Effect Of Intrinsic Defects And Cu Ion Implantation On The Magnetic Properties Of Single Crystal Rutile TiO₂ And Wurtzite ZnO

Due to its potential applications in the field of information processing and storage,Spintronics devices that take advantage of both electronic charge and spin properties have become the focus of interdisciplinary research in electronics,materials science and physics.Diluted magnetic semiconductors（DMS）have attracted much attention as a potential material for preparing spintronics devices.Because of their excellent photoelectric properties,TiO₂ and ZnO have become popular host materials for DMSs.At present,both doped and undoped TiO₂ and ZnO have been reported to exhibit room temperature ferromagnetism（RTFM）.Since the effect of the doped magnetic elements（such as Fe,Co,Ni,Mn,etc.）may format magnetic second phase or clusters and intrinsic defects cannot be excluded,the understanding of RTFM origin in DMS is still controversial.Metallic copper and its oxides are non ferromagnetic at room temperature,which is a proper doping particle to helpprobe the internal mechanism of RTFM in DMS.In order to explain the origin of RTFM of undoped and Cu ion implantation single crystal rutile TiO₂ and wurtzite ZnO observed by our group.In this paper,CASTEP module of Material Studio software based on the first-principles method of density functional theory was adopted.The effects of intrinsic defects and Cu ion implantation on the magnetic properties of rutile TiO₂ and wurtzite ZnO-based DMS were calculated using the Perdew-Burke-Ernzerhof function（PBE）of the generalized gradient approximation（GGA）.The intrinsic mechanism of ferromagnetism origin was analyzed and explained from the aspects of geometrical structure,electronic structure,density of states and Mulliken population.In the calculation of the intrinsic defect system,the Hubbard model strong correlation system correction method was used to study the magnetism of the neutral intrinsic point defect and the main composite defect systems.The effect of Cu ion implantation on the ferromagnetism of single crystal rutile TiO₂ and wurtzite ZnO was studied by calculating the pure Cu implantation systems with different substituted Cu valence states and concentrations,as well as several possible substituted Cu and vacancy compound defect systems.For the undoped single crystal TiO₂,the Calculation results showed that supercells withoxygen vacancy（V_O）,titanium interstitial（Ti_i）or titanium vacancy(V_Ti)were ferromagnetic in room temperature.The magnetic moment（M_M）of V_O system mainly contributed by the occupied electrons of Ti 3d and O 2p hybrid orbitals which extended to the vacancy.The M_M of V_Ti system mainly came from O 2p orbital electrons without formingπbond around the vacancy.Interstitial atoms and neighboring Ti atoms provided the M_M in Ti_i defect system.The ferromagnetic properties of V_O and oxygen interstitial complex defect system were similar to that of the V_O system.Since V_Ti and Ti_i were antiferromagnetically coupled in the same supercell,the V_Ti and Ti_i complex defect system just presented weak M_M.For the Cu ion implantation of single crystal TiO₂,the contribution of Cu substituent atoms to the RTFM of the sample was regulated by vacancy and substituent concentration.When the concentration of substituted Cu was low,only the zero-valence substituted Cu system was ferromagnetic.When the concentration was increased,it was found that the residual M_M also appeared in substituted Cu²⁺system.The system was in the ferromagnetic ground state when the two Cu substituent atoms were close to each other（no other interval atom）.But the ferromagnetic coupling was directional.In the complex defect system with Cu substituent and V_O,the high carrier concentration was beneficial to the polarization of the system with low Cu substituent concentration,and tended to effect the bonding of Ti atoms around the defect.The carrier contribution to the magnetic moment has a saturation limit,and V_O leaded to the formation of anti-ferromagnetic coupling between substituted Cu atoms.The composite defect of substituted Cu and V_Ti inducedthe weakening of p-d hybridization and the reduction of the M_M of the system.The lattice distortion caused by a point defect or a complex defect and the strong interaction between the Cu-O bond were the origin of ferromagnetism.For the undoped single crystal ZnO,the Calculation results showed that Zn vacancy(V_Zn)and oxygen interstitial（octahedron）systems were ferromagnetic.The M_M of V_O system is zero,while a cluster including three V_Os leaded to a M_M of¹μ_B.None of zinc interstitial and anti-site defects cause spin polarization in the systems.The M_M of V_Zn system was contributed by the electrons which come from 2p orbital of the adjacent O.When two neutral V_Zns in different relative distances were created in respective supercells,the systems showed different M_M.It was due to the unequal level between the highest electron occupied orbital of the defect state(introduced by different V_Zn sites)and the valence band maximum.The M_M of oxygen interstitial system was mainly provided by the extra electrons of interstitial oxygen atom.The system with a complex defect of V_O and V_Zn was magnetic when those vacancies were adjacent but still do not cause the compensation effect.The oxygen interstitial defect is unstable,and V_Zn easily turns into the compound defect.We suggest that V_O clusters and V_Zn complex defects could likely be the origin of ferromagnetism in undoped ZnO.The V_O cluster supercell presented ferromagnetism,which explained the experimental phenomenon of RTFM in undoped ZnO,was related to oxygen vacancy.For the Cu ion implantation of of single crystal ZnO,the value of the saturation M_M of the sample is regulated by substituted Cu valence state,substituted Cu pair relative distance and V_O concentration.The polarization of the system can be caused by+1,+2 and+3 valence Cu substituent atoms,but the M_M induced by+1 substituent atoms was far less than that of other valence states.It was drawing to the incomplete filling electronic configuration of the substituted Cu ion and Cu 3d hybridized with O2p contributing to the magnetism.The larger relative distance of the two substituted Cu²⁺,the weaker the ferromagnetic coupling tendency and the smaller the effective magnetic moment.The calculation results of composite defects system also showed that the extra electrons introduced by V_O occupied the 3d orbital of Cu to form a full band,or V_O’s orbital extended to the space of Cu\Zn atom,leading to the weakening of the hybridization of d-p and a dramatic reduction of the magnetic moment of Cu+V_O and Cu+2V_O systems.At the same time,V_O reduced the effective ferromagnetic coupling between substituent atoms.The decrease of the ratio of Cu^2+,3+/Cu¹⁺,excessive concentration of V_O and the diffusion of Cu clusters after Ar⁺post-irradiation were the reasons for the saturation magnetization of Cu ion implanted single crystal ZnO sample decreased sharply.In conclusion,although the origin of the local M_M in various intrinsic defects or Cu implantation system of both TiO₂ and ZnO based DMSs were different,the calculation results indicated that the hybrid between 3d and 2p orbitals,compensation effect,carrier mediation can illuminate the origin of magnetism of undopd and Cu implantation TiO₂ and ZnO based DMSs.The difference of lattice structure result in the ferromagnetism of undoped TiO₂ more drivedfrom the defect type that provides charge carriers,while the ferromagnetism of undoped ZnO tends to occur inthe defect typethat provides vacancy carriers.The direct contribution of substituted Cu to ferromagnetism was small,but the formation energy of adjacent vacancy defects can be reduced.The above results provide a design idea for the preparation process of TiO₂ and ZnO-based DMSs.