闪锌矿Ga_(1-x)Cr_xN铁磁性的密度泛函理论(英文)

采用基于密度泛函的第一性原理方法计算了闪锌矿GaN掺Cr的电子结构和磁性.考虑不同掺杂浓度和位置,计算结果表明,Ga1-xCrxN呈现铁磁基态,Cr原子间是铁磁性耦合并团簇于N原子,Cr原子与最近邻N原子为反铁磁性耦合.我们采用双交换机制解释了磁性来源和机制.计算结果和最近闪锌矿GaN掺Cr的实验结果吻合.     

First-Principles Study of Ferromagnetism in Mn-Doped GaN

We investigate the magnetic properties of Mn-doped GaN through first-principles pseudopotential calculations within the spin-density-functional approximation. We examine the nature of magnetic interactions between Mn ions, and find that the ferromagnetic coupling has a short-range nature, effective for Mn–Mn distances up to about 7~{Å}. For Mn concentrations of about 6%, we find that the ferromagnetic solution is more stable than the antiferromagnetic state, while the stability of the ferromagnetic state is weakened by electron doping. Based on the calculated exchange coupling and the percolation approach, we estimate the Curie temperature lying above room temperature. Analyzing the Mn d levels, we suggest that the d–d hybridization between Mn ions is the main reason for stabilizing the ferromagnetic state. We also find that the formation of small Mn nanoclusters consisting of a few Mn atoms is energetically favorable. Since these small clusters are stable in the ferromagetic state, offering large magnetic moments, we do not rule out a possibility that small Mn nanoclusters are responsible for the ferromagnetism observed in Mn-doped GaN.     

Room-temperature ferromagnetism in Cu doped GaN nanowires

We report magnetism in Cu doped single-crystalline GaN nanowires. The typical diameter and the length of the Ga1-xCuxN nanowires (x = 0.01, 0.024) are 10-100 nm and tens of micrometers, respectively. The saturation magnetic moments are measured to be higher than 0.86 microB/Cu at 300 K, and the Curie temperatures are far above room temperature. Anomalous X-ray scattering and X-ray diffraction measurement make it clear that Cu atoms substitute the Ga sites, and they largely take part in the wurtzite network of host GaN. X-ray absorption and X-ray magnetic circular dichroism spectra at Cu L(2,3) edges show that doped Cu has local magnetic moment and the electronic configuration of it is mainly 3d9 but mixed with a small portion of trivalent component. It seems that the ionocovalent bonding nature of Cu 3d orbital with surrounding semiconductor medium makes Cu atom a mixed electron configuration and local magnetic moments. These outcomes suggest that the Ga1-xCuxN system is a room-temperature ferromagnetic semiconductor.     

LDA + U Study of Induced Half Metallicity in Cr-Doped GaN

Half-metallic ferromagnetism in the Ga_{1 ? x}Cr _x N compound at different concentrations, x = 25, 12.5 and 6.25 %, have been investigated using density functional theory as implemented in code Spanish Initiative for Electronic Simulations with Thousands of Atoms (SIESTA) using LDA + U as exchange-correlation (XC) potential, to find out the possibility of new diluted DMSs. The outcomes reveal that transition metal atom (Cr) doping in GaN induces ferromagnetism. The 3d levels of the TM ion originate a half-metallic gap at the Fermi level in the majority spin channel for all concentrations. Moreover, diluted magnetic semiconductor compounds retain the half-metallic nature at all concentrations, i.e., x = 0.25, 0.125 and 0.0625, with 100 % spin polarization at the Fermi level (E _F). The total magnetic moment of these compounds is due to Cr-3d states, and the existence of a small magnetic moment on Ga and N, non-magnetic atoms, for all doping concentrations is a consequence of p-d hybridization of Cr-d and N-p states. The calculated values of s-d exchange constant N α and p-d exchange constant N βconfirm the ferromagnetic character of the Cr-doped GaN compound.     

Strong d-d electron interaction inducing ferromagnetism in Mn-doped LiNbO3

Mn-doped LiNbO₃ was prepared by ion beam implantation with Mn content of 1 to 5 at.%. The samples were ferromagnetic. The maximum atomic magnetic moment was 5.83 μ_B/Mn for the samples with Mn content of 3 at.% in the implanted layer. Structure characterization using X-ray absorption near edge structure determined that the Mn atoms substituted principally the Li atoms in the LiNbO₃ lattice. The magnetic mechanism was understood with the aid of electronic structure calculation using local density approximations plus U method. The calculated results demonstrated that the Mn:LiNbO₃ with Mn atom on the Li site is half-metallic ferromagnetic. The calculated magnetic moment per cell agreed well with the experimental results. Spin splitting of the d-states occurred for both the Mn dopant and the Nb atoms. The doped Mn atom interacts strongly with its neighboring Nb atoms. This strong d-d electron interaction can work at long range through the whole crystalline cell.     

General synthesis of manganese-doped II-VI and III-V semiconductor nanowires

A general approach for the synthesis of manganese-doped II-VI and III-V nanowires based on metal nanocluster-catalyzed chemical vapor deposition has been developed. High-resolution transmission electron microscopy and energy-dispersive X-ray spectroscopy studies of Mn-doped CdS, ZnS, and GaN nanowires demonstrate that the nanowires are single-crystal structures and homogeneously doped with controllable concentrations of manganese ions. Photoluminescence measurements of individual Mn-doped CdS and ZnS nanowires show characteristic pseudo-tetrahedral Mn2+ ((4)T1-->(6)A1) transitions that match the corresponding transitions in bulk single-crystal materials well. Photoluminescence studies of Mn-doped GaN nanowires suggest that manganese is incorporated as a neutral (Mn3+) dopant that partially quenches the GaN band-edge emission. The general and controlled synthesis of nanowires doped with magnetic metal ions opens up opportunities for fundamental physical studies and could lead to the development of nanoscale spintronic devices.     

Investigations on Magnetic Properties of Cr-Doped LiZnAs by First-Principle Calculations

First-principle calculations were performed to investigate the electronic structures and magnetic properties of Cr-doped LiZnAs. The Cr-doped LiZnAs system with Cr concentration of 6.25 % should be nonmagnetic, while the system favors spin-polarized ground states with Cr-doped concentration up to 12.5 %. The magnetic coupling results show that Cr-doped LiZnAs prefer the ferromagnetic stable state. The ferromagnetic interactions in the Cr–Cr pair can be attributed to the p-d exchange interactions as Cr_↑–As_↓–Cr_↑. The calculations on the formation energies of vacancies reveal that V _Li requires a much lower energy than V _Zn and V _As. V _Li, V _Zn, and V _As can induce ferromagnetism in Cr-doped LiZnAs.     

Electronic and Magnetic Investigations of Rare-Earth Tm-doped AlGaN Ternary Alloy

Electronic structure and magnetic interaction of substitutional thulium rare earth-doped wurtzite Al_0.5Ga_0.5N ternary alloy have been performed using density-functional theory within local spin-density approximation with Hubbard-U corrections (LSDA+U) approach. The LSDA+U method is applied to the rare earth (RE) 4f states. The calculation of formation energy shows that it is more energetically favorable for a substitutional Tm atom to replace the Al atom than the Ga atom. For AlGaN:Tm, the lattices parameters are expanded due to larger ionic radius of Tm than that of Al atoms. The energy band gap of AlGaN:Tm has direct character and its width becomes small compared with that of AlGaN. The magnetic coupling between Tm ions in the nearest neighbor sites is ferromagnetic. Magnetic interaction of rare earth ion with the host states at the valence and conduction band edges has been investigated and compared to those of GaN:Mn and has been found to be relatively small.     

Effect of Magnetic Transition Metal (TM = V,Cr, and Mn) Dopant on Characteristics of Copper Nitride

Pure and magnetic transition metal (TM = V, Cr, and Mn)-doped copper nitride (Cu₃N) films with a preferred orientation of (100) were successfully prepared by reactive rf magnetron sputtering with an appropriate optical band gap for potential application in solar energy conversion. The incorporation of V or Mn can make the lattice constant smaller while it becomes larger after the inset of Cr into Cu₃N. The calculated equilibrium structural parameters reveal that the unit cell of Cu₃N could expand by inserting TM in the center or Cu vacancy of Cu₃N. The SEM shows boundaries disappeared grains of V-doped sample, small spherical-like grains of Cr-doped sample, and greater glomerate crystal particles of Mn-doped Cu₃N. The band gap becomes minor (1.50 to 1.22 eV) after doping with V or Cr and larger (1.50 to 1.56 eV) for Mn-doped samples, which realized the adjustable optical band gap of Cu₃N films by doping with TM. What is more, TM in the center vacancy of Cu₃N can improve magnetic properties more effectively than that of TM in the Cu vacancy of Cu₃N. And Cr doping in the center gives the biggest magnetic moment of 0.2656 μ _B.     

A Comparative First-Principles Study of Fe-, Co- and FeCo-Doped ZnO with Wurtzite and Zinc Blende Structures

First-principles study of the electronic and magnetic properties of zinc-blende and wurtzite structures of Fe-, Co-, and FeCo-doped ZnO is presented. It is found that after doping, this diamagnetic material becomes ferromagnetic and half-metallic. It is also shown that the half-metallicity may be obtained for ZnFeO, ZnCoO, and ZnFeCoO. The analysis of the spin density reveals that the ferromagnetic phase is due to the ferromagnetic coupling between the p?Cd states. The effects of Fe on the magnetic properties of ZB and WZ Fe-doped ZnO compound have been investigated with the GGA calculations. In order to understand the role of Fe atom in the ferromagnetism, the density of states both in the presence and absence of Co doping, were calculated. The obtained results show the presence of coupling between Co and Fe atoms through the spin-split impurity band exchange mechanism. More importantly, the calculations show that the magnetic moment changes sensitively with the type of structure of ZnO, zinc-blende, or wurtzite. A discussion by comparing the results obtained in this study and the experimental results reported in the literature of similar systems show a very good agreement.     

Ferromagnetism above room temperature in bulk and transparent thin films of Mn-doped ZnO

The search for ferromagnetism above room temperature in dilute magnetic semiconductors has been intense in recent years. We report the first observations of ferromagnetism above room temperature for dilute (<4 at.%) Mn-doped ZnO. The Mn is found to carry an average magnetic moment of 0.16 mu(B) per ion. Our ab initio calculations find a valance state of Mn(2+) and that the magnetic moments are ordered ferromagnetically, consistent with the experimental findings. We have obtained room-temperature ferromagnetic ordering in bulk pellets, in transparent films 2-3 microm thick, and in the powder form of the same material. The unique feature of our sample preparation was the low-temperature processing. When standard high-temperature (T > 700 degrees C) methods were used, samples were found to exhibit clustering and were not ferromagnetic at room temperature. This capability to fabricate ferromagnetic Mn-doped ZnO semiconductors promises new spintronic devices as well as magneto-optic components.     

Mn- and Cr-Doped InN: A Promising Diluted Magnetic Semiconductor Material

Cr- and Mn-doped InN films were successfully grown by plasma-assisted molecular beam epitaxy on c-plane sapphire substrates. Low temperature GaN buffer layers grown by metal-organic vapor-phase epitaxy were used to accommodate the large lattice mismatch between InN and sapphire. A high n-type carrier concentration of 1.5×10²⁰ cm^–3 was measured in InN films with 3% Cr-doping. Films of this type exhibit a well-defined in-plane magnetic hysteresis loop and remanence for temperatures varying from 5 to 300K. The Mn-doped films, however, turned out to exhibit less clear magnetic properties. Thus, ferromagnetism in Cr-doped InN can be concluded from our measurements.     

Structural and magnetic properties of GaGdN/GaN superlattice structures

GaGdN/GaN superlattices (SLs) having various GaGdN and GaN layer thicknesses were grown on Al₂O₃ (0001) substrates by radio frequency molecular beam epitaxy and their structural and magnetic properties were investigated. By changing the thickness of GaGdN and GaN layer in the GaGdN/GaN SL, we find that the lattice constant in the c-direction of GaGdN is larger than that of GaN. All SLs samples exhibited ferromagnetic characteristics at 10 K and 300 K. Comparing with the GaGdN single layer sample, the magnetization of SL samples is larger at the same unit volume of GaGdN. Magnetization data show that the SL samples having relatively thinner GaGdN layer and thicker GaN layer have larger magnetic moment per Gd atom. This phenomenon can be explained by considering that the carriers (electrons) flow from the GaN layers and are accumulated in the GaGdN layers; thinner GaGdN layer has higher carrier density, suggesting the carrier-enhanced magnetization in SL structure.     

Ferromagnetic properties,electronic structures,and formation energies of Zn vacancy monodoping and (Zn vacancy,Li) codoped ZnO by first principles study

Using the first-principles method based on the density functional theory, we investigated the ferromagnetic properties, electronic structures, and formation energies of Zn vacancy monodoping and (Zn vacancy, Li) codoped ZnO. The results indicate that both cases prefer the ferromagnetic ground state. It was found that the Zn vacancy defect brings a spin polarized state in the nearest neighbor oxygen atoms, and the magnetic moments mainly come from the O atoms surrounding the defect centers, which are different from the conventional diluted magnetic semiconductor. In addition, we found that the spin polarized oxygen atoms have a metallic feature in both spin states and the ferromagnetic exchange interaction among oxygen atoms is mediated by Zn 3d state. Furthermore, it was observed that the replacement of one Zn atom in the system of Zn₁₅O₁₆ by one Li atom can generate holes and reduce the formation energy of Zn vacancy, and then stabilizes the zinc vacancy-including system, resulting in a larger magnetic moment.     

The magnetic properties of Gd doped ZnO nanowires

We present a study of the ferromagnetic properties of Gd doped ZnO nanowires (Nws) fabricated by means of a chemical vapor deposition process. The sample was grown with a Gd mole ratio 5% in a mixed Zn/Mn source under a constant O₂/Ar gas mixture flowing at 580 °C followed by annealing at 800 °C. We found that the magnetic properties of ZnO:Gd Nws are a function of the external magnetic field and temperature. An average value of the moment per Gd atom is as high as 3278 μB as compared to its atomic moment of 8 μB, showing that the ZnO:Gd Nws are an intrinsic diluted magnetic semiconductor. The unprecedented colossal moment is attributed to the effective Ruderman-Kittel-Kasuya-Yosida exchanging interaction.     

Effects of Transition Metal (TM = V,Cr, Mn,Fe, Co,and Ni) Elements on Magnetic Mechanism of LiZnP with Decoupled Charge and Spin Doping

The electronic structure and magnetism of a series of 111-type diluted magnetic semiconductors Li(Zn,TM)P (TM = V, Cr, Mn, Fe, Co, and Ni) are investigated on the basis of density functional theory. Our results indicate that V-, Cr-, Mn-, and Fe-doped LiZnP are magnetic while Co- and Ni-doped LiZnP systems show no magnetisms. But all TM-doped LiZnP systems prefer antiferromagnetic behavior by magnetic coupling calculations. In contrast, V/Li- and Cr/Li-codoped LiZnP prefer ferromagnetic ordering, and Mn/Li-, Fe/Li- and Co/Li-codoped LiZnP display antiferromagnetic spin ordering. Hence, Li dopant is very vital for the ferromagnetic formation of Li(Zn,TM)P materials. It is revealed that the magnetic moments come mainly from the TM 3d orbitals. The ferromagnetic coupling between the TM atoms is explained by through-bond spin polarization. Our work demonstrates that the magnetic properties of Li(Zn,TM)P can be mediated by doping different TM atoms. These results may provide theoretical guidance for further experimental research on DMS.     

Structural, Electronic and Magnetic Properties of Zinc-Blende Ga1−x TM x N (TM = Cr, Mn, Fe, V)

In this paper, we present a theoretical study of structural, electronic and magnetic properties for zinc-blende Ga_1?x TM _x N(TM = Cr, Fe, Mn, V) using the full-potential augmented plane wave (FP-APW) method with local-spin density approximation (LSDA). We have analysed the dependence of structural parameters values on the composition x in the range of x=0.25, x=0.50. Also, the role of p–d hybridisation is analysed by partial (PDOS) and total density of states (TDOS). The magnetic moment of Ga_1?x TM _x N has been studied by increasing the concentration of TM atom. The TM atom is the most important source of the total magnetic moment in these alloys, while the contributions from Ga and N are minor. In addition our results verify the half-metallic ferromagnetic character of TM doped GaN.     

Nickel-related defect in diamond: A tight-binding molecular-dynamics study

Structural, electronic, and magnetic properties of isolated Ni impurities at point defects and a dislocation core in diamond are investigated using tight-binding molecular-dynamics simulations. The results show the structural stabilization associated by lowering local symmetry in the cases of point defects. The segregation energies of Ni impurity for a substitutional site and for an interstitial site in the dislocation core are estimated to be in the same order within 0.2 eV. The local electronic density-of-states reveals that the gap states appeared by the insertion of Ni impurity are strongly localized around the Ni sites. Magnetic moments on neighboring C atoms are induced so as to screen the moment on the Ni atom except for the case of interstitial Ni impurity in which the total magnetic moment remains non-zero. Analyses indicate that localized atomic d states on the Ni atom and the p–d coupling between Ni and neighboring C atoms are responsible for the residual magnetic moment in the system with an interstitial Ni defect. In the other systems investigated, on the other hand, the bonding states between Ni impurity and its neighboring C atoms are dominated by the s–p coupling.     

The piezoelectric properties and the stability of the resonant frequency in Mn–Cr Co-doped PSZT ceramics

Changes of microstructure and piezoelectric properties by Mn-doping, Cr-doping, and Mn–Cr co-doping were investigated in (Pb_0.9Sr_1.0)(Zr_0.5Ti_0.5)O₃ (PSZT) ceramics. Temperature stability and ageing characteristics of a resonant frequency were also studied in Mn-doped, Cr-doped, and Mn–Cr co-doped PSZT ceramics. Both Mn-doping and Mn–Cr co-doping decreased average grain size and both Cr-doping and Mn–Cr co-doping made the piezoelectric properties hard. Mn–Cr co-doped PSZT ceramics had a similar microstructure and temperature dependence of the resonant frequency to Mn-doped ones, but similar piezoelectric properties and ageing characteristics of the resonant frequency to Cr-doped ones. The changes of the resonant frequency with temperature and with ageing time in PSZT ceramics resulted from different origins. The ageing characteristic of the resonant frequency is closely related to an acceptor doping effect.     

Ferromagnetism and the Optical Properties of Mn-Doped CdSe with the Wurtzite Structure

The geometrical structure of CdSe was optimized by using the ultrasoft pseudopotential method of a total energy plane wave based on density functional theory. The band structure, density of states, and optical properties were calculated and discussed in detail. The Mn-doped CdSe is found to be a half-metallic ferromagnet with 100% carrier spin polarization at the Fermi level. At a Mn concentration of 12.5%, the calculated total energy of the spin-polarized state is 614 meV lower than that of the nonspin-polarized state. The net magnetic moment of 5 μ _B is found per supercell for 12.5% Mn-doped CdSe. The estimated Curie temperature of 748.6 K for Mn-doped CdSe is above room temperature. The ferromagnetic ground state in Mn-doped CdSe can be explained in terms of the p ? d hybridization mechanism. These results suggest that Mn-doped CdSe may present a promising dilute magnetic semiconductor, and may have potential applications in the field of spintronics.