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Coexistence of intrinsic ferromagnetism and piezoelectricity, namely piezoelectric ferromagnetism (PFM), is crucial to advance multifunctional spintronic technologies. In this work, we demonstrate that Janus monolayer YBrI is a PFM, which is dynamically, mechanically and thermally stable. Electronic correlation effects on physical properties of YBrI are investigated by using generalized gradient approximation plus $U$ (GGA+$U$) approach. For out-of-plane magnetic anisotropy, YBrI is a ferrovalley (FV) material, and the valley splitting is larger than 82 meV in considered $U$ range. The anomalous valley Hall effect (AVHE) can be achieved under an in-plane electric field. However, for in-plane magnetic anisotropy, YBrI is a common ferromagnetic (FM) semiconductor. When considering intrinsic magnetic anisotropy, the easy axis of YBrI is always in-plane with magnetic anisotropy energy (MAE) from 0.309 meV to 0.237 meV ($U$=0.0 eV to 3.0 eV). However, the magnetization can be adjusted from the in-plane to off-plane direction by external magnetic field, and then lead to the occurrence of valley polarization. Moreover, missing centrosymmetry along with mirror symmetry breaking results in both in-plane and out-of-plane piezoelectricity in YBrI monolayer. At a typical $U$=2.0 eV, the $d_{11}$ is predicted to be -5.61 pm/V, which is higher than or compared with ones of other two-dimensional (2D) known materials. The electronic and piezoelectric properties of YBrI can be effectively tuned by applying a biaxial strain. For example, tensile strain can enhance valley splitting and $d_{11}$ (absolute value). The predicted Curie temperature of YBrI is higher than those of experimentally synthesized 2D ferromagnetic materials $\mathrm{CrI_3}$ and $\mathrm{Cr_2Ge_2Te_6}$., 10 pages, 12 figures

The combination of piezoelectricity with nontrivial topological insulating phase in two-dimensional (2D) systems, namely piezoelectric quantum spin Hall insulator (PQSHI), is intriguing for exploring novel topological states toward the development of high-speed and dissipationless electronic devices. In this work, we predict a PQSHI Janus monolayer VCClBr constructed from $\mathrm{VCCl_2}$, which is dynamically, mechanically and thermally stable. In the absence of spin orbital coupling (SOC), VCClBr is a narrow gap semiconductor with gap value of 57 meV, which is different from Dirac semimetal $\mathrm{VCCl_2}$. The gap of VCClBr is due to built-in electric field caused by asymmetrical upper and lower atomic layers, which is further confirmed by external-electric-field induced gap in $\mathrm{VCCl_2}$. When including SOC, the gap of VCClBr is improved to 76 meV, which is larger than the thermal energy of room temperature (25 meV). The VCClBr is a 2D topological insulator (TI), which is confirmed by $Z_2$ topological invariant and nontrivial one-dimensional edge states. It is proved that the nontrivial topological properties of VCClBr are robust against strain (biaxial and uniaxial cases) and external electric field. Due to broken horizontal mirror symmetry, only out-of-plane piezoelectric response can be observed, when biaxial or uniaxial in-plane strain is applied. The predicted piezoelectric strain coefficients $d_{31}$ and $d_{32}$ are -0.425 pm/V and -0.219 pm/V, which are higher than or compared with ones of many 2D materials. Finally, another two Janus monolayer VCFBr and VCFCl (dynamically unstable) are constructed, and they are still PQSHIs. Moreover, their $d_{31}$ and $d_{32}$ are higher than ones of VCClBr, and the $d_{31}$ (absolute value) of VCFBr is larger than one., 10 pages, 12 figures

Heterostructures composed of dissimilar oxides with different properties offer opportunities to develop emergent devices with desired functionalities. A key feature of oxide heterostructures is interface electronics and orbital reconstructions. Here, we combined infinite-layered SrCuO2 and perovskite SrRuO3 into heterostructures. A rare high spin state as large as 3.0 μB f.u-1 and an increase in Curie temperature by 12 K are achieved in an ultrathin SrRuO3 film capped by a SrCuO2 layer. Atomic-scale lattice imaging shows the uniform CuO2-plane-to-RuO5-pyramid connection at the interface, where the regularly arranged RuO5 pyramids were elongated along the out-of-plane direction. As revealed by theoretical calculations and spectral analysis, these features finally result in an abnormally high spin state of the interfacial Ru ions with highly polarized eg orbitals. The present work demonstrates that oxygen coordination engineering at the infinite-layer/perovskite oxide interface is a promising approach towards advanced oxide electronics.

Structural, electronic, ferroelectric, and optical properties of two-dimensional (2D) BiN monolayer materials with phosphorene-like structure are studied in terms of the density functional theory a...

Two-dimensional electron gas due to semiconductor interfaces can have high mobility and exhibits superconductivity, magnetism, and other exotic properties that are unexpected in constituent bulk materials. We study crystal structures, electronic states, and magnetism of short-period (BTO)m/(GTO)2 (m=2 and 4) superlattices consisting of insulating BaTiO3 (BTO) and GdTiO3 (GTO) by first principles calculations. Our investigation shows that the middle Ti-O monolayer in the GTO layer becomes metallic. Although both GTO and BTO share a common constituent, the Ti-O atomic layer, the M-O (M=Ba, Ti, and Gd) displacements along the c axis in each monolayer reflect the asymmetry of two interfacial Ti-O monolayers and the large distortion of the TiO6 octahedra in such superlattices, against electronic reconstruction at the interfaces and thus differentially changing the d energy levels of the three Ti-O monolayers related with the GTO layer. Such superlattices are interesting for potential spintronics applications because of their unique asymmetrical two-dimensional electron-gas properties and possible useful spin-orbit effects.

The manipulation of Rashba effects in two-dimensional (2D) electron systems in semiconductors is highly desirable for controllable electronic and optical applications. Here, combining a first-principles investigation and model analysis, we use uniaxial stress to control monolayer BiTeI as a Rashba 2D semiconductor. We find that the stress-driven electron system can be described by an effective anisotropic Rashba model including all three Pauli matrices, and uniaxial stress allows an out-of-plane spin component because of rotational symmetry breaking. When appropriate electron carriers are introduced into the monolayer, an in-plane electric field can induce a charge current and three spin current components (including that based on the out-of-plane spin) because of the reduced symmetry. Therefore, uniaxial stress can be used to control Rashba 2D electron systems such as monolayer BiTeI for promising spintronic devices.

The structural, magnetic, and electronic properties of (STO)$_4$/(YTO)$_2$ superlattice consisting of Mott insulator YTiO$_3$ (YTO) and band insulator SrTiO$_3$ (STO) under strain are investigated by the density-functional-theory plus \emph{U} method. It is found that an insulator-metal transition occurs when a compressive strain of 0.2\% is applied. The structural analyses reveal that the presence of metallic state in such superlattices accompanies structural phase transition with restoring of inversion symmetry. Further study shows that this strain-induced structural transition makes the $d$ energy level of the interfacial Ti atoms of the YTO layer move upward due to the decreasing of the TiO$_{6}$ octahedral volume and induces the electron reconstruction in the whole superlattice systems. In addition, when the on-site interaction $U$ is changed from 5 to 4 eV, a similar insulator-metal transition also occurs in such superlattices due to the weakened electron correlation. These findings can improve our understanding of the insulator-metal transitions in such oxide superlattices.

The electronic, magnetic, and topological properties of CoBr2 monolayer are studied in the frame-work of the density-functional theory (DFT) combined with tight-binding (TB) modeling in terms of Wannier basis. Our DFT investigation and Monte Carlo simulation show that there exists intrinsic two-dimensional ferromagnetism in the CoBr2 monolayer thanks to large out-of-plane magnetocrystalline anisotropic energy. Our further study shows that the spin-orbits coupling makes it become a topologically nontrivial insulator with quantum anomalous Hall effect and topological Chern number C=4, and its edge states can be manipulated by changing the width of its nanoribbons and applying strains. The CoBr2 monolayer can be exfoliated from the layered CoBr2 bulk material because its exfoliation energy is between those of graphene and MoS2 monolayer and it is dynamically stable. These results make us believe that the CoBr2 monolayer can make a promising spintronic material for future high-performance devices.

Recent temperature-dependent x-ray diffraction and Raman spectroscopy experiment proved that single-crystalline BiMnO 3 assumes a centrosymmetric monoclinic structure ( C 2/ c space group). Here we investigate magnetic structure and electronic structure of this centrosymmetric BiMnO 3 phase by using the modified Becke-Johnson (mBJ) exchange functional within the density functional theory (DFT). Our mBJ calculated semiconductor gap, magnetic moment, and other aspects of the electronic structure, in contrast with previous DFT results, are in good agreement with recent experimental values. This satisfactory description of the electronic structure and magnetism of the BiMnO 3 is because mBJ reasonably captures the kinetic property and correlation of electrons. Our calculated results with mBJ approach are both useful to study such Bi-based perovskite oxide materials for spintronics applications.

We use first-principles methods to study oxygen-octahedra rotations in ABO3 perovskite oxides. We focus on the short-period, perfectly antiphase or in-phase, tilt patterns that characterize most compounds and control their physical (e.g., conductive, magnetic) properties. Based on an analytical form of the relevant potential energy surface, we discuss the conditions for the stability of polymorphs presenting different tilt patterns, and obtain numerical results for a collection of thirty-five representative materials. Our results reveal the mechanisms responsible for the frequent occurrence of a particular structure that combines antiphase and in-phase rotations, i.e., the orthorhombic Pbnm phase displayed by about half of all perovskite oxides and by many non-oxidic perovskites. The Pbnm phase benefits from the simultaneous occurrence of antiphase and in-phase tilt patterns that compete with each other, but not as strongly as to be mutually exclusive. We also find that secondary antipolar modes, involving the A cations, contribute to weaken the competition between different tilts and play a key role in their coexistence. Our results thus confirm and better explain previous observations for particular compounds. Interestingly, we also find that strain effects, which are known to be a major factor governing phase competition in related (e.g., ferroelectric) perovskite oxides, play no essential role as regards the relative stability of different rotational polymorphs. Further, we discuss why the Pbnm structure stops being the ground state in two opposite limits, for large and small A cations, showing that very different effects become relevant in each case. Our work thus provides a comprehensive discussion on these all-important and abundant materials, which will be useful to better understand existing compounds as well as to identify new strategies for materials engineering.

Manganese oxides are good candidates of strongly correlated electron materials due to the uniqueness of electronic structure of manganese and the mobility of oxygen among lattice sites under external impacts. Here, we used electron beam as the excitation source to explore the structural evolution of YMnO3 and identified a new phase under the radiation of electron beam in the transmission electron microscope. Analyses of the electron energy-loss spectra reveal that this phase originates from ordered oxygen vacancy. We applied the first principles calculation to pick out the optimized stable structure with a lower polarization, and verified its correctness by electron diffraction and image simulations. Analyses of density of states indicate that weak Y–O covalence is favorable for the existence of ferroelectricity, supporting the electrostatic nature of ferroelectricity in the YMnO3.

It is highly desirable to combine the full spin polarization of carriers with modern semiconductor technology for spintronic applications. For this purpose, one needs good crystalline ferromagnetic (or ferrimagnetic) semiconductors with high Curie temperatures. Rutile CrO$_2$ is a half-metallic spintronic material with Curie temperature 394 K and can have nearly-full spin polarization at room temperature. Here, we find through first-principles investigation that when a biaxial compressive stress is applied on rutile CrO$_2$, the density of states at the Fermi level decreases with the in-plane compressive strain, there is a structural phase transition to an orthorhombic phase at the strain of -5.6\%, and then appears an electronic phase transition to a semiconductor phase at -6.1\%. Further analysis shows that this structural transition, accompanying the tetragonal symmetry breaking, is induced by the stress-driven distortion and rotation of the oxygen octahedron of Cr, and the half-metal-semiconductor transition originates from the enhancement of the crystal field splitting due to the structural change. Importantly, our systematic total-energy comparison indicates the ferromagnetic Curie temperature remains almost independent of the strain, near 400 K. This biaxial stress can be realized by applying biaxial pressure or growing the CrO$_2$ epitaxially on appropriate substrates. These results should be useful for realizing full (100\%) spin polarization of controllable carriers as one uses in modern semiconductor technology., Comment: 7 pages, 7 figures

We investigate the electronic structures of the cubic and tetragonal phases of Sr 2 MgWO 6 . Calculated electronic structures show that the two phases are both wide indirect band gap semiconductors. Our phonon calculations indicate that the structural phase transition from cubic to tetragonal phase is because the phonon frequencies of Γ 4 + modes become imaginary. Our variable cell-shape relaxation results imply that a symmetry-allowed intermediate trigonal phase could be realized.

It is important to spintronics to achieve fully-spin-polarized magnetic materials that are stable and can be easily fabricated. Here, through systematical density-functional-theory investigations, we achieve high and even full spin polarization for carriers in the ground-state phase of CrTe by applying tensile biaxial stress. The resulting strain is tensile in the xy plane and compressive in the z axis. With the in-plane tensile strain increasing, the ferromagnetic order is stable against antiferromagnetic fluctuations, and a half-metallic ferromagnetism is achieved at an in-plane strain of 4.8%. With the spin-orbit coupling taken into account, the spin polarization is equivalent to 97% at the electronic phase transition point, and then becomes 100.0% at the in-plane strain of 6.0%. These make us believe that the full-spin-polarized ferromagnetism in this stable and easily-realizable hexagonal phase could be realized soon, and applied in spintronics., Comment: 5 pages, 5 figures

We report a systematic first-principles study of structural properties, electronic structures and magnetic properties of Cr doped ZnTe with the Cr concentration up to 0.5. We fully optimize all the structures concerned, both ferromagnetic and antiferromagnetic, and their internal atomic positions. Our calculations show that they are half-metallic ferromagnets with half-metallic gaps over 0.6 eV. Our calculated results are consistent with the fact that only those bulk samples with very small doping concentrations can be stable thanks to the entropy effect. On the other hand, up to 20% Cr doping concentrations have been achieved experimentally in thin films fabricated with nonequilibrium methods. Our calculated results indicate that half-metallic ferromagnetism can be realized in high-quality thin film samples of CrxZn1−xTe with nonequilibrium methods. We explain the half-metallic ferromagnetism in terms of a pd hybridization and resulting large exchange splitting of d-dominated bands. Compatible with appropriate semiconductors, these materials, at least some of them, could be useful in spintronics.

The Kinetic Monte Carlo (KMC) method based on the transition-state theory, powerful and famous for simulating atomic epitaxial growth of thin films and nanostructures, was used recently to simulate the nanoferromagnetism and magnetization dynamics of nanomagnets with giant magnetic anisotropy. We present a brief introduction to the KMC method and show how to reformulate it for nanoscale spin systems. Large enough magnetic anisotropy, observed experimentally and shown theoretically in terms of first-principle calculation, is not only essential to stabilize spin orientation but also necessary in making the transition-state barriers during spin reversals for spin KMC simulation. We show two applications of the spin KMC method to monatomic spin chains and spin-polarized-current controlled composite nanomagnets with giant magnetic anisotropy. This spin KMC method can be applied to other anisotropic nanomagnets and composite nanomagnets as long as their magnetic anisotropy energies are large enough.

We report systematical density-functional calculations of ternary transition-metal compounds based on zincblende ZnTe and CdTe semiconductor. Some of them are half-metallic at their optimized cell volumes. The effect of atomic position optimization (and cell volume re-optimization) on the electronic structures is to widen the energy bands near the Fermi energy and to reduce the density of states there. As a result, the Fermi level moves upward and the energy gap of the minority-spin bands at the Fermi energy becomes narrower. Therefore, the half-metallic gaps are reduced, two of them even being closed. These compounds are compatible with the II–VI semiconductors, and could be useful in spin-based electronics.

The spin-s quantum Heisenberg ferromagnetic model is investigated in the physical magnon theory. The effect of the extra unphysical magnon states on every site is completely removed in the magnon Hamiltonian and during approximation procedure so that the condition = 0(n greater than or equal to 2s + 1) is rigorously satisfied. The physical multi-approximation occupancy (1 less than or equal to n less than or equal to 2s) is proportional to T-3n/2 at low temperature and is equivalent to 1/(2s + 1) at the Curie temperature. The magnetization not only unified but also well-behaved from zero temperature to Curie temperature is obtained in the framework of the magnon theory for the spin-s quantum Heisenberg ferromagnetic model. The ill-behaved magnetizations at high temperature in earlier magnon theories are completely corrected. The relation of magnon (spin wave) theory with spin-operator decoupling theory is clearly understood. (C) 2001 Elsevier Science B.V. All rights reserved.

We explore the effects of various basic atomic processes on the island shapes of diffusion-limited-aggregate growth on a triangular lattice using kinetic Monte Carlo simulation. Edge diffusion and bond-reducing relaxation of double-coordinated adatoms, as well as single-coordinated adatoms, are proved to be essential to the island shape transition from fractal to irregularly compact and from irregularly compact to hexagonal islands, respectively. Shape transitions of adatom islands on fcc (111) substrates can be understood naturally by this model.

Electronic structures and magnetic properties of various possible ternary transition-metal compounds based on zincblende ZnTe and CdTe semiconductors are systematically explored by using an accurate density-functional method. Half-metallic (HM) ferromagnetism is found in the ZnTe and CdTe heavily doped with Cr, V, and Mn. Quite high Curie temperatures would be expected in most of these HM ferromagnets. These HM ferromagnetic phases, structurally stable and compatible with the two semiconductors, would be useful in spintronics.

We investigate the electronic structures of the ferromagnetic semiconductor HgCr(2)Se(4) by using a modified Becke and Johnson exchange potential. The energy gap calculated with the experimental lattice structure is in good agreement with the experimental value. When the pressure reaches 9.0 GPa, HgCr(2)Se(4) becomes a so-called spin gapless semiconductor due to the closure of the spin non-conservation energy gap, and when the pressure increases to 11.4 GPa, the spin conservation energy gap closes, leading to a semiconductor-semimetal transition.

We explore how much the RKKY spin interaction can contribute to the high-temperature ferromagnetism in cubic (Ga, Mn)N diluted magnetic semiconductors. The usual coupling constant is used and effective carriers are considered independent of doped magnetic atoms, as is shown experimentally. Our Monte Carlo simulated results show that maximal Curie temperature is reached at the optimal carrier concentration for a given Mn concentration, equaling 373 K for 5% Mn and 703 K for 8% Mn. Because such a Monte Carlo method does not overestimate transition temperatures, these calculations indicate that the RKKY spin interaction alone can yield high-enough Curie temperatures in cubic (Ga, Mn)N under optimized conditions.

An accurate full-potential linear augmented plane waves method has been used to investigate the structural stability, half-metallic ferromagnetism of the zinc-blende phase of the ferromagnetic semiconductor alloy Al1-xCrxAs. The relative stability of the zinc-blende phase with respect to the NiAs one and the ferromagnetism to the antiferromagnetism are confirmed by comparing total energy of various structures. Full optimization for all these structures is performed, including the cell volume, c/a for tetrahedral cells and internal parameters. Our calculation shows that when the Cr composition is less than 30%, the zinc-blende phase is more stable, whereas the NiAs one is more stable for more Cr composition. Therefore, bulk Al1-xCrxAs with zinc-blende structure can be obtained when x is smaller than 0.3, whereas only epitaxial films can exist for larger x. For all these compounds with x ranging from 0.125 to 1.0, robust half-metallic ferromagnetism is found in the zinc-blende phase. In addition, the effects of the spin-orbital interaction are considered and the total magnetic moments including both the spin and orbital contributions are presented. The spin-orbital interaction will destroy the perfect half-metallic ferromagnetism by reflecting some majority-spin states into the minority-spin gap, however, the decrease of the spin-polarization is only about 0.2% for all substitutional cases. At last, we give the exchange parameters and consecutively the Curie temperature within the mean-field approximation. Our systematical calculation for Al1-xCrxAs is useful to elucidate the half-metallic ferromagnetism in Al1-xCrxAs and to explore high-performance spintronic devices based on Al1-xCrxAs in future information technologies.

Using a nonequilibrium Monte Carlo method suitable to nanomagnetism, we investigate representative systems of interacting sub-10nm grained nanomagnets with large uniaxial anisotropy. Various magnetization memory and aging effects are found in such systems. We explain these dynamical effects using the distributed relaxation times of the interacting nanomagnets due to their large anisotropy energies., Physics Letters A, in press

High Curie temperature of 900 K has been reported in Cr-doped AlN diluted magnetic semiconductors prepared by various methods, which is exciting for spintronic applications. It is believed that N defects play important roles in achieving the high-temperature ferromagnetism in good samples. Motivated by these experimental advances, we use a full-potential density-functional-theory method and supercell approach to investigate N defects and their effects on ferromagnetism of (Al,Cr)N with N vacancies (V-N). We investigate the structural and electronic properties of V-N, single Cr atom, Cr-Cr atom pairs, Cr-V-N pairs, and so on. In each case, the most stable structure is obtained by comparing different atomic configurations optimized in terms of the total energy and the force on every atom, and then it is used to calculate the defect formation energy and study the electronic structures. Our total-energy calculations show that the nearest substitutional Cr-Cr pair with the two spins in parallel is the most favorable and the nearest Cr-V-N pair makes a stable complex. Our formation energies indicate that V-N regions can be formed spontaneously under N-poor condition because the minimal V-N formation energy equals -0.23 eV or Cr-doped regions with high enough concentrations can be formed under N-rich condition because the Cr formation energy equals 0.04 eV, and hence real Cr-doped AlN samples are formed by forming some Cr-doped regions and separated V-N regions and through subsequent atomic relaxation during annealing. Both of the single Cr atom and the N vacancy create filled electronic states in the semiconductor gap of AlN. N vacancies enhance the ferromagnetism by adding mu(B) to the Cr moment each but reduce the ferromagnetic exchange constants between the spins in the nearest Cr-Cr pairs. These calculated results are in agreement with experimental observations and facts of real Cr-doped AlN samples and their synthesis. Our first-principles results are useful to elucidate the mechanism for the ferromagnetism and to explore high-performance Cr-doped AlN diluted magnetic semiconductors.

The island growths during the epitaxy of differently polar ZnO thin films under O-rich condition are simulated in terms of a phase-field model. The different island nucleation modes as well as the different atom migration and influx sticking ability are considered on the Zn-polar surfaces and the O-polar ones. By reproducing the morphological variation from the smooth two-dimensional island growth on the Zn-polar surfaces to the rough three-dimensional islands on the O-polar surfaces, our simulated results indicate that the surface morphology of the ZnO thin films is dependent not only on the migration ability and influx rate but also on the island nucleation kinetics. For the Zn-polarity, the easy island nucleation contributes to the smooth morphology as a result of the limited migration and fast influx. However, the reduced nucleation kinetics on the O-polar surfaces causes the developed three-dimensional island growth. This phase-field modeling should be applicable to other semiconductor epitaxial growths.

Motivated by recent reports on high Curie temperatures in Cr- and Mn-doped aluminum nitrides and the fabrication of heavily transition-metal-doped ones with room-temperature ferromagnetism, we study systematically 12.5% transition-metal substituted aluminium nitrides, hexagonal CrAl7N8 and MnAl7N8, using a full-potential density-functional method. We optimize fully the crystal structures and prove the stability of their ferromagnetism against possible antiferromagnetic orders. Their volumes are expanded only by about 1% compared with that of AlN. Every d electron of +3 cations of Cr and Mn contributes one Bohr magneton to the moment. Our calculations show that both of them are half-metallic ferromagnets with half-metallic gaps larger than 1 eV. The transition-metal substitution creates 10 transition-metal d-dominated impurity bands in the semiconductor gap of AlN. The hybridization of the d states with the N p ones yields a large spin exchange splitting of the d states which drives the ferromagnetism. The half-metallicity is attributed to the wide minority-spin gap across the Fermi level and the favorable majority-spin density of states of the d-dominated bands in addition to the above factors. 25% Cr and Mn substituted cubic aluminum nitrides, cubic CrAl3N4 and MnAl3N4, are studied in the same way and proved to share almost all the properties of the two hexagonal ones. The mechanism for the ferromagnetism should be useful for understanding nitride-based diluted magnetic semiconductors. These half-metallic transition-metal aluminum nitrides, at least some of them, could be useful in spintronics.

It is highly desirable to explore robust half-metallic ferromagnetic materials compatible with important semiconductors for spintronic applications. A state-of-the-art full potential augmented plane wave method within thedensity-functional theory is reliable enough for this purpose. In this chapter we review theoretical research on half-metallic ferromagnetism and structural stability of transition metal pnictides and chalcogenides. We show that some zincblende transition metal pnictides are half-metallic and the half-metallic gap can be fairly wide, which is consistent with experiment. Systematic calculations reveal that zincblende phases of CrTe, CrSe, and VTe are excellent half-metallic ferromagnets. These three materials have wide half-metallic gaps, are low in total energy with respect to the corresponding ground-state phases, and, importantly, are structurally stable. Half-metallic ferromagnetism is also found in wurtzite transition metal pnictides and chalcogenides and in transition-metal doped semiconductors as well as deformed structures. Some of these half-metallic materials could be grown epitaxially in the form of ultrathin films or layers suitable for real spintronic applications.

A generalized tight-binding bond Stoner model satisfying charge self-consistence is used to study Mn-doped Ge. We fit the tight-binding parameters to first-principles nonmagnetic band structures of zinc blende MnGe, and then calculate the ferromagnetic and antiferromagnetic band structures with the local stoner model. The predicted band structures match the first-principles ferromagnetic and antiferromagnetic calculations of the MnGe well and reveal that the MnGe is a half-metallic ferromagnet with integer magnetic moment in Bohr magneton. Applied to low concentrations of Mn doping, this model produces good magnetic moments, spin-dependent density of states, and energetics for Mn doping concentrations down to 0.03125. These results are in agreement with corresponding first-principles calculations. This method can be applied to other transition-metal doped semiconductors.

It is highly desirable to explore robust half-metallic ferromagnetic materials compatible with important semiconductors for spintronic applications. A state-of-the-art full potential augmented plane wave method within thedensity-functional theory is reliable enough for this purpose. In this chapter we review theoretical research on half-metallic ferromagnetism and structural stability of transition metal pnictides and chalcogenides. We show that some zincblende transition metal pnictides are half-metallic and the half-metallic gap can be fairly wide, which is consistent with experiment. Systematic calculations reveal that zincblende phases of CrTe, CrSe, and VTe are excellent half-metallic ferromagnets. These three materials have wide half-metallic gaps, are low in total energy with respect to the corresponding ground-state phases, and, importantly, are structurally stable. Half-metallic ferromagnetism is also found in wurtzite transition metal pnictides and chalcogenides and in transition-metal doped semiconductors as well as deformed structures. Some of these half-metallic materials could be grown epitaxially in the form of ultrathin films or layers suitable for real spintronic applications.

Phonon-dispersion curves for a series of single-wall carbon nanotubes (SWCNT's) have been obtained by ab initio supercell approach. Force constants are calculated using norm-conserving pseudopotential plane-wave method in the framework of the density-functional theory and local-density approximation. Cumulant force-constant method is used instead of interatomic force-constant method to construct the dynamical matrices. We get much better low-frequency dispersions than existing results with no soft phonon mode presented. Good accuracy is shown in the full frequency range. Residue forces in the relaxed SWCNT structures are filtered out by a linear fitting force-constant scheme. Raman and IR active modes are reanalyzed by D-2nd group symmetry. The frequencies are found to agree excellently with the experimental observations. Our results confirm the reported frequency drop of the A(1g) mode. Some issues in the low-temperature specific-heat experiment and the neutron-scattering experiment are well explained by our phonon dispersions. Phonon dispersions of graphite are also calculated using the same method, being in good agreement with experiment. Theoretical analysis is also presented to explain the advantages of cumulant force-constant method to get better low-frequency dispersions.

The full-potential augmented plane wave plus local orbital method within density-functional theory is used to predict that MnBi in the zinc-blende structure is a true half-metallic ferromagnet with a magnetic moment of $4.000{\ensuremath{\mu}}_{B}$ per formula unit. This contrasts with the zinc-blende phase of MnAs, which is only a nearly half-metallic ferromagnet. This half-metallic ferromagnetic behavior of zinc-blende MnBi is found to be robust against compressive volume changes of up to 15%, its stability being enhanced by the relativistic shift of the valence s state energy levels. Zinc-blende MnBi could possibly be grown epitaxially on the important binary semiconductors such as InSb or CdTe, although it remains to be seen whether epitaxially it retains its half-metallic ferromagnetic state.

Giant anomalous Hall effect in a perpendicular CoFeB thin film has been reported, which satisfies large anomalous Hall resistivity and low switching field at same time. The largest sensitivity of linear anomalous Hall effect reaches 2376 Ω/kOe, which is 21 times larger than that of the best semiconductor. Our results suggest the low cost MgO/CoFeB/Ta thin film can be a potential candidate for highly sensitive magnetic field detecting.

We study the spin-$s$ quantum Heisenberg antiferromagnetic models in a magnon theory free of any unphysical magnon state. Because the unphysical magnon states are completely removed in the magnon Hamiltonians and during the approximation process, we derive spin-s (s>1/2) reliable Neel temperature $T_N$ and reasonable sublattice magnetization unified for $T\leq T_N$., 4 papges with 2 figures included, revtex

TiO2 has been recently used to realize high-temperature ferromagnetic semiconductors. In fact, it has been widely used for a long time as white pigment and sunscreen because of its whiteness, high refractive index, and excellent optical properties. However, its electronic structures and the related properties have not been satisfactorily understood. Here, we use Tran and Blaha's modified Becke-Johnson (TB-mBJ) exchange potential (plus a local density approximation correlation potential) within the density functional theory to investigate electronic structures and optical properties of rutile and anatase TiO2. Our comparative calculations show that the energy gaps obtained from mBJ method agree better with the experimental results than that obtained from local density approximation (LDA) and generalized gradient approximation (GGA), in contrast with substantially overestimated values from many-body perturbation (GW) calculations. As for optical dielectric functions (both real and imaginary parts), refractive index, and extinction coefficients as functions of photon energy, our mBJ calculated results are in excellent agreement with the experimental curves. Our further analysis reveals that these excellent improvements are achieved because mBJ potential describes accurately the energy levels of Ti 3d states. These results should be helpful to understand the high temperature ferromagnetism in doped TiO2. This approach can be used as a standard to understand electronic structures and the related properties of such materials as TiO2.

We investigate the electronic structures and optical dielectric functions of the room temperature phases of SrTiO3 and BaTiO3 by using Tran and Blaha’s modified Becke and Johnson exchange potential. Calculated energy gaps are substantially better than previous first-principles results with respect to experimental values, further calculated optical dielectric functions as functions of photon energy are in good agreement with experimental curves, and calculated zero frequency refractive indexes are also consistent with experiment. This substantial improvement is achieved because the energy levels of the Ti d states are correctly calculated with this new exchange potential. These make a reasonable, reliable understanding of the electronic structures and optical properties of room temperature phases of SrTiO3 andBaTiO3. This approach should be applicable to other semiconducting materials.

Motivated by high-temperature ferromagnetism in transition-metal doped gallium nitrides, we study 12.5% transition-metal-substituted wurtzite GaN, XGa(7)N(8) (X = Cr, Mn, Fe, Co, Ni), using a full-potential density-functional method. Our calculated results show that CrGa(7)N(8) is a metallic ferromagnet and MnGa(7)N(8) is a typical half-metallic ferromagnet with a large half-metallic gap of 0.75 eV. They both have high spin polarization at the Fermi levels, 96.8% for CrGa(7)N(8) and 99.9% for MnGa(7)N(8) with spin - orbit coupling taken into account. Their ferromagnetism is very robust against the corresponding antiferromagnetic fluctuations and is attributed to the right positions of the d levels and the large spin exchange splitting driven by strong p - d hybridization. Antiferromagnetic orders are shown to be favourable in the cases of Fe, Co and Ni because their d levels are much lower in energy than that of both Cr and Mn and the majority-spin parts of them are merged in the valence bands of wurtzite GaN. The two highly-spin-polarized ferromagnetic phases should play some important roles in the ferromagnetism in GaN-based diluted magnetic semiconductors and could be realized in future high-quality samples.