Magnetism in boron nitride monolayer: Adatom and vacancy defect

Using the full potential linearized augmented plane wave (FLAPW) method, we have investigated the adatom or vacancy defect induced magnetic properties of hexagonal boron nitride (h-BN) monolayer. It has been observed that the N vacancy defect has no influence on the magnetic property of h-BN, whereas the B vacancy defect caused spin polarization in the nearest three N atoms. The total magnetic moment is about 0.87 μ_B within muffin-tin radius (0.29 μ_B per N atom) and the spin polarized N atoms show metallic feature. In the presence of B adatom defect, we have obtained rather weak spin polarization about 0.1 μ_B. However, the sizable magnetic moment of 0.38 μ_B appears in N adatom defect. Both B and N adatom defect systems preserve very close to semiconducting feature with a finite band gap. We have found that the DOS and the XMCD spectral shapes are strongly dependent on the defect type existing in the h-BN monolayer and this finding may help reveal the origin of magnetism in the h-BN layer if one performs surface sensitive experiment such as spin polarized scanning tunneling microscopy or XMCD measurement in the near future.     

A novel optical property induced by MO,S vacancy and V-doped in monolayer MoS2

The electronic structure and optical properties of Mo, S vacancy and V doping in MoS₂ monolayer will be investigated through first-principles calculations based on the density functional theory. The results indicate that the MoS₂ with Mo, S vacancy and V doping (Mo₁₄VS₃₂, Mo₁₅VS₃₁ and Mo₁₄VS₃₁) will gain the property of magnetic semiconductor with the magnetic moment of 1 μB, 1 μB and 0.95 μB, respectively. The optical properties of these V-doped and vacancy defect structures all reflect the phenomenon of red shift. The absorption edge of pure monolayer molybdenum disulfide is 0.8 eV, whereas the absorption edges of Mo₁₄VS₃₂, Mo₁₅VS₃₁ and Mo₁₄VS₃₁ become 0 eV, 0.2 eV and 0.16 eV, respectively. As a potential material, MoS₂ is widely used in many fields such as the production of optoelectronic devices, military devices and civil devices.     

Strain-dependent electronic and magnetic of Co-doped monolayer of WSe2

We perform first-principles calculation to investigate electronic and magnetic properties of Co-doped WSe₂ monolayer with strains from −10% to 10%. We find that Co can induce magnetic moment about 0.894 μ_B, the Co-doped WSe₂ monolayer is a magnetic semiconductor material without strain. The doped system shows half-metallic properties under tensile strain, and the largest half-metal gap is 0.147 eV at 8% strain. The magnetic moment (0.894 μ_B) increases slightly from 0% to 6%, and jumps into about 3 μ_B at 8% and 10%, which presents high-spin state configurations. When we applied compressive strain, the doped system shows a half-metallic feature at −2% strain, and the magnetic moment jumps into 1.623 μ_B at −4% strain, almost two times as the original moment 0.894 μ_B at 0% strain. The magnetic moment vanishes at −7% strain. The Co-doped WSe₂ can endure strain from −6% to 10%. Strain changes the redistribution of charges and magnetic moment. Our calculation results show that the Co-doped WSe₂ monolayer can transform from magnetic semiconductor to half-metallic material under strain.     

Electronic and magnetic behaviors of graphene with 5d series transition metal atom substitutions: A first-principles study

The electronic structures and magnetic behaviors of graphene with 5d series transition metal atom substitutions are investigated by performing first-principles calculations. All the impurities are tightly bonded to single vacancy in a graphene sheet. The substitutions of La and Ta lead to Fermi level shifting to valence and conduction band, respectively. Both the two substitutions result in metallic properties. Moreover, the Hf, Os and Pt-substituted systems exhibit semiconductor properties, while the Re and Ir-substituted ones exhibit robust half-metallic properties. Interestingly, W-substituted system shows dilute magnetic semiconductor property. On the other hand, the substitution of Ta, W, Re and Ir induce 0.86 μ_B, 2 μ_B, 1 μ_B and 0.99 μ_B magnetic moment, respectively. Our studies demonstrate that the 5d series transition metal substituted graphene have potential applications in nanoelectronics, spintronics and magnetic storage devices.     

Magnetic properties and Eu hyperfine interactions in EuSr2Ru1−xO6 (x=0 and 0.2)

Magnetization studies show that nonsuperconducting RSr₂Ru_1−xCu_xO₆ (R=Eu and Gd) compounds are magnetically ordered below T_N=31 K regardless of R and Cu concentration. The magnetic ordering is due to the Ru sublattice. Mössbauer effect studies reveal that the Ru magnetism induces a magnetic moment (0.35 μ_B) and a magnetic hyperfine field (270 kOe) in the otherwise nonmagnetic Eu³⁺ ions, and that the Ru magnetization lies in the basal plane.     

Magnetic properties of Ni doped gallium nitride with vacancy induced defect

Investigations have been carried out to study the ferromagnetic properties of transition metal (TM) doped wurtzite GaN from first principle calculations using tight binding linear muffin-tin orbital (TBLMTO) method within the density functional theory. The present calculation reveals ferromagnetism in nickel doped GaN with a magnetic moment of 1.13 μ_B for 6.25% of Ni doping and 1.32 μ_B for 12.5% of nickel doping, there is a decrease of magnetic moment when two Ni atoms are bonded via nitrogen atom. The Ga vacancy (V_Ga) induced defect shows ferromagnetic state. Here the magnetic moment arises due to the tetrahedral bonding of three N atoms with the vacancy which is at a distance of 3.689 Å and the other N atom which is at a distance of 3.678 Å .On the other hand the defect induced by N vacancy (V_N) has no effect on magnetic moment and the system shows metallic character. When Ni is introduced into a Ga vacancy (V_Ga) site, charge transfer occur from the Ni ‘d’ like band to acceptor level of V_Ga and formed a strong Ni–N bond. In this Ni–V_Ga complex with an Ni ion and a Ga defect, the magnetic moment due to N atom is 0.299 μ_B .In case of Ni substitution in Ga site with N vacancy, the system is ferromagnetic with a magnetic moment of 1 μ_B.     

A DFT study of substrate effect on the magnetism of V(001) surface

The effect of isoelectric transition metals (TM) Nb and Ta on the magnetism of the V(001) surface is investigated from first principles using Density functional theory (DFT), with the generalized gradient approximation (GGA). Ferromagnetic (FM) moments of 2.5 μ_B and 2.2 μ_B are obtained for the relaxed surface V monolayer (ML) in the V/Nb(001) and the V/Ta(001) systems respectively, at T = 0 K. The values are almost twice of those obtained with Mo and W of group VIB and can be attributed to the comparatively smaller bandwidths of the substrates Nb and Ta. Small induced magnetic moments are present on the Nb and Ta interfacial layers, which are coupled anti-ferromagnetically with the V ML.     

Low-magnetic moment and strong anisotropy in non-doped iron-based superconductors

We perform first-principles calculations with a specific LDA + U scheme for non-doped iron based superconductor LaFeAsO, which exhibits an antiferromagnetic ordering with an unexpected low-magnetic moment compared to the one predicted by standard first-principles calculations. Consequently, we find two stable electronic structures, one of which has the high magnetic moment ～2.0 μ_B equivalent with the previous calculation results, and another of which reproduces the low magnetic moment ～0.3 μ_B as obtained experimentally. The former is stable in U = 0, whereas the latter becomes stable in an intermediate U range. The latter well explains various anomalous features as observed experimentally.     

Ab initio studies of isolated hydrogen vacancies in graphane

We present a density functional study of various hydrogen vacancies located on a single hexagonal ring of graphane (fully hydrogenated graphene) considering the effects of charge states and the position of the Fermi level. We find that uncharged vacancies that lead to a carbon sublattice balance are energetically favorable and are wide band gap systems just like pristine graphane. Vacancies that do create a sublattice imbalance introduce spin polarized states into the band gap, and exhibit a half-metallic behavior with a magnetic moment of 1.00 μ_B per vacancy. The results show the possibility of using vacancies in graphane for novel spin-based applications. When charging such vacancy configurations, the deep donor (+1/0) and deep acceptor (0/−1) transition levels within the band gap are noted. We also note a half-metallic to metallic transition and a significant reduction of the induced magnetic moment due to both negative and positive charge doping.     

Spin and orbital magnetic moments and spin anisotropy energies of light rare earth atoms embedded in graphene: A first-principles study

The geometries, electronic structures, spin magnetic moments (SMMs), orbital magnetic moments (OMMs) and spin anisotropy energies (SAEs) of light rare earth atoms (La, Ce, Pr, Nd, Pm, Sm, Eu, and Gd) embedded in graphene were studied by using first-principles calculations based on Density Functional Theory (DFT). The spin-orbital coupling effect was taken into account and GGA+U method was adopted to describe the strongly localized and correlated 4f electrons. There is a significant deformation of the graphene plane after doping and optimization. The deformation of Gd doped graphene is the largest, while Eu the smallest. The results show that the valence is +3 for La, Ce, Pr, Nd, Pm, Sm and Gd, and +2 for Eu. Except Eu and Gd, there are obvious OMMs. When the spin is in the Z direction, the OMMs are −0.941 μ_B, −1.663 μ_B, −3.239 μ_B, −3.276 μ_B and −3.337 μ_B for Ce, Pr, Nd, Pm and Sm, respectively, and point the opposite direction of SMMs. All the doped systems except Gd show considerable SAEs. For Ce, Pr, Nd, Pm, Sm, and Eu, the SAEs are −0.928 meV, 20.941 meV, −8.848 meV, 7.855 meV, 75.070 meV and 0.810 meV, respectively. When the spin orientation is different, different orbital angular moments lead to apparent charge density difference of the 4f atoms, which can also explain the origin of SAEs.     

Intrinsic magnetism induced by vacancy in GaN

We performed total energy electronic-structure calculations based on DFT that clarify the intrinsic magnetism of undoped GaN. The magnetism is due to Ga, instead of N, vacancies. The origin of magnetism arises from the unpaired 2p electrons of N surrounding Ga vacancy. At a vacancy concentration of 5.6%, the ferromagnetic state is 181 meV lower than the antiferromagnetic state. Our findings are helpful to gain a more novel understanding of structural and spin properties of Ga vacancy in wurtzite GaN and also provide a possible way to generate magnetic GaN by introducing Ga vacancies instead of doping with transition-metal atoms.     

Ga vacancy induced ferromagnetism enhancement and electronic structures of RE-doped GaN

Because of their possible applications in spintronic and optoelectronic devices, GaN dilute magnetic semiconductors (DMSs) doped by rare-earth (RE) elements have attracted much attention since the high Curie temperature was obtained in RE-doped GaN DMSs and a colossal magnetic moment was observed in the Gd-doped GaN thin film. We have systemically studied the GaN DMSs doped by RE elements (La, Ce–Yb) using the full-potential linearized augmented plane wave method within the framework of density functional theory and adding the considerations of the electronic correlation and the spin-orbital coupling effects. We have studied the electronic structures of DMSs, especially for the contribution from f electrons. The origin of magnetism, magnetic interaction and the possible mechanism of the colossal magnetic moment were explored. We found that, for materials containing f electrons, electronic correlation was usually strong and the spin–orbital coupling was sometimes crucial in determining the magnetic ground state. It was found that GaN doped by La was non-magnetic. GaN doped by Ce, Nd, Pm, Eu, Gd, Tb and Tm are stabilized at antiferromagnetic phase, while GaN doped by other RE elements show strong ferromagnetism which is suitable materials for spintronic devices. Moreover, we have identified that the observed large enhancement of magnetic moment in GaN is mainly caused by Ga vacancies (3.0μB per Ga vacancy), instead of the spin polarization by magnetic ions or originating from N vacancies. Various defects, such as substitutional Mg for Ga, O for N under the RE doping were found to bring a reduction of ferromagnetism. In addition, intermediate bands were observed in some systems of GaN:RE and GaN with intrinsic defects, which possibly opens the potential application of RE-doped semiconductors in the third generation high efficiency photovoltaic devices.     

DFT-GGA study of NO adsorption on the LaO (001) surface of LaFeO3

In order to demonstrate the adsorption of the nitrogen monoxide molecule (NO) on the LaO (001) surface of LaFeO₃, we perform simulations based on density functional theory. The generalized gradient approximation (GGA) for the exchange-correlation energy functional indicates that the electronic state of the LaFeO₃ bulk is an anti-ferromagnetic insulator with a local magnetic moment of 4.1 μ_B at each Fe atom. Using the ultrasoft pseudo-potential method with spin-polarized GGA, fully optimized internal parameters as well as charge and spin density are determined for the NO-adsorbed structure prepared in a slab model. The calculated adsorption energy of NO is around ? 1.4 eV on the LaO (001) surface of LaFeO₃. This value decreases down to ? 4.46 eV at an oxygen vacancy site, where the nitrogen atom of NO is embedded in the 1st LaO layer forming a bond with Fe in the 2nd FeO layer.     

Electronic and magnetic properties of Mn-doped WSe2 monolayer under strain

Electronic and magnetic properties of Mn-doped WSe₂ monolyer subject to isotropic strain are investigated using the first-principles methods based on the density functional theory. Our results indicate that Mn-doped WSe₂ monolayer is a magnetic semiconductor nanomaterial with strong spontaneous magnetism without strain and the total magnetic moment of Mn-doped system is 1.038μ_B. We applied strain to Mn-doped WSe₂ monolayer from -10% to 10%. The doped system transforms from magnetic semiconductor to half-metallic material from −10% to −2% compressive strain and from 2% to 6% tensile strain. The largest half-metallic gap is 0.450 eV at −2% compressive strain. The doped system shows metal property from 7% to 10%. Its maximum magnetic moment comes to 1.181μ_B at 6% tensile strain. However, the magnetic moment of system decreases to zero sharply when tensile strain arrived at 7%. Strain changes the redistribution of charges and arises to the magnetic effect. The coupling between the 3d orbital of Mn atom, 5d orbital of W atom and 4p orbital of Se atom is analyzed to explain the strong strain effect on the magnetic properties. Our studies predict Mn-doped WSe₂ monolayers under strain to be candidates for thin dilute magnetic semiconductors, which is important for application in semiconductor spintronics.     

First-principles study on the magnetism and electronic structure in 3d transition metal (X=Sc,V, Cr,Mn, Fe,Ni, Cu) doped CoO

We have studied the electronic structure and magnetism of the single transitional metal element X=Sc, V, Cr, Mn, Fe, Ni, Cu-doped CoO systems by first-principles calculations. At X=Sc, Cr, Cu, the binding energy of the doped systems is lower than pure CoO, suggesting that these systems are energetically stable. In the Sc, V, Cr, Mn, Fe, Ni, Cu-doped 2×2×2 CoO supercells, the total magnetic moments are 3.03, 5.64, 6.80, 7.70, 6.93, 2.30 and 1.96 μ_B, respectively. At X=Cr and Fe, the doped CoO systems are half-metallic with a high spin polarization. The large magnetic moment and high spin polarization in the Cr and Fe-doped CoO are important for the design of the spintronic devices.     

Ferromagnetism driven by cation vacancy in GaN thin films and nanowires

The first-principles calculations have been performed to understand the origin of magnetism in undoped GaN thin films. The results show that Ga vacancy, rather than that of N contributes the observed magnetism, and the magnetic moments mainly come from the unpaired 2p electrons at nearest-neighbor N atoms of the Ga vacancy. Calculations and discussions are also extended to bare and passivated GaN nanowires, We find that per Ga vacancy on the surface sites products the total magnetic moment of 1.0  while that inside of the nanowires can lead to the formation of a net moment of 3.0 . The coupling between two Ga vacancies is also studied and we found that the coupling is ferromagnetic coupling. The surface passivation with hydrogen is shown to strongly enhance the ferromagnetism. Our theoretical study not only demonstrates that GaN nanowire can be magnetic even without transition-metal doping, but also suggests that introducing Ga vacancy is a natural and an effective way to fabricate low-dimensional magnetic GaN nanostructures.     

Stabilization of high spin FCC Fe in (Fe/Pd)n multilayers

Polycrystalline (Fe/Pd)_n multilayers are grown onto sapphire substrates at room temperature in a UHV system. The number of periods n=40 and the thickness of Pd layers of t_Pd=4 nm are kept constant, whereas the thickness of the Fe layers is varied from 1.5 to 5 nm. Structural properties are studied by in situ reflection high energy diffraction (RHEED), scanning tunnelling microscopy (STM) and ex situ by X-ray diffraction at small angles and large angles. Analyzing the experimental data using the program SUPREX we obtain interplanar distances of d_Fe=2.03±0.01 Å for an Fe layer thickness larger than about 2.5 nm as expected for (1 1 0) planes of BCC Fe. For Fe layers with thicknesses less than about 2.5 nm the interplanar distance is d_Fe=2.1±0.01 Å, which is close to the distance between (1 1 1) planes of FCC Fe with a lattice parameter of a=3.64 Å. Magnetic susceptibility measurements at temperatures between 1.5 and 300 K for (Fe/Pd)_n multilayers with FCC Fe yield a magnetic moment per Fe atom of μ=2.7±0.1 μ_B, which is about 20% larger compared to μ=2.2 μ_B for BCC Fe. We show that the occurrence of the large magnetic moment originates from FCC Fe being in the high spin (HS) state rather than from polarization effects of Pd at Fe/Pd interfaces.     

Effect of Ni deficit on the electronic structure and magnetic properties of PrNi2−xSb2

The influence of a Ni deficit in the nickel sublattice on the electronic and magnetic properties of PrNi_2−xSb₂ compound is investigated. The band structure is calculated using the LMTO method for x=0, 0.50, 1.0 and 1.5. At T=0 K the compound is antiferromagnetic with a magnetic moment on Pr close to 2.0 μ_B.     

Influence of local environment on electronic properties of Co atoms in the Tm3Co11B4 compound

The electronic structure of the Tm₃Co₁₁B₄ compound has been studied by X-ray photoemission spectroscopy and ab initio self-consistent tight binding linear muffin tin orbital (TB LMTO) method. This compound crystallizes in the hexagonal Ce₃Co₁₁B₄-type structure (P6/mmm). We have found a good agreement between the experimental XPS valence band spectra and theoretical ab initio calculations. The calculated total magnetic moment is equal to 13.635 μ_B/f.u. The magnetic moments on the Co atoms are antiparallel to the moments of the Tm atoms. Their values are depended on the local environment, especially on the number of the Co neighbors. The theoretical results are compared with other calculations, saturation magnetization measurements as well as neutron diffraction data for R₃Co₁₁B₄ (R=Y, Nd, Gd, Tb).     

Ab initio determination of atomic structure and energy of surface states of bare and hydrogen covered GaN (0001) surface — Existence of the Surface States Stark Effect (SSSE)

Density Functional Theory (DFT) calculations indicate that energetically stable structure of clean GaN(0001) surface posses (2 × 1) reconstruction, having every second row of Ga located near plane of N atoms, that gives rise to Ga-related dispersionless surface electronic state, already identified by angle resolved photoelectron spectroscopy (ARPES) measurements [S.S. Dhesi et al. Phys. Rev. B 56 (1997) 10271, L. Plucinski et al. Surf. Sci 507-10 (2002) 223, S. M. Widstrand et al. Surf. Sci. 584 (2005) 169]. The energy reduction in reconstruction proceeds via change of the hybridization of the occupied Ga surface states from sp³ to sp², transforming the empty states to p_z type. It is also shown that the electric subsurface field, modeled in new slab model which allows to simulate electric fields at the semiconductor surfaces [P. Kempisty et al., J. Appl. Phys. 106 (2009) 054901], strongly affects the energy of electronic states of GaN(0001) surfaces. The change of the field may shift the energy of surface states of bare and hydrogen covered GaN(0001) surface, by several eV with respect to the band states. The phenomenon, denoted as Surface States Stark Effect (SSSE), explains various band bending values, measured at differently doped n-type GaN(0001) surfaces. It is shown also that, for the adsorbate density up to one H atom for each Ga surface atom i.e. 1 monolayer coverage (1 ML), the hydrogen adatoms are located at the on-top positions, i.e. directly above Ga atoms. For these adsorbate densities, the H-related quantum surface state is located slightly below the valence band maximum (VBM) in the case of p-type GaN surface. For n-type GaN, the H-related surface state is located deeply in the valence band, about 2 eV below VBM. For higher, 1.25 ML hydrogen coverage, the two H adatoms create either surface attached H₂ ad-molecule (energetically stable) or triple bridge configuration is created (metastable). The H₂ ad-molecule is weekly attached to the surface, having the desorption energy barrier equal to 0.16 eV. For 1.25 ML coverage the DFT results were obtained for p-type GaN only. They show that in the ad-molecule case, a new surface electronic state arises which is located about 6.7 eV below VBM. In the case of the bridge configuration, the bridge related surface state is located closely to the conduction band minimum (CBM).