Magnetic and optical properties of Cu-doped ZnO nanosheet: First-principles calculations

We performed first-principles calculations within density-functional theory to study the magnetic and optical properties of Cu-doped ZnO nanosheet (NS). We found that Cu atom prefers to substitute for Zn site and can induce a local magnetic moment of 1.00 μ_B per unit in ZnO NS. When two Zn atoms are substituted by two Cu dopants, they tend to form a cluster and ferromagnetic (FM) ordering becomes energetically more favorable. In addition, localized states appear within the band gap due to the introduction of Cu dopant to ZnO NS. With increasing Cu concentrations, both the imaginary part of dielectric function and the absorption spectrum exhibit a red-shift behavior, which are in good agreement with the recent experimental results. The ferromagnetic coupling can be attributed to the p–d hybridization mechanism. The intriguing properties of Cu-doped ZnO NS may be promising for designing novel multifunctional nanodevice.     

The role of Co impurities and oxygen vacancies in the ferromagnetism of Co-doped SnO2: GGA and GGA+U studies

The electronic structure and ferromagnetic stability of Co-doped SnO₂ are studied using the first-principle density functional method within the generalized gradient approximation (GGA) and GGA+U schemes. The addition of effective U_Co transforms the ground state of Co-doped SnO₂ to insulating from half-metallic and the coupling between the nearest neighbor Co spins to weak antimagnetic from strong ferromagnetic. GGA+U_Co calculations show that the pure substitutional Co defects in SnO₂ cannot induce the ferromagnetism. Oxygen vacancies tend to locate near Co atoms. Their presence increases the magnetic moment of Co and induces the ferromagnetic coupling between two Co spins with large Co-Co distance. The calculated density of state and spin density distribution calculated by GGA+U_Co show that the long-range ferromagnetic coupling between two Co spins is mediated by spin-split impurity band induced by oxygen vacancies. More charge transfer from impurity to Co-3d states and larger spin split of Co-3d and impurity states induced by the addition of U_Co enhance the ferromagnetic stability of the system with oxygen vacancies. By applying a Coulomb U_O on O 2 s orbital, the band gap is corrected for all calculations and the conclusions derived from GGA+U_Co calculations are not changed by the correction of band gap.     

Effect of codoping with C or N on electronic structures and magnetic properties of ZnO:Cu

Using first-principles calculations based on density functional theory, we investigated systematically the electronic structures and magnetic properties of ZnO:Cu. The results indicate that Cu-doped ZnO prefers a ferromagnetic ground state and behaves like a half-metallic ferromagnet. The magnetic moment mainly localizes at Cu atom and the rest mainly comes from the spin polarized O atoms. It has been found that the ferromagnetic stability can be enhanced slightly by substituting an oxygen atom with one N atom; while the ferromagnetic stability can be weakened by replacing one O atom with a C atom. Due to absence of magnetic ion and the 100% spin polarization of the carriers in ZnO:Cu, one can expect that Cu-doped ZnO would be a useful half-metallic ferromagnet both in practical application and in theoretical studies.     

Prediction of band gap reduction and magnetism in (Cu,S)-codoped ZnO

By employing a density functional theory plane-wave pseudopotential method, we investigated band gap reduction and magnetism as well as electronic structures of (Cu, S)-codoped ZnO. Our calculations indicated that Cu and/or S-doped ZnO can reduce the band gap of ZnO. The (Cu, S)-codoped ZnO has a large band gap reduction of 0.37 eV, two times larger than that in Cu-doped ZnO. S atom has no contribution for the total magnetic moment of (Cu, S)-codoped ZnO, whereas it plays a central role in spin-polarizing of both Cu and S dopants due to strong coupling between Cu 3d and S 3p states. This would offer a new strategy for designing narrow band gap devices with magnetism.     

Half-metallic ferromagnetism in Cu-doped zinc-blende ZnO from first principles study

Electronic structures and magnetism of Cu-doped zinc-blende ZnO have been investigated by the first-principle method based on density functional theory (DFT). The results show that Cu can induce stable ferromagnetic ground state. The magnetic moment of supercell including single Cu atom is 1.0 μ_B. Electronic structure shows that Cu-doped zinc-blende ZnO is a p-type half-metallic ferromagnet. The half-metal property is mainly attribute to the crystal field splitting of Cu 3d orbital, and the ferromagnetism is dominated by the hole-mediated double exchange mechanism. Therefore, Cu-doped zinc-blende ZnO should be useful in semiconductor spintronics and other applications.     

The magnetic phase transition in Cu-doped ZnO: From bulk to nanocluster

Using the first-principles calculations based on the density functional theory, we have investigated the magnetic properties of Cu-doped ZnO both in bulk and nanocluster. The calculated results show that the substitutional Cu ions are spin polarized and have a tendency to assemble. It is found that the ground state has shown a change from ferromagnetic phase to antiferromagnetic phase as the size for the doping system decreases from bulk to nanocluster. In bulk ZnO, the ferromagnetism is attributed to the strong hybridization between Cu 3d and O 2p states. In ZnO nanocluster, however, the antiferromagnetic exchange interaction is dominant because of the very close Cu–Cu distance.     

First-principles study on ferromagnetism in C-doped AlN

First-principles calculations are performed to study the electronic structures and magnetic properties of C-doped AlN. Both generalized gradient approximation (GGA) and GGA+U calculations show that a substitutional C atom introduces magnetic moment of about 1.0 μB, which comes from the partially occupied 2p orbitals of the C, its first neighboring Al and first neighboring N atoms (GGA) or out-of-plane first and fifth neighboring N atoms (GGA+U), among which the atomic moment of the C is the biggest. The U correction for the anion-2p states obviously changes the magnetic moment distribution of Al and N atoms and transforms the ground state of C-doped AlN to insulating from half-metallic. The C atoms can induce ferromagnetic ground state with long-range couplings between the moments in C-doped AlN. The ferromagnetic coupling can be explained in terms of the two band coupling model.     

Synthesis,structural, optical and magnetic properties of Cu-doped ZnO nanorods prepared by a simple direct thermal decomposition route

Cu-doped ZnO nanorods (i.e. Cu = 1.75, 3.55, 5.17 and 6.39 at.%) have been successfully synthesized by simple, direct, thermal decomposition of zinc and copper acetates in air at 300 °C for 6 h. The prepared samples were characterized by X-ray diffraction (XRD) and transmission electron microscopy. The XRD results indicate that the 1.75 at.% Cu-doped ZnO sample has a pure phase with the ZnO wurtzite structure, while the impurity phases are detected with increasing Cu concentrations. It was found that the doping of Cu results in a reduction of the preparation temperature. The optical properties of the samples were also investigated by UV–visible spectroscopy and photoluminescence measurements. The results show that the Cu doping causes the change in energy-band structures and effectively adjusts the intensity of the luminescence properties of ZnO nanorods. X-ray photoelectron spectroscopy analysis implies that there are some oxygen vacancies in the samples and also indicates that all the doped samples are associated with the mixture of Cu ion states (Cu²⁺ and Cu¹⁺/Cu⁰). Magnetic measurements by vibrating sample magnetometry indicate that undoped ZnO is diamagnetic, whereas all of the Cu-doped ZnO samples exhibit room temperature ferromagnetic behavior. We suggest that Cu substitution and density of oxygen vacancies (V _o) may play a major role in the room temperature magnetism of the Cu-doped ZnO samples.     

First principles investigation on the band gap of the ground state of LaCoO3

The band gap of LaCoO₃ in ground state is obtained from the generalized gradient approximation (GGA) with on-site Coulomb correction (GGA+U) calculations (U=3.4 eV, J=0.49 eV ), which agrees with the experimental result very well. A series of local spin density approximation (LSDA) with on-site Coulomb corrections (LSDA+U) and GGA+U calculations are performed with various U and J parameters to understand the recently published band gaps of 1.43 eV from LSDA+U (U=8.33 eV ) and 1.0 eV from GGA+U (U=2.7) calculations. The partial density of states (PDOSs) are presented to investigate the origin of the band gap.     

First-principles linear muffin-tin orbital investigations in the electronic and magnetic structures of double perovskites Ba2TMoO6 (T=V,Cr, Mn,Fe and Co)

The electronic and magnetic structures of ordered double perovskites Ba₂TMoO₆ (T=V, Cr, Mn, Fe and Co) are systematically investigated by means of the first-principle linear muffin-tin orbitals with the atomic-sphere approximation (LMTO-ASA) method. The calculations are performed by using the both local spin density approximation (LSDA) and the LSDA+U Coulomb interaction schemes. The results show a half-metallic ferrimagnetic ground states for T=Cr, Fe and Co in LSDA+U treatment, whereas half-metallic ferromagnetic character is observed for T=V. For T=Mn, insulating ground state is obtained, stabilized in the antiferromagnetic state. The LSDA+U calculations yield better agreement with the theoretical and the experimental results than do the LSDA.     

Cu掺杂的AlN铁磁性和光学性质的第一性原理研究

采用基于密度泛函理论（DFT）的总体能量平面波超软赝势方法,结合广义梯度近似（GGA）,对Cu掺杂AlN 32原子超原胞体系进行了几何结构优化,计算了Cu掺杂AlN的晶格常数,能带结构,电子态密度和光学性质.结果表明,Cu掺杂AlN会产生自旋极化状态,能带结构显示半金属性质,掺杂后带隙变窄,长波吸收加强,能量损失明显减小.同传统的稀磁半导体（DMS）相比,Cu掺杂AlN不会有铁磁性沉淀物的问题,因为Cu本身不具有磁性.因而,Cu掺杂的AlN也许是一种非常有前途的稀磁半导体. 关键词： AlN 第一性原理 铁磁性 光学性质     

First-principles study of ferromagnetism in Zn- and Cd-doped SnO2

The electronic structures and magnetic properties of Zn- and Cd-doped SnO₂ are investigated using first-principles calculations within the generalized gradient approximation (GGA) and GGA+U scheme. The substitutional Zn and Cd atoms introduce holes in the 2p orbitals of the O atoms and the introduced holes are mostly confined to the minority-spin states. The magnetic moment induced by doping mainly comes from the 2p orbitals of the O atoms, among which the moment of the first neighboring O atoms around the dopant are the biggest. The U correction for the anion-2p states obviously increases the moment of the first neighboring O atoms and transforms the ground states of the doped SnO₂ from half-metallic to insulating. The magnetic coupling between the moments induced by two dopants is ferromagnetic and the origin of ferromagnetic coupling can be attributed to the p–d hybridization interaction involving holes.     

Electronic structures and magnetic properties in Cu-doped two-dimensional dichalcogenides

We explore the electronic structures and magnetic properties in Cu-doped MX₂ (=MoS₂, MoSe₂, MoTe₂, and WS₂) based on density functional theory. A Cu dopant leads to a net moment of 5.0 or 1.0 μ_B in MX₂, which mainly depend on the size of crystal-field splitting relative to that of the spin splitting. No magnetism is observed in Cu-doped MoTe₂. The local distortion around the Cu atom reduces the total magnetic moment in two-Cu-doped MX₂. The magnetic coupling between the nearest neighboring Cu atoms is ferromagnetic for all the cases, but they demonstrate various magnetic ground states with the increasing distance between Cu atoms: the Cu-doped MoS₂ and WS₂ exhibit anti-ferromagnetic and nonmagnetic ground state, respectively. A long-range ferromagnetic or ferrimagnetic coupling is attributed to double-exchange interaction in Cu-doped MoSe₂. Half-metallic ferromagnetism with Curie temperature above room temperature in Cu-doped MoSe₂ provides a useful guidance to engineer the magnetic properties of MoSe₂ in experiments.     

Effect of Ce and Cu co-doping on the structural,morphological, and optical properties of ZnO nanocrystals and first principle investigation of their stability and magnetic properties

Ce, Cu co-doped ZnO (Zn_1−2xCe_xCu_xO: x=0.00, 0.01, 0.02, 0.03, 0.04 and 0.05) nanocrystals were synthesized by a microwave combustion method. These nanocrystals were investigated by using X-ray diffraction (XRD), UV–visible diffuse reflectance spectroscopy (DRS), scanning electron microscopy (SEM), and vibrating sample magnetometer (VSM). The stability and magnetic properties of Ce and Cu co-doped ZnO were probed by first principle calculations. XRD results revealed that all the compositions are single crystalline. hexagonal wurtzite structure. The optical band gap of pure ZnO was found to be 3.22 eV, and it decreased from 3.15 to 3.10 eV with an increase in the concentration of Cu and Ce content. The morphologies of Ce and Cu co-doped ZnO samples confirmed the formation of nanocrystals with an average grain size ranging from 70 to 150 nm. The magnetization measurement results affirmed the antiferro and ferromagnetic state for Ce and Cu co-doped ZnO samples and this is in agreement with the first principles theoretical calculations.     

Competition between ferromagnetic and anti-ferromagnetic couplings in Co doped ZnO with vacancies and Ga co-dopants

Spin-polarized first-principles electronic structure and total energy calculations have been performed to better understand the magnetic properties of Co doped ZnO (ZnO:Co) with vacancies and Ga co-dopants. The paramagnetic state of ZnO:Co, in which Co ions lose their magnetic moments, has been found to be unstable. The total energy results show that acceptor-like Zn vacancies and donor-like Ga co-dopants render the anti-ferromagnetic (AFM) and ferromagnetic (FM) states to be more favorable, respectively. With O vacancies, ZnO:Co has been found to be in the weak FM state. These magnetic properties can be understood by the calculated O- and Zn-vacancies and Ga co-dopant induced changes of the electronic structure, which suggest that AFM and FM Co-Co couplings are mediated by O 2p-Co majority (↑)-spin 3d hybridized states in the valence band of ZnO and O-vacancy-derived p states or Ga sp states in the ZnO band gap, respectively. For ZnO:Co with Zn vacancies (Ga co-dopants) the AFM (FM) coupling outweighs the FM (AFM) coupling and results in the AFM (FM) state, while for ZnO:Co with O vacancies, both the FM and AFM couplings are enhanced by similar degrees and result in the weak FM state. This study reveals a competition between FM and AFM couplings in ZnO:Co with vacancies and Ga co-dopants, the detailed balancing between which determines the magnetic properties of these materials.     

Ferromagnetic properties of Cu-doped ZnS: A density functional theory study

Using plane-wave pseudopotential (PWPP) method, the magnetism and spin-resolved electronic properties of Cu-doped ZnS system are studied. Our calculations indicate that ferromagnetic (FM) state is ground state in Cu-doped ZnS. The FM coupling strength in ZnS doping with Cu fluctuates with the variation of distance between two dopants and the fluctuation gets larger with increase in distance. Room temperature ferromagnetism can be observed in Cu-doped ZnS with high dopant concentration. Formation energy calculation implies that the clustering effect is not obvious in Cu-doped ZnS. Thus, Cu-doped ZnS can be a promising dilute magnetic semiconductor (DMS), which promises to be free of magnetic precipitates.     

Theoretical study of Mn doping effects and O or Zn vacancies on the magnetic properties in wurtzite ZnO

The effects of including the exchange interaction (J) and Hubbard on-site Coulombic interaction (U) on the structural parameters and magnetic moment of Mn-doped ZnO were explored. The calculations were performed with the plane-wave pseudopotential method along with generalized-gradient approximations (GGA). Using the GGA+U + +J method by applying Hubbard corrections U_d to the Zn 3d states and U_p to the O 2p states, the lattice constants were calculated for various reported Hubbard parameters. The difference in the lattice constants between the calculated results and experimental measurements is within 1% for pure ZnO and pure MnO. This study considers three cases: (i) substitution of Mn for Zn, (ii) substitution of Mn for Zn combined with Zn vacancy, and (iii) substitution of Mn for Zn with O vacancy. Results are shown that the system is ferromagnetic (FM) when zinc vacancies are present. For three cases with oxygen vacancies, only one of them is FM. It was also found that the Hubbard U and exchange interaction J improved the calculated results, allowing it to exhibit good agreement properties for Mn-doped ZnO with the experimental data.     

Room temperature ferromagnetism in Cu–Gd co-doped GaN nanowires: A first-principles study

The electronic structure and room temperature ferromagnetism of wurtzite Cu–Gd co-doped GaN nanowires have been investigated by means of the first-principles calculations within the density functional theory, including the on-site Coulomb energy U. The magnetic coupling between Gd atoms in the Gd-doped GaN nanowire is paramagnetic instead of ferromagnetic (FM) as in the bulk structure. After replacing Ga with Cu atom we find a stable FM coupling between Gd magnetic moments in this p-type system. p–d coupling between Cu-3d and N-2p states pushes N-2p states up to Fermi level due to the existence of hole states introduced by Cu dopants. While the p–d coupling between host N-2p and Gd-5d states near Fermi level stabilizes a FM phase of Gd magnetic moments. Furthermore, we get a FM state above room temperature by increasing the holes concentration.     

Theoretical study of magnetic ordering and electronic properties of AgxAl1−x N compounds

Ferromagnetic ordering of silver impurities in the AlN semiconductor is predicted by plane-wave ultrasoft pseudopotential and spin-polarized calculations based on density functional theory (DFT). It was found that an Ag impurity atom led to a ferromagnetic ground state in Ag_0.0625Al_0.9375N, with a net magnetic moment of 1.95 μ_B per supercell. The nitrogen neighbors at the basal plane in the AgN₄ tetrahedron are found to be the main contributors to the magnetization. This magnetic behavior is different from the ones previously reported on transition metal (TM) based dilute magnetic semiconductor (DMS), where the magnetic moment of the TM atom impurity is higher than those of the anions bonded to it. The calculated electronic structure band reveals that the Ag-doped AlN is p-type ferromagnetic semiconductor with a spin-polarized impurity band in the AlN band gap. In addition, the calculated density of states reveals that the ferromagnetic ground state originates from the strong hybridization between 4d-Ag and 2p-N states. This study shows that 4d transition metals such as silver may also be considered as candidates for ferromagnetic dopants in semiconductors.