First Principle Study On Magnetism Of Doped ZnO Based Diluted Magnetic Semiconductors

Spintronics seek to exploit both spin and charge attributes of information carriers totransfer,process and store data,which has become a focused interdisciplinary fieldinvolving electronics,physics,materials science and other disciplines.As a crucial materialin this field,ZnO based diluted magnetic semiconductor has received a great deal ofattention.Zinc Oxide semiconductors base materials with high doping concentration and the3-d band of doped ions could generate strong ferromagnetic coupling interaction throughthe intrinsic defect states of the base material,which led to Zinc Oxide become the mostemerging hotspot in this field.Two key scientific problems in spintronics to both look formagnetic semiconductors with high Curie temperature and analyse the origin andmechanism of magnetism in these semiconductors could be sought for solution.Therefore,the electronic structure of diluted magnetic semiconductors through first-principlecalculation was firstly carried out,and the results were utilized to analyze and explain theorigin of ferromagnetism.Base on the result achieved in first-principles,a new couplingintegration algorithm to calculate the Curie temperature of ZnO DMS and discusse thecontrolling method of it was put forward.Then the effects of interaction mechanism ofdifferent single-doping and co-doping diluted magnetic semiconductor systems wereextensively discussed in detail.Firstly,the electronic structure of ZnO-based diluted magnetic semiconductor wascalculated by first-principles according to the Pseudopotential Plane Wave method (PP-PW)and Full Potential Augmented Plane Wave method (FP-LAPW).The coupling interactionbetween transition metals in oxide magnetic semiconductors were analysed fromperspectives of energy bands theory,which described the origin of magnetism about dilutedmagnetic semiconductor.Furthermore,the coupling computations of first-principles andMonte Carlo method were developed by calculation code in our study,which was used toestimate Curie temperature of several diluted magnetic semiconductor systems and studiedthe further controlling method of it.Secondly,the electronic structure of carbon doped ZnO was calculated by the full potential linearized augmented plane wave method (FP-LAPW) with general gradualapproximation (GGA).The results showed that the single carbon doping led toferromagnetism both in Oxygen position and in interstitial position,and they both result inhalf-metallic property,which theoretically had a high rate of spin-polarization.The Curietemperature of carbon doped ZnO was also predicted through a combination of first-principles and Monte Carlo coupling integrated calculation method.The results showedthat the Curie temperature of carbon doped ZnO system changed ranges from 251 to 439 Kas the concentration variation.As a result,room-temperature ferromagnetism of carbondoped ZnO could be derived in proper preparing conditions.With full potential linearized augmented plane wave method (FP-LAPW) and Coulombpotential amendment,electronic structure and magnetism of neodymium doped ZnO,theneodymium doped ZnO with an oxygen vacancy ( Vo ) and neodymium doped ZnO a Zincvacancy ( V_Zn) were calculated,respectively.The results indicated that neodymium dopedZnO with V_Zn defects might have high Curie temperature,while those without intrinsicdefects are paramagnetic,and those with Vo defects get weak anti-ferromagnetism.Furthermore,bound magnetic polaron theory can be employed to explain the origin ofmagnetism in systems with Zn vacancy.Finally,after the single doped ZnO was presented,the effect of metal-transition metalco-doped ZnO systems were extensively examined to described the mechanism of the inter-exchange and the origin of magnetism.Hence,Al-Co co-doped ZnO systems were alsoinvestigated;the calculated results showed that by co-doping with Al,the systems weretransferred from anti-ferromagnetism to ferromagnetism.The origin of ferromagnetismmainly attributes to the interaction between Al-2p electron and nearest Co-3d electronmodulated by extra electrons,and the inter-exchange effect can be explained by p-dexchange model.Al-Fe co-doped ZnO system is also investigated,and the results indicatethat by co-doping with Al the system is transferred from anti-ferromagnetism toferromagnetism.But the Al-2p and Fe-3d electrons do not have interaction with each other,and the origin of ferromagnetism can be explained by long range Ruderman-Kittel- Kasuya-Yoshida (RKKY) exchange interaction.In order to describe the different interaction mechanism of DMS,the influence of metal-non metal co-doping system were analyzed.Hence,the Cu-N co-doped ZnO systemswere also investigated.By introducing nitrogen,the interaction between N-2p and Cu-3delectrons stabilize the magnetism of the system.In the meanwhile,both before and after co-doping,the VBM is always occupied by O-2p electrons,while in conduction band states aremostly occupied by Cu-3d and 4s electrons.Carrier mediated model was employed toexplain the origin of magnetism.As nitrogen co-doping improved the density of chargecarriers in system,interaction mediated by free carriers between magnetic ions led toferromagnetic order as a whole.To obtain the Curie temperature controling method and the interaction mechanism,metal-nonmetal co-doped systems were also studied by co-doping nitrogen the Manganesedoped system showed stable ferromagnetism.Based on Heisenberg model and mean fieldtheory,with Monte Carlo and first-principles method,we predict that room-temperatureferromagnetism can achieve in ideal conditions theoretically.As a result,through the combination of first-principles and Monte Carlo method,single-doped and co-doped systems were extensively discussed.The results showed thatdifferent doped system has different inter-exchange mechanism.Our calculation can explainthe origin of magnetism in some doped systems and the way to increase the Curietemperature.All these can give a guide for the preparation techniques of ZnO dilutedmagnetic semiconductors.