First-principles study on the magnetism in ZnS-based diluted magnetic semiconductors

The electronic and magnetic properties of wurtzite ZnS semiconductor doped with transition metal (Cr, Mn, Fe, Co, and Ni) atoms are studied by using the first-principle’s method in this paper. The ZnS bulk materials doped with Cr, Fe, and Ni are determined to be half-metallic, while those doped with Mn and Co impurities are found to be semiconducting. These doped transition metal ions have long range interactions mediated through the induced magnetic moments in anions and cations of host semiconductors. These doped ZnS-based diluted magnetic semiconductors seem to be good candidates for the future spintronic applications.     

Hyperfine Interactions and Magnetism of 3d Transition-Metal-Impurities in II–VI and III–V Compound-Based Diluted Magnetic Semiconductors

The electronic structure of II–VI and III–V compound-based diluted magnetic semiconductors is calculated based on the local density approximation (LDA) using the Korringa–Kohn–Rostoker method combined with the coherent potential approximation. The magnetism of 3d transition-metal-atom-doped ZnO, ZnS, ZnSe, ZnTe, GaN, GaAs is investigated from first-principles. It is suggested that the double exchange mechanism stabilizes the ferromagnetism in these DMSs. In order to obtain microscopic information on the electronic structure of transition-metal-impurities in semiconductors, the hyperfine field of respective impurities in each host material is calculated. It is found that the agreement with the experimental values is not good, probably because the LDA is not sufficient to describe the core states of transition metals. However, it is suggested that the hyperfine fields clearly reflect the local magnetic moments for 3d impurities. This revised version was published online in September 2006 with corrections to the Cover Date.     

Hyperfine interactions in intermetallic compounds between Gd and 3d transition metals

Measurements of hyperfine interactions at ¹⁵⁵Gd nuclei in metallic compounds between Gd and 3d transition metals and at ⁶¹Ni in GdNi compounds by Mössbauer spectroscopy are reported. The results are discussed in terms of various models proposed for the electronic structure of these compounds. The Gd isomer shifts with respect to metallic Gd are at variance with the model of a strong d electron transfer from rare earths to transition metal ions. From the observation of a linear relation between magnetic hyperfine fields at Gd and at Dy nuclei in corresponding compounds it is inferred that crystal-field induced variations of Dy moments are neglible and that the conduction electron polarization induced by 4f moments is directly related to that caused by 3d moments.     

Ab initio exploration of d<Superscript>0</Superscript> digital magnetic heterostructures: the case of MgO and CaO <Emphasis Type="Italic">δ</Emphasis>-doped with potassium

Previous studies of digital magnetic heterostructures have mainly focused on 3d transition metal δ-doped semiconductors. In this work, sp-electron digital magnetic heterostructures without magnetic ions are proposed. Based on a theoretical density functional investigation, electronic structures and magnetic properties of MgO and CaO δ-doped with K were reported. The results show that these heterostructures are half-metallic ferromagnetic materials having semiconducting up spins and metallic down spins, with an exchange interaction much stronger than that of a random alloy with similar K concentration. Our first-principles calculations show that the carriers at the Fermi level are strongly confined within a few monolayers around the KO plane. This strong confinement is responsible for the large exchange coupling and the two-dimensional half-metallic behavior. The thickness of the host semiconducting spacer does not significantly change the global electric and magnetic features.     

Transition metal encapsulated hydrogenated silicon nanotubes: Silicon-based half-metal

One-dimensional hydrogenated silicon nanotubes (H-SiNTs) with transition metal atom encapsulated were systematically studied by using density functional theory. The band structures and magnetic properties of the H-SiNTs can be tailored by doping transition metal (TM) (TM = Cr, Mn, Fe, Co) atoms within the tube. The hydrogenated silicon nanotubes are semiconductors with wide band gaps. TM doping turns H-SiNTs to be metals or semiconductors with a very small gap, and TM atoms at the center of the tubes keep large magnetic moments. Robust half-metallicity is observed in Mn-doped H-SiNTs and it is free from Peierls distortion. Thus, H-SiNTs with encapsulated magnetic elements may find important applications in spintronic devices.     

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.     

First-principles study of electronic structure and magnetic properties of SrTi1−xMxO3 (M= Cr,Mn, Fe,Co, or Ni)

We used first-principles calculations to conduct a comparative study of the structure and the electronic and magnetic properties of SrTiO₃ doped with a transition metal (TM), namely, Cr, Mn, Fe, Co, or Ni. The calculated formation energies indicate that compared with Sr, Ti can be substituted more easily by the TM ions. The band structures show that SrTi_0.875Cr_0.125O₃ and SrTi_0.875Co_0.125O₃ are half metals, SrTi_0.875Fe_0.125O₃ is a metal, and SrTi_0.875Mn_0.125O₃ is a semiconductor. The 3d TM-doped SrTiO₃ exhibits various magnetic properties, ranging from ferromagnetism (Cr-, Fe-, and Co-doped SrTiO₃) to antiferromagnetism (Mn-doped SrTiO₃) and nonmagnetism (Ni-doped SrTiO₃). The total magnetic moments are 4.0_μB, 6.23_μB, and 2.0_μB for SrTi_0.75Cr_0.25O₃, SrTi_0.75Fe_0.25O₃, and SrTi_0.75Co_0.25O₃, respectively. Room-temperature ferromagnetism can be expected in Cr-, Fe-, and Co-doped SrTiO₃, which agrees with the experimental observations. The electronic structure calculations show that the spin polarizations of the 3d states of the TM atoms are responsible for the ferromagnetism in these compounds. The magnetism of TM-doped SrTiO₃ is explained by the hybridization between the TM-3d states and the O-2p states.     

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.     

Phenomenological band structure model of magnetic coupling in semiconductors

A unified band structure model is proposed to explain the magnetic ordering in Mn-doped semiconductors. This model is based on the p-d and d-d level repulsions between the Mn ions and host elements and can successfully explain magnetic ordering observed in all Mn doped II-VI and III-V semiconductors such as CdTe, GaAs, ZnO, and GaN. The model can also be used to explain the interesting behavior of GaMnN, which changes from ferromagnetic ordering to antiferromagnetic ordering as the Mn concentration increases. This model, therefore, is useful to provide a simple guideline for future band structure engineering of magnetic semiconductors.     

Features of the semiconductor-metal transition in GaAs at ultrahigh pressures: New intermediate phases

Using the thermopower method (Seebeck effect), the semiconductor-metal transition that occurs in gallium arsenide single crystals of n and p types at ultrahigh pressures P above ～11–18 GPa has been studied. It has been found that the transition in n-type samples begins at lower pressures. In the region of the semiconductor-metal phase transition, features have been observed on the thermopower dependences S(P). These features indicate that lattices intermediate between the initial semiconductor structure of zinc blende and the Cmcm high-pressure orthorhombic metallic phase are formed. By analogy with ZnTe, one intermediate phase (semiconductor with hole conductivity) is suggested to have the cinnabar structure and the second intermediate phase (semimetallic with electron conductivity) possibly has the SC16 structure. A model of the semiconductor-metal transition is discussed. The behavior of the thermoelectric properties in GaAs under pressure is compared with the behavior of these properties in other A_NB_8?N semiconductors, which also undergo the transition to the metallic state.     

Specific features of the phonon thermal conductivity of II–VI compound semiconductors doped with 3<Emphasis Type="Italic">d</Emphasis> transition metal ions

The phonon thermal conductivity of II–VI compound semiconductors doped with 3d transition metal ions is studied. The observed temperature anomalies are analyzed in terms of the dynamic Jahn-Teller effect.     

Magnetic metastability in tetrahedrally bonded magnetic III-nitride semiconductors

Results of density-functional calculations for isolated transition metal (TM = V, Cr, Mn, Fe, Co, Ni on cation sites) doped GaN demonstrate a novel magnetic metastability in dilute magnetic semiconductors. In addition to the expected high spin ground states (4muB/Mn and 5muB/Fe), there are also metastable low spin states (0muB/Mn and 1muB/Fe)--a phenomenon that can be explained in simple terms on the basis of the ligand field theory. The transition between the high spin and low spin states corresponds to an intraionic transfer of two electrons between the t2 and e orbitals, accompanied by a spin-flip process. The results suggest that TM-doped wideband semiconductors (such as GaN and AlN) may present a new type of light-induced spin-crossover material.     

Properties of InGaAs/GaAs quantum wells with a δ〈Mn〉-doped layer in GaAs

A method of formation of two-dimensional structures containing a δ〈Mn〉-doped layer in GaAs and an In_xGa_1?x As quantum well (QW) separated by a GaAs spacer of thickness d = 4–6 nm is developed using laser evaporation of a metallic target during MOS hydride epitaxy. It is shown that, up to room temperature, these structures have ferromagnetic properties most likely caused by MnAs clusters. At low temperatures (T _m ～ 30 K), the anomalous Hall effect is revealed to occur. This effect is related to hole scattering by Mn ions in GaAs and to the magnetic exchange between these ions and QW holes, which determines the spin polarization of the holes. The behavior of the negative magnetoresistance of these structures at low temperatures indicates the key role of quantum interference effects.     

Study of magnetic and optical properties of Zn_(1-x)TM_xTe(TM = Mn,Fe,Co,Ni) diluted magnetic semiconductors:First principle approach

We present structural,magnetic and optical characteristics of Zn_(1-x)TM_xTe(TM = Mn,Fe,Co,Ni and x = 6.25%),calculated through Wien2 k code,by using full potential linearized augmented plane wave(FP-LAPW) technique.The optimization of the crystal structures have been done to compare the ferromagnetic(FM) and antiferromagnetic(AFM) ground state energies,to elucidate the ferromagnetic phase stability,which further has been verified through the formation and cohesive energies.Moreover,the estimated Curie temperatures T_c have demonstrated above room temperature ferromagnetism(RTFM) in Zn_(1-x)TM_xTe(TM =Mn,Fe,Co,Ni and x= 6.25%).The calculated electronic properties have depicted that Mn- and Co-doped ZnTe behave as ferromagnetic semiconductors,while half-metallic ferromagnetic behaviors are observed in Fe- and Ni-doped ZnTe.The presence of ferromagnetism is also demonstrated to be due to both the p-d and s-d hybridizations between the host lattice cations and TM impurities.The calculated band gaps and static real dielectric constants have been observed to vary according to Penn's model.The evaluated band gaps lie in near visible and ultraviolet regions,which make these materials suitable for various important device applications in optoelectronic and spintronic.     

Structural,electronic,and magnetic behaviors of 5d transition metal atom substituted divacancy graphene:A first-principles study

Structural, electronic, and magnetic behaviors of 5d transition metal(TM) atom substituted divacancy(DV) graphene are investigated using first-principles calculations. Different 5d TM atoms(Hf, Ta, W, Re, Os, Ir, and Pt) are embedded in graphene, these impurity atoms replace 2 carbon atoms in the graphene sheet. It is revealed that the charge transfer occurs from 5d TM atoms to the graphene layer. Hf, Ta, and W substituted graphene structures exhibit a finite band gap at high symmetric K-point in their spin up and spin down channels with 0.783 μB, 1.65 μB, and 1.78 μB magnetic moments,respectively. Ir and Pt substituted graphene structures display indirect band gap semiconductor behavior. Interestingly, Os substituted graphene shows direct band gap semiconductor behavior having a band gap of approximately 0.4 e V in their spin up channel with 1.5 μB magnetic moment. Through density of states(DOS) analysis, we can predict that d orbitals of 5d TM atoms could be responsible for introducing ferromagnetism in the graphene layer. We believe that our obtained results provide a new route for potential applications of dilute magnetic semiconductors and half-metals in spintronic devices by employing 5d transition metal atom-doped graphene complexes.     

Effects of enhanced electronic correlation on magnetic properties of light non-metallic element (B,C, N,and O)-doped CuCl: A first-principles study

Based on density functional calculations within both standard generalized gradient approximation and plus on-site Coulomb interactions approaches, we have investigated the electronic structure and magnetic properties of the first-row element-doped CuCl semiconductors. The electronic correlations in both 2p and 3d orbitals are enhanced by adding the on-site Coulomb repulsion (Hubbard U and Hund exchange J). After a comparative study, we find that, for both standard and beyond approaches, B-doped CuCl is a half-metallic magnet with majority-spin impurity bands touching the Fermi level, C-doped CuCl is a magnetic semiconductor, and N-doped CuCl is a half-metallic magnet with minority-spin impurity bands crossing the Fermi level. Nevertheless, for O-doped CuCl, it transforms from a nonmagnetic semiconductor to a half-metallic magnet with metallic up-spins by considering the correlation effects. The calculation shows that the enhanced electronic correlation not only corrects the error of band-gap, but also influences the magnetic ground state and the distribution of local magnetic moments. The location of impurity bands with different dopants was understood based on the elements' electronegativity and interaction between dopant and host atoms. Strong hybridization between the dopant's 2p states and the filled 3d orbitals of adjacent Cu yields the main contribution to magnetization.     

Manifestation of magnetically induced spatial dispersion in the cubic semiconductors ZnTe, CdTe, and GaAs

Nonreciprocal birefringence due to magnetically induced spatial dispersion was observed in the T _d-class cubic semiconductors ZnTe, CdTe, and GaAs near the fundamental absorption edge. The dispersion of the parameters A and g, describing the contributions from terms of the type B _ik_j to the diagonal and off-diagonal components of the permittivity tensor ε _ij(ω,B,k), is determined for ZnTe and CdTe. Analysis of the dispersion and anisotropy of the nonreciprocal birefringence shows that in ZnTe, CdTe, and GaAs, in contrast to magnetic semiconductors of the type Cd_1−x Mn_xTe, it is due excitonic mechanisms. Pis’ma Zh. éksp. Teor. Fiz. 69, No. 7, 514–519 (10 April 1999)     

Spin-polarized calculations of electronic structures in ferromagnetic and antiferromagnetic Zn0.75TM0.25Se (TM=Cr,Fe, Co and Ni)

In this work, we aim to examine the spin-polarized electronic band structures, the local densities of states as well as the magnetism of Zn_1−xTM_xSe (TM=Cr, Fe, Co and Ni) diluted magnetic semiconductors in the ferromagnetic (FM) and antiferromagnetic (AFM) phases, and with 25% of TM. The calculations are performed by the developed full-potential augmented plane wave plus local orbitals method within the spin density functional theory. As exchange-correlation potential we used the generalized gradient approximation (GGA) form. We treated the ferromagnetic and antiferromagnetic phases and we found that all compounds are stable in the ferromagnetic structure. Structural properties are computed after total energy minimization. Our results show that the cohesive energies of Zn_0.75TM_0.25Se are greater than that of zinc blende ZnSe. We discuss the electronic structures, total and partial densities of states, local moments and the p–d exchange splitting. Furthermore, we found that p–d hybridization reduces the local magnetic moment of TM and produces small local magnetic moments on the nonmagnetic Zn and Se sites. We found also that in the AFM phase the TM local magnetic moments are smaller than in the FM phase; this is due to the greater interaction of the TM d-up and d-down orbitals.     

Half-metallicity in Rh-doped TiO<Subscript>2</Subscript> from ab initio calculations

First-principles calculations based on density functional theory (DFT) are performed to study the electronic structures and magnetic properties of Rh-doped TiO₂ crystals. The hybridization between Rh-4d and O-2p results in Rh becoming ferromagnetic with a magnetic moment of about 1.0 μ _B per supercell. The Rh-doped TiO₂ system exhibits half-metallic ferromagnetism based both DFT and DFT + U. The strong ferromagnetic couplings between local magnetic moments can be attributed to both the p-d hybridization and double-exchange mechanisms, as well as superexchange interaction. These results suggest an alternative approach to achieve promising dilute magnetic semiconductors by doping non-magnetic transition metals in a TiO₂ host.     

The Electronic and Magnetic Properties of FCC Iron Clusters in FCC 4D Metals

The electronic and magnetic structures of small FCC iron clusters in FCC Rh, Pd and Ag were calculated using the discrete variational method as a function of cluster size and lattice relaxation. It was found that unrelaxed iron clusters, remain ferromagnetic as the cluster sizes increase, while for relaxed clusters antiferromagnetism develops as the size increases depending on the host metal. For iron in Rh the magnetic structure changes from ferromagnetic to antiferromagnetic for clusters as small as 13 Fe atoms, whereas for Fe in Ag antiferromagnetism is exhibited for clusters of 24 Fe atoms. On the hand, for Fe in Pd the transition from ferromagnetism to antiferromagnetism occurs for clusters as large as 42 Fe atoms. The difference in the magnetic trends of these Fe clusters is related to the electronic properties of the underlying metallic matrix. The local d densities of states, the magnetic moments and hyperfine parameters are calculated in the ferromagnetic and the antiferromagnetic regions. In addition, the average local moment in iron-palladium alloys is calculated and compared to experimental results.