finally the electronic structure of Fe-doped SnO₂ has been systematically investigated. Ferrimagnetic exchange interaction between the dopants,which is rarely found in dilute magnetic semiconductors,is predicted for the ground doping configuration in which two Fe ions are adjacently substituting the Sn sites in a distorted rutile structure.Spin density results reveal the direct exchange interaction between O-2p and Fe-3d electrons with anti-parallel spins,which leads to the competition between ferromagnetic and antiferromagnetic superexchange interactions since the doping ions tend to form a special 120°Fe-O-Fe bond. Therefore,negligible energy difference between the parallel and anti-parallel spin alignments is predicted for the ground state configuration.Moreover,the calculated density of states with both parallel and anti-parallel spins show the nearly 100%spin polarized states at the Fermi level.An interesting half-metallic antiferromagnetic state reported recently has been also obtained in one of the calculated doping configurations.In conclusion,the electronic structures of Co doped ZnO,Fe doped In₂O₃ and Fe-doped SnO₂ magnetic semiconductors have been systemically investigated by first-principles calculation.We proposed the combination of V_o and V_Znstructure as the origin of ferromagnetism in Co-doped ZnO.It was found that the dopants are ferrimagnetically coupled together in Fe-doped SnO₂.The calculation results show that the defect states induced by V_o play the key role in mediating the ferromagnetic interactions in magnetic oxide semiconductors.The results provide direct evidence for the F-center mediated exchange interaction in oxide-based magnetic semiconductors.