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1.
The real-space recursion method and unrestricted Hartree-Fock approximation have been applied to calculate the density of states of various Co perovskite, CeCoO3, SrCoO3 and Sr1−xCexCoO3. We have studied the magnetically ordered states of these Co perovskites in an enlarged double cell, and find its various magnetic structures due to the occupancy of 3d band and its interaction with neighboring Co ions. In this study, we have studied the p-d hybridization of the three Co perovskites, we find t2g electrons are localized and the flat eg band is responsible for the itinerant behavior, and although the rare earth elements itself contribute little to the DOS at the Fermi energy, the DOS at Fermi energy and the magnetic moment changed consequently because of different valence of Co ions in these compounds and p-d hybridization effect is very important. 相似文献
2.
The crystal structure, magnetic and magnetotransport properties of the variation of B′-site transition metal in Sr2FeMO6 (M=Mo, W) with double perovskites structure have been investigated systematically. Measurements of magnetization vs. temperature at H=5 T show that Sr2FeMoO6 is a ferromagnet and Sr2FeWO6 is an antiferromagnet with TN∼35 K. Additionally, the large magnetoresistance ratio (MR) of ∼22% (H=3 T) at room temperature (RT) was observed in the Sr2FeWO6 compound. However, the Sr2FeMoO6 compound did not show any significant MR even at high fields and RT (MR∼1%; H=3 T and 300 K). The implications of these findings are supported by band structure calculations to explain the interaction between the 4d(Mo) and 5d(W) orbitals of transition metal ions and oxygen ions. 相似文献
3.
Q. Zhang G.H. Rao Y.G. Xiao G.Y. Liu Y. Zhang J.K. Liang 《Applied Physics A: Materials Science & Processing》2006,84(4):459-463
Hole-doped double perovskite compounds (Sr,Na)2FeMoO6 with the Na content of x=0,0.02,0.03,0.07 and 0.17 have been synthesized by sol–gel method. Effects of hole doping on the crystal structure and magnetic properties of Sr2FeMoO6 have been investigated by means of X-ray powder diffraction (XRD) and magnetic measurements. The XRD pattern indicates that all the samples are of single phase and belong to the space group I4/m. Due to the smaller ionic radius of Na+ than that of Sr2+ ions, the lattice constants and unit cell volume of the compound decrease slightly with x. The degree of cation ordering in the Na-doped Sr2FeMoO6 compounds shows a non-monotonic variation with the doping level, increasing from x=0 to x=0.03 and decreasing slightly with further increase of the doping. In contrast to the composition dependence of the degree of ordering, the Curie temperature of the compound decreases at low doping level and increases at high doping level. The saturation magnetization of the compound increases with x for x<0.17. Similar to the electron-doped Sr2FeMoO6, provided that the doped hole enters selectively the spin-down band, the variation of the saturation magnetization can be explained in light of the ferrimagnetic model (FIM). PACS 61.72.Ww; 75.50.-y; 75.30.Cr; 75.50.Gg 相似文献
4.
Pedro A. Montano Yinwan Li J.F. Mitchell P.E. Mijnarends S. Kaprzyk A. Bansil 《Journal of Physics and Chemistry of Solids》2004,65(12):1999-2004
Magnetic Compton profiles have been measured for the colossal magnetoresistance manganites La1.2Sr1.8Mn2O7 and La0.7Sr0.3MnO3, and for magnetite Fe3O4, along various crystallographic directions, over a wide range of temperatures and magnetic fields. The experimental results are interpreted via first-principles computations for the double layer manganite, La1.2Sr1.8Mn2O7, and by using a simple model involving atomic d-orbitals and free electrons for the other two compounds. For all three materials a preference for the occupation of eg orbitals is found, particularly, for orbitals of dx2−y2 symmetry. An itinerant electron contribution is adduced at all temperatures in magnetite; such a contribution also appears in La1.2Sr1.8Mn2O7, but it is present only at low temperatures in La0.7Sr0.3MnO3. 相似文献
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The EPR-spectra of the2 S 1/2-state of Pb3+-ions in different host lattices are characterized by a pronounced hyperfine splitting constantA, a smallg-shift from the free spin valueg e and an appreciable amount of superhyperfine interaction. The unpaired 6s-electron of the Pb3+-ion is predominantly found in molecular orbitals built from the central 6s-function and orbitals of the sorrounding ligand ions. This assumption leads to correct estimates of theg-shift (Watanabe theory) and the reduction ofA due to covalent bonding effects. Theg- andA-values are shown to be correlated. 相似文献
7.
X.J. Liu Q.J. Huang S.Y. Zhang A.H. Luo C.X. Zhao 《Journal of Physics and Chemistry of Solids》2004,65(7):1247-1251
The thermal diffusivity has been investigated in double perovskite Sr2MMoO6 (M=Fe, Mn and Co) by means of the mirage effect. We have found that the thermal diffusivity of metallic Sr2FeMoO6 is 0.39 cm2/s, which is larger than that (0.33 cm2/s) of insulating Sr2MnMoO6 and Sr2CoMoO6. We further investigate the substitution effects of the La3+ ions for the Sr2+ ions in Sr2FeMoO6 and Sr2MnMoO6, and have found that the thermal diffusivities of both samples significantly increase with the La concentration. Such an enhancement of the thermal diffusivities has been ascribed to occupation of the extra itinerant electrons on the conduction Mo4d band. 相似文献
8.
Magnetoresistance material Sr2FeMoO6 with double perovskite structure was synthesized by microwave sintering method using SrCO3, Fe2O3 and MoO3 as raw materials, with MnO2 for microwave absorber. The phase structure, magnetic and electrical transport properties were investigated by X-ray powder diffraction (XRD) and vibrating-sample magnetometer. XRD analysis shows that the as-synthesized sample is Sr2FeMoO6 with tetragonal crystal structure and I4/mmm space group. The unit cell parameters are a=0.5587 nm, c=0.7894 nm, volume=0.2464 nm3. The calculated grain size of the sample is 31.62 nm, which is obtained by the Scherrer formula using the diffraction data. Magnetism testing results show that the sample Sr2FeMoO6 is ferromagnetic with the magnetic transition temperature of about 380 K. Under 1.0 T magnetic field, the saturation and spontaneous magnetization of Sr2FeMoO6 is 1.25 μB/f.u. and 1.00 μB/f.u. at room temperature. The magnetoresistance ratio of the sample is 28%. Electrical transport properties testing results indicate that the sample exhibits typical semiconductor behavior. The conductive mechanism of Sr2FeMoO6 is highly dependent on temperature: within the temperature range of 100–300 K, the mechanism is attributed to the small polaron variable-range hopping model; while it is ascribed to the adiabatic small polaron model within the temperature range of 80–100 K. 相似文献
9.
Q. Zhang X.M. Feng G.Y. Liu Y.G. Xiao Y. Zhang J.K. Liang 《Solid State Communications》2005,133(4):223-227
We have investigated the crystal structure, magnetization and magnetoresistance of the double perovskite compounds Sr2(Fe1−xVx)MoO6 (0≤x≤0.1). The lattice constants and the cation ordering decrease monotonously with the V content. The Curie temperature, saturation magnetization and low field magnetoresistance of the compounds decrease with increasing x due to the reduced degree of ordering. The resistivity of Sr2FeMoO6 and lightly doped samples shows semiconductive behavior, while the samples with higher doping levels exhibit a semiconductor-metal transition around 80 K. 相似文献
10.
The detailed orbital-decomposed electronic structures and magnetic properties of the double perovskite Sr2FeReO6 have been studied using the first-principles projector augmented wave (PAW) potential within the generalized gradient approximation (GGA). Both occupied and unoccupied s and three p states of Fe3+ ion are located far away from the Fermi level, while all up-spin states and most down-spin states are completely filled for the s and three p states of Re5+ ion. The octahedral crystal field of the oxygen atoms around transition-metal (TM) sites splits the five-fold degenerate d states of the free TM atoms into triply degenerate t2g states with smaller bonding-antibonding splitting and doubly degenerate eg states with larger bonding-antibonding splitting. The Fe3+ and Re5+ ions are in the states (3d5, S=5/2) and (5d2, S=1) with magnetic moments 3.70 and −0.86μB, respectively and thus antiferromagnetic coupling via oxygen between them. There are no direct interactions between two nearest Fe-Fe or Re-Re pairs, whereas along each Fe-O-Re-O-Fe or Re-O-Fe-O-Re chains, the hybridizations between Fe 3d and 4s, O 2s and 2p, as well as Re 5p, 5d and 6s orbitals are fairly significant. 相似文献
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The structural, electronic and magnetic properties of the double perovskite Pb2FeMoO6 have been studied by using the first-principle projector augmented wave (PAW) potential within the generalized gradient approximation (GGA) as well as taking into account the on-site Coulomb repulsive interaction (GGA+U). Similar to Sr2FeMoO6 and Pb2FeReO6, the optimized crystal structure of Pb2FeMoO6 is a body-centered tetragonal (BCT) with the lattice constants a=b=5.60 Å and c=7.94 Å. The two axial TM?O distances are slightly larger than the four equatorial TM?O distances, which shows that the Jahn–Teller structural distortion exists in FeO6 and MoO6 octahedra. The half-metallic ferromagnetic nature implies a potential application of this new compound in spintronics devices. The Fe3+ and Mo5+ ions are in the states (3d5, S=5/2) and (4d1, S=1/2) with magnetic moments 3.87 and ?0.38μB respectively and thus there exists an antiferromagnetic coupling via oxygen between them. 相似文献
13.
X.M. Feng G.H. Rao G.Y. Liu W.F. Liu Z.W. Ouyang J.K. Liang 《Solid State Communications》2004,129(11):753-755
The effects of A-site average cation size 〈rA〉 and anti-site defects on Curie temperature (TC) and room-temperature magnetoresistance (MR) in (Sr2−xBax)FeMoO6 (x=0, 0.4 and 1.6) have been investigated. By Ba doping, not only the room-temperature MR but also the TC have been enhanced. The larger MR in the Ba-doped samples compared with the prototype Sr2FeMoO6 is associated with the lower saturation field. The optimization of TC and MR in (Sr1.6Ba0.4)FeMoO6 other than in the reported (Sr0.4Ba1.6)FeMoO6 can be understood according to the two competing effects: anti-site defects and chemical pressure. 相似文献
14.
A. Koizumi T. Nagao N. Hiraoka T. Arima Y. Murakami 《Journal of Physics and Chemistry of Solids》2005,66(12):2183-2191
Magnetic Compton profiles (MCP) have been measured in the [100], [110] and [001] directions on the single crystals of La2−2xSr1+2xMn2O7 (x=0.30, 0.35 and 0.42) at 10 K. The occupation numbers in t2g and two eg type orbitals (x2−y2 and 3z2−r2) of Mn-3d state are evaluated from the line-shape analysis of MCP's in the [001] direction by using theoretical profiles derived from the ab initio calculations for (MnO6)8−cluster. It has been found that the eg state is dominated by the x2−y2 type orbital at every hole concentration, x, and the 3z2−r2 type orbital population decreases with increasing x. From the result, the connections of eg orbital state with the electron correlation effect, exchange interactions, lattice distortion and electronic inhomogeneity are discussed. 相似文献
15.
SCF-Xα SW MO calculations on metal core ion hole states and X-ray emission (XES) and X-ray photoelectron (XPS) transition states of the non- transition metal oxidic clusters MgO610?, AlO45? and SiO44? show relative valence orbital energies to be virtually unaffected by the creation of valence orbital or metal core orbital holes. Accordingly, valence orbital energies derived from XPS and XES are directly comparable and may be correlated to generate empirical MO diagrams. In addition, charge relaxation about the metal core hole is small and valence orbital compositions are little changed in the core hole state. On the other hand, for the transition metal oxidic clusters FeO610?, CrO69? and TiO68? relative valence orbital energies are sharply changed by a metal core orbital or crystal field orbital hole, the energy lowering of an orbital increasing with its degree of metal character. Consequently O 2p nonbonding → M 3d-O 2p antibonding (crystal field) energies are reduced, while M 3d bonding → O 2p nonbonding and M 3d-O 2p antibonding → M 4s,p-O 2p antibonding (conduction band) energies increase. Charge relaxation about the core hole is virtually complete in the transition metal oxides and substantial changes are observed in the composition of those valence orbitals with appreciable M 3d character. This change in composition is greater for e g than for t2g orbitals and increases as the separation of the eg crystal field (CF) orbitals and the O 2p nonbonding orbital set decreases. Based on the hole state MO diagrams the higher energy XPS satellite in TiO2 (at about 13 eV) is assigned to a valence → conduction band transition. The UV PES satellites at 8.2 eV in Cr2O3 and 9.3 eV in FeO are tentatively assigned to similar transitions to conduction band orbitals, although the closeness in energy of the crystal field and O 2p nonbonding orbitals in the valence orbital hole state prevents a definite assignment on energy criteria alone. However the calculations do clearly show that charge transfer transitions of the eg bonding → eg crystal field orbital type would generally occur at lower energy than is consistent with observed satellite structure.A core electron hole has little effect upon relative orbital energies and is only slightly neutralized by valence electron redistribution for MgO and SiO2. For the transition metal oxides a core hole lowers the relative energies of M3d containing orbitals by large amounts, reducing O → M charge transfer and increasing M 3d crystal field → conduction band energies. Large and sometimes overcomplete neutralization of the core hole is observed, increasing from CrO69? to FeO610? to TiO68?. as the O → M charge transfer energy declines.High energy XPS satellites in TiO2 may be assigned to O 2p nonbonding → conduction band transitions while lower energy UV PES satellites in FeO and Cr2O3 arise from crystal field or O 2p nonbonding → conduction band excitations. Our “shake-up” assignment for FeO610?, CrO69? and TiO68? are less than definitive because no procedure has yet been developed to calculate “shake-up” intensities resulting from transitions of the type described. However the results do allow a critical evaluation of earlier qualitative predictions of core and valence hole effects. First, we find that the comparison of hole or valence state ionic systems with equilibrium distance systems of higher nuclear and/or cation charge (e.g. the comparison of the FeO610? Fe 2p core hole state to Co3O4) is dangerous. For example, larger MO distances in the ion states substantially reduce crystal field splittings. Second, core and CF orbital holes sharply reduce O → M charge transfer energies, giving 2eg → 3eg energy separations which are generally too small to match observed satellite energies. Third, highest occupied CF-conduction band energies are only about 4–5 eV in the ground states, but increase to about 7–11 eV in the core and valence hole states of the transition metal oxides studied. The energetic arguments presented thus support the idea of CF and/or O 2p nonbonding → conduction band excitations as assignments for “shake-up” satellites, at least in oxides of metals near the beginning of the transition series. 相似文献
16.
A. Takahashi H. Shiba 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,5(3):413-417
The optical absorption in ferromagnetic metal La1-xSrxMnO3 is anomalous; it has a wide-range absorption up to about 1 eV even at zero temperature. Since 3d electrons in La1-xSrxMnO3 partially fill doubly degenerate eg orbitals, the orbital degrees of freedom are crucial to understand this metallic system. We argue that the interband transition
within eg orbitals is important in the optical absorption. The optical spectrum is modified also by the inter-orbital Coulomb interaction.
We have examined perturbatively the effect of the Coulomb interaction on the spectrum. Available experiments are discussed
by comparing with the present results.
Received: 13 February 1998 / Accepted: 17 March 1998 相似文献
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A tight-band model is employed for thee 2g orbitals in manganites. It is shown that a large intra-atomic Hund couplingJ H and the resulting double-exchange mechanism lead to antiferromagnetic ordering along one of the cubic axes, stabilized by the cooperative Jahn-Teller effect, which further decreases the band energy, of the electrons. As a result, LaMnO3 is a band insulator built of 2D ferromagnetic layers. The critical concentration (x c ?0.16) for the onset of ferromagnetic and metallic behavior at low temperatures in La1?x Sr x MnO3 and the phase transition are treated in a percolation approach. 相似文献
19.
Electronic structures and magnetoelectric properties of tetragonal BaFeO3: an ab initio density functional theory study
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First-principles calculations have been performed to investigate the ground state electronic properties of BaFeO3 (BFO). Local spin density approximation (LSDA) plus U (LSDA+U) treatment modified the metallic behaviour to insulated one with a band gap of 4.12eV. The spontaneous polarization was found to be 89.3\muC/cm2 with Berry phase scheme in terms of the modern theory of polarization. Fe-3d eg were split into two singlet states (dz2 and d x2-y2}), and Fe-3d t2g were split into one doublet states(dxz and dyz) and one singlet states(dxy) after Fe and O displaced along the c axis. Meanwhile the occupation numbers of dz2, dyz, dxz and OT pz (on the top of Fe) were increased at the expense of those in xy plane. Our results showed that it was the sensitivity of hybridization to ferroelectric distortions, not just the total change of hybridization, that produced the possibility of ferroelectricity. Moreover, the increasing occupation numbers of OT pz and Fe dz2 favoured the 180o coupling between Fe-3d eg and Fe-3d t2g, leading to ferromagnetic ordering, which has been confirmed by the increase of magnetic moment by 0.13μB per formula unit in the polarized direction. Hence, the magnetization can be altered by the reversal of external electric field. 相似文献
20.
Toshiaki Yamaguchi Hironori Suwaki Koichi Kikuta Shin-ichi Hirano 《Solid State Communications》2005,133(2):71-75
We fabricated the ordered Sr2FeMoO6 (SFMO) thin film with a double perovskite structure using the chemical solution deposition (CSD) method. The highly c-axis oriented SFMO thin film with a high degree of Fe/Mo ordering was successfully synthesized on an MgO (001) substrate by optimizing processing conditions. The precise preparation process control of the SFMO precursor solution leads to a typical magnetoresistance effect in a low magnetic field at room temperature. 相似文献