Electronic structure and magnetic properties of the perovskite cerium manganese oxide from ab initio calculations

We have performed first-principle calculations of the structural, electronic and magnetic properties of cerium manganese oxide (CeMnO)₃, using full-potential linearized augmented plane-wave (FP-LAPW) scheme within GGA and GGA+U approaches. Features such as the lattice constant, bulk modulus and its pressure derivative are reported. Also, we have presented our results of the band structure and the density of states. The results show a half-metallic ferromagnetic ground state for CeMnO₃ in GGA+U treatment, whereas semi-metallic ferromagnetic character is observed in GGA. The results obtained, make the cubic CeMnO₃ a candidate material for future spintronic application.     

Electronic structure and magnetic properties of Ca3Co2O6 cobaltites: Intrachain magnetic ordering

The present study is focussed on the intrachain magnetic interactions between the trigonal prismatic (TP) and octahedral (OCT) Co sites in Ca₃Co₂O₆ cobaltites from their electronic structure determined using the first principle full potential linearized augmented plane wave method within the GGA+U approximation. The occurence of various magnetic solutions as a function of the Coulomb integral U of the GGA+U approach is investigated. Various magnetic configurations, corresponding to different total magnetization, are found only when the insulating state is reached for U > 2.5 eV and the most stable solution corresponds to a magnetic spin moment on the TP (respectively OCT) site around 2.6 (respectively 1) Bohr magneton.     

Ab initio prediction of structural,electronic, magnetic and optical properties of Ba2GdSbO6

A first-principles approach is used to study the structural, electronic, optic and magnetic properties of Ba₂GdSbO₆, using full-potential linearized augmented plane wave (FP-LAPW) scheme within GGA+U approach. Features such as the lattice constant, bulk modulus and its pressure derivative are reported. The calculated band structure and density of states show that the material under load has an indirect energy band gap L→X for majority-spin direction and Г→X for the minority spin channel. The analysis charge densities show that bonding character as a mixture of covalent and ionic nature. The optical properties are analyzed and the origin of some peaks in the spectra is described. Besides, the dielectric function, refractive index and extinction coefficient for radiation up to 14 eV have also been reported.     

First principles study of structural,electronic, elastic and magnetic properties of gadolinium hydride system GdHx (x=1, 2, 3)

The structural, electronic, elastic and magnetic properties of gadolinium and its hydrides GdH_x (x=1, 2, 3) are investigated by using Vienna ab-initio simulation package with the generalized gradient approximation parameterized by Perdew, Burke and Ernzerhof (GGA-PBE) plus a Hubbard parameter (GGA-PBE+U) in order to include the strong Coulomb correlation between localized Gd 4f electrons. At ambient pressure all the hydrides are stable in the ferromagnetic state. The calculated lattice parameters are in good agreement with the experimental results. The bulk modulus is found to decrease with the increase in the hydrogen content for the gadolinium hydrides. A pressure-induced structural phase transition is predicted to occur from cubic to hexagonal phase in GdH and GdH₂ and from hexagonal to cubic phase in GdH₃. The electronic structure reveals that mono and di-hydrides are metallic, whereas trihydride is half-metallic at normal pressure. On further increasing the pressure, a half-metallic to metallic transition is also observed in GdH₃. The calculated magnetic moment values of GdH_x (x=1, 2, 3) are in accord with the experimental values.     

Energy-band structure of the A(Sn1−x Mx)O3 (A = Ca, Sr, Ba; M = Mn, Fe, Co) perovskite-type phases: A search for new magnetic semimetals

The full-potential LAPW method in the local spin-density approximation (LSDA) is applied to the theoretical search for new magnetic semimetals on the basis of a nonmagnetic SrSnO₃ semiconductor doped with magnetic ions M = Mn, Fe, Co. It is found that, in contrast to Sr₂SnMnO₆ (a ferromagnetic semiconductor) and Sr₂SnCoO₆ (a ferromagnetic metal), Sr₂SnFeO₆ perovskite is a ferromagnetic semimetal: its electronic spectrum has a band gap (～0.7 eV) for the low-spin subsystem and is metal-type for the high-spin subsystem. For isoelectronic analogs SrSnO₃-CaSnO₃ and BaSnO₃, it is shown that their doping by iron (Ca₂SnFeO₆ and Ba₂SnFeO₆ compositions) also makes it possible to realize the semimetal state of these systems. It is assumed that A(Sn_1?x Fe_x)O₃ (A = Ca, Sr, Ba) solid solutions can become the first representatives of a new group of materials for spin electronics.     

Influence of Superexchange Interaction on the Ferromagnetic Properties of Manganites and Cobaltites

The role of superexchange interaction in the formation of the ferromagnetic state of cobaltites in the La_0.5Sr_0.5Co_1–xMexO₃ (Me = Cr, Ga, Fe) systems and manganites La_0.7Sr_0.3Mn_0.85M_0.15O₃ (M–Nb, Mg) with a perovskite structure has been studied. It was found that the ferromagnetic state in cobaltites can be implemented in some compositions without the mixed-valence effect of cobalt ions. The initial compound (x = 0) is ferromagnetic (Т_С = 247 K) with the saturation magnetization close to 2μB (at T = 30 K) per formular unit. It has been shown that the chemical substitution of cobalt by chromium reduces the spontaneous magnetization to 0.3μB (at х = 0.2), while the substitution of cobalt by iron ions (х = 0.2) does not alter the magnetization. The obtained data are interpreted in the model of positive superexchange interactions between cobalt and iron ions and negative ones between cobalt and chrome. It has been shown that the La_0.7Sr_0.3Mn_0.85Nb_0.15O₃ composition is ferromagnetic with ТС=145K, with a magnetic moment of 3.1 μB/Mn at 10 K, and no evidence of a cooperative orbital ordering in the manganite compounds has been revealed. Partial chemical substitution of Nb⁵⁺ ions by Mg²⁺ ones leads to the formation of Mn⁴⁺ ions, while it does not strengthen the ferromagnetic state. The strengthening of the structural distortions reduces the ferromagnetic component. It is assumed that the ferromagnetic state is caused by a significant hybridization of eg-orbitals of the manganese and oxygen ions, which strengthens the positive component of the superexchange interactions.     

Investigation of the Properties of Ba-Substituted La0.7Sr0.3−x Ba x MnO3 Perovskite Manganite Films for Resistive Switching Applications

La_0.7Sr_0.3?x Ba _x MnO₃ (LSBMO: x = 0.09, 0.18, and 0.27) thin films were prepared on Pt-coated Si substrates using a radiofrequency magnetron sputtering technique at a substrate heating temperature of 450°C. The effects of varying the amount of substituted Ba²⁺ on the physical, chemical, and electrical properties of the perovskite manganite films were systematically investigated. X-Ray diffraction showed that the growth orientation and crystallinity of films were not affected by the amount of substituted Ba cations. Raman spectroscopy was used to determine the tilt of MnO₆ octahedra and the Jahn–Teller-type distortion variation of the manganite films. The change in covalent characteristics of Mn–O bonds with increasing amounts of Ba²⁺ substituent was analyzed by x-ray photoelectron spectroscopy, specifically to examine the effects of bond characteristics on the resistive switching properties of LSBMO. The resistance of the LSBMO films increased with increasing Ba²⁺ content due to an increase in the covalent nature of Mn–O bonds. The resistive switching ratio increased with increasing Ba²⁺ amount, and relationships among resistive switching, Jahn–Teller distortion, and Mn–O bond character of LSBMO films were interpreted.     

Bi(2223) thick films (TC(0)=109K) on Ba2GdNbO6: a new perovskite ceramic substrate for BSCCO superconductor

Ba₂GdNbO₆ has been developed as a new substrate suitable for the BSCCO superconductor. Ba₂GdNbO₆ has a complex cubic perovskite (A₂BB′O₆) structure with a lattice constant a=8.587 Å. The DTA studies revealed that there is no phase transition occurring in Ba₂GdNbO₆ in the temperature range of 30–1300°C. The thermal expansion coefficient of Ba₂GdNbO₆ is found to be 7.913×10⁻⁶°C⁻¹. The dielectric constant and loss factor of Ba₂GdNbO₆ are in a range suitable for its use as a substrate for microwave applications. The Bi(2223) superconductor does not show any detectable chemical reaction with Ba₂GdNbO₆ even under extreme processing conditions. The thick films of Bi(2223) dip-coated on polycrystalline Ba₂GdNbO₆ substrate gave a T_C(0) of 109 K and a current density of approximately 4×10³ A/cm² at 77 K and zero magnetic field.     

Photoluminescence properties of YVO4:Eu3+,Ba2+ nanoparticles prepared by an ion exchange method

YVO₄:Ba²⁺ nanoparticles with a Ba²⁺ doping concentration x=0%, 1%, 3%, 5%, 7% and 9% were synthesized by a solvothermal method and then they were codoped with Eu³⁺ ions by an ion exchange method to form the YVO₄:Eu³⁺,Ba²⁺ nanoparticles. It was found that the photoluminescence intensity of the as-prepared YVO₄:Eu³⁺,Ba²⁺ nanoparticles steadily increased with x until x=7%, and then decreased for higher x. Thermal annealing resulted in considerable enhancement in their photoluminescence, and higher annealing temperature led to stronger photoluminescence enhancement. The emission intensity of the YVO₄:Eu³⁺,Ba²⁺ (x=7%) nanoparticles annealed at 500 °C was about 205% stronger than the sample without Ba²⁺ doping. Thermal annealing of the ion-exchanged YVO₄:Eu³⁺,Ba²⁺ nanoparticles at 500 °C and 700 °C resulted in photoluminescence enhancement of about 14 times and 27 times, respectively. The asymmetric ratio of Eu³⁺ in the ion-exchanged YVO₄:Eu³⁺,Ba²⁺ nanoparticles was found to increase after annealing.     

Single crystal growth and solidification processing of RE123 superconductive oxides

Single crystal growth and bulk solidification processing in air are reviewed for the RE_1+xBa_2−xCu₃O_6+d solid solutions (RE123; RE=Sm, Nd). By changing the solvent and/or the initial composition from the usual composition of RE123—Sm₂Ba₁Cu₁O₅ (Sm211)/Nd₄Ba₂Cu₂O₁₀ (Nd422) to the Ba enriched side, RE123 crystals with a small amount of substitution were produced, which resulted in high T_c values of about 94–96 K for both single crystals and bulk materials. Another unique and interesting finding for the bulk processing includes the reduction of the RE211 particle size with increasing the BaO/CuO ratio in the initial composition. Uniform dispersion of the fine RE211 particles was attained in the RE123 matrix, which led to improve J_c values in relatively low magnetic fields of about 1 T. Changing the initial compositions toward the Ba enriched side is a simple and fundamental process for obtaining the bulk RE123 superconductor with high T_c and J_c values even in air, which in turn is an important and useful approach from the industrial point of view.     

Tailoring the electronic properties of zigzag graphene nanoribbons via sp2/sp3 edge functionalization with H/F

Functionalizing the edges of graphene nanoribbons (GNR) plays a vital role to alter their electronic properties. In present work, the effect of sp²∕sp³ edge functionalization of zigzag GNR (ZGNR) via H/F is investigated to reveal their structural stability and spin dependent electronic properties. Depending upon the way of functionalization, nanoribbons under considerations are categorized in three groups viz. Group A: one edge is sp² functionalized while other edge is sp³ functionalized, Group B: both the edges are sp³ functionalized and Group C: both edges are sp³ functionalized, however, number of H and F atoms are different. All the group A and group C structures settled in magnetic ground state while group B nanoribbons are all nonmagnetic except F₂-ZGNR-H₂ which prefers ferromagnetic state. Further, it is noticed that F functionalization enhances the stability and is equivalent to p-type doping of nanoribbon along with affecting the electronic properties significantly. Interestingly, for selective structures, magnetic ground state has been obtained and the observed magnetic stabilization is found to be greater than room temperature thermal excitations which ensures the potential of considered ZGNR for practical applications. Present findings provide a viable way for tailoring the spintronic properties of ZGNR and absolute shifting of Fermi level which can play a crucial role for designing future nano-devices.     

Atomic‐Scale Control of Electronic Structure and Ferromagnetic Insulating State in Perovskite Oxide Superlattices by Long‐Range Tuning of BO6 Octahedra

Control of BO₆ octahedral rotations at the heterointerfaces of dissimilar ABO₃ perovskites has emerged as a powerful route for engineering novel physical properties. However, its impact length scale is constrained at 2–6 unit cells close to the interface and the octahedral rotations relax quickly into bulk tilt angles away from interface. Here, a long‐range (up to 12 unit cells) suppression of MnO₆ octahedral rotations in La_0.9Ba_0.1MnO₃ through the formation of superlattices with SrTiO₃ can be achieved. The suppressed MnO₆ octahedral rotations strongly modify the magnetic and electronic properties of La_0.9Ba_0.1MnO₃ and hence create a new ferromagnetic insulating state with enhanced Curie temperature of 235 K. The emergent properties in La_0.9Ba_0.1MnO₃ arise from a preferential occupation of the out‐of‐plane Mn d_3z²_?r² orbital and a reduced Mn e_g bandwidth, induced by the suppressed octahedral rotations. The realization of long‐range tuning of BO₆ octahedra via superlattices can be applicable to other strongly correlated perovskites for exploring new emergent quantum phenomena.     

Ferromagnetic Y2CoMnO6: Spin-Glass-Like Behavior and Dielectric Relaxation

The magnetic and dielectric properties of a Y₂MnCoO₆ ceramic have been investigated. The x-ray diffraction pattern shows that the sample has the monoclinic structure (space group P2₁/n). Magnetic measurements show that Y₂MnCoO₆ becomes ferromagnetic (FM) below T _C ~75 K. More importantly, the temperature dependence of magnetization (zero field cooled and field cooled behavior) and the M(H) behavior are indicative of spin-glass-like behavior of Y₂MnCoO₆, which may be caused by competition between the FM and antiferromagnetic interaction. Dielectric relaxor behavior at low temperature is also observed; this arises from dipolar effects associated with the charge ordering of Co²⁺ and Mn⁴⁺.     

Intrinsic Half‐Metallicity in 2D Ternary Chalcogenides with High Critical Temperature and Controllable Magnetization Direction

Searching for 2D ferromagnetic materials with a high critical temperature, large spin polarization, and controllable magnetization direction is a key challenge for their broad applications in spintronics. Here, through a systematic study on a series of 2D ternary chalcogenides with first‐principles calculations, it is demonstrated that a family of experimentally available 2D CoGa₂X₄ (X = S, Se, or Te) are half‐metallic ferromagnets, and they exhibit high critical temperature, fully polarized spin state, and strain‐dependent magnetization direction simultaneously. Following the Goodenough–Kanamori rules, the half‐metallic ferromagnetism of CoGa₂X₄ family is caused by superexchange interaction mediated by Co? X? Co bonds. The half‐metal gaps are large enough (>0.5 eV) to ensure that the half‐metallicity is stable against the spin flipping at room temperature. Magnetocrystalline anisotropy energy calculations indicate that CoGa₂X₄ favor easy plane magnetization. Under achievable biaxial tensile strain (2–6%), the magnetization directions of CoGa₂X₄ can change from in‐plane to out‐of‐plane, providing a route to control the efficiency of spin injection/detection. Further, the critical temperatures T_c of ferromagnetic phase transition for CoGa₂X₄ are close to room temperature. Belonging to the big family of layered AB₂X₄ compounds, the proposed CoGa₂X₄ systems will enrich the available 2D candidates and their heterojunctions for various applications.     

Symmetry-Mismatch-Induced Ferromagnetism in the Interfacial Layers of CaRuO3/SrTiO3 Superlattices

By modifying the entangled multi-degrees of freedom of transition-metal oxides, interlayer coupling usually produces interfacial phases with unusual functionalities. Herein, a symmetry-mismatch-driven interfacial phase transition from paramagnetic to ferromagnetic state is reported. By constructing superlattices using CaRuO₃ and SrTiO₃, two oxides with different oxygen octahedron networks, the tilting/rotation of oxygen octahedra near interface is tuned dramatically, causing an angle increase from ≈150° to ≈165° for the Ru O Ru bond. This in turn drives the interfacial layer of CaRuO₃, ≈3 unit cells in thickness, from paramagnetic into ferromagnetic state. The ferromagnetic order is robust, showing the highest Curie temperature of ≈120 K and the largest saturation magnetization of ≈0.7 µ_B per formula unit. Density functional theory calculations show that the reduced tilting/rotation of RuO₆ octahedra favors an itinerant ferromagnetic ground state. This work demonstrates an effective phase tuning by coupled octahedral rotations, offering a new approach to explore emergent materials with desired functionalities.     

A density functional study of structural,electronic and optical properties of titanium dioxide: Characterization of rutile,anatase and brookite polymorphs

Study of fundamental physical properties of titanium dioxide (TiO₂) is crucial to determine its potential for different applications, such as study of electronic band gap energy is essential to exploit it for optoelectronics and solar cell technology. We present here investigations pertaining to structural, electronic and optical properties of rutile, anatase and brookite polymorphs of TiO₂ by employing state of the art full potential (FP) linearized (L) augmented plane wave plus local orbitals (APW+lo) approach realized in WIEN2k package and framed within density functional theory (DFT). To incorporate exchange correlation(XC) energy functional/potential part into total energy, these calculations were carried out at the level of PW–LDA, PBE–GGA, WC–GGA, EV–GGA, and mBJ–GGA which are exploited as the manipulated variables in this work. From our computations, the obtained structural parameters results were found to be consistent with the available experimental results. The analysis of electronic band gap structure calculations point to TiO₂ as a semiconducting material in all three phases, whereas band gap character around Fermi level was found to be indirect for anatase, and direct for rutile and brookite phases. Density of state (DOS) profiles showed a substantial degree of hybridation between O 2p and Ti 3d in conduction and valence band regions, illustrating a strong interaction between Ti and O atoms in TiO₂ compund. In addition, our investigations of the optical properties also endorse the interband transitions from O 2p in valence band to Ti 3d in conduction band.     

Effect of Hf and Hf-Ca substitution on the superconductivity of GdBa2Cu3O7 − δ

The structural and superconducting properties of (Gd_1−x−yCa_yHf_x)Ba₂Cu₃O_z samples are investigated using X-ray diffraction, resistivity, AC susceptibility and oxygen content measurements. The effect of increasing Hf concentration in (Gd_1−xHf_x)Ba₂Cu₃O_z lowers the oxygen content and decreases T_c which is attributed to hole filling by Hf. The substitution of Ca for Gd in (Gd_0.85−yCa_yHf_0.15)Ba₂Cu₃O_z provides proper matching between the ionic radius and valence of Gd³⁺ (0.94 Å) and the average ionic radius and valence of Hf⁴⁺ (0.78 Å) and Ca²⁺ (0.99 Å). As the Ca content increases, the T_c increases from 81 K (y = 0.05) to 86.5 K (y = 0.20, compensated oxide), closer to the value of 91 K for pure GdBa₂Cu₃O_7−δ due to the balance between the hole filling by Hf and hole doping by Ca. A comparative study of Hf doped samples of (R_1−xHf_x)Ba₂Cu₃O_z (R = Y, Er, Gd) indicates that the magnetic moment carried by R-ion plays an important role in the suppression of superconductivity and T_c.     

Fluorescence enhancement of single-phase red-blue emitting Ba3MgSi2O8:Eu2+,Mn2+ phosphors via Dy3+ addition for plant cultivation

Fluorescence enhancement of red and blue concurrently emitting Ba₃MgSi₂O₈:Eu²⁺,Mn²⁺ phosphors for plant cultivation has been investigated by Dy³⁺ addition. The Ba₃MgSi₂O₈:Eu²⁺,Mn²⁺,Dy³⁺(BMS-EMD) phosphors have two-color emissions at the wavelength peak values of 437 nm and 620 nm at the excitation of 350 nm. The two emission bands are coincident with the absorption spectrum for photosynthesis of plants. An obvious enhancement effect has been observed upon addition of Dy³⁺ with amount of 0.03 mol%, in which the intensities of both blue and red bands reach a maximum. The origin of red and blue emission bands is analysed. The photochromic parameters of the samples at the nearly UV excitation are tested. This fluoresence enhancement is of great significance for special solid state lighting equipment used in plant cultivation. This work has been supported by National Natural Science Foundation of China (Grant No 50872091) and the Natural Science Foundation of Tianjin, China (06YFJMJC02300, 06TXTJJC14602).