Electronic and magnetic structure studies of double perovskite Sr2CrReO6 by first-principles calculations

First-principles calculations have been performed to study the electronic band structure and ferromagnetic properties of the double perovskite Sr₂CrReO₆. The density of states (DOS), the total energy, and the spin magnetic moment were calculated. The calculations reveal that the Sr₂CrReO₆ has a stable ferromagnetic ground state and the spin magnetic moment per molecule is 1.0 μ_B, in good agreement with the experimental value. By analysis of the band structure, we propose that the ordered double perovskite Sr₂CrReO₆ is a strong candidate for half-metallic ferromagnet.     

Crystal structural, magnetic and electrical transport properties of CeKFeMoO6 double perovskite

The crystal structural, magnetic and electrical transport properties of double perovskite CeKFeMoO₆ have been investigated. The crystal structure of the compound is assigned to the monoclinic system with space group P2₁/n and its lattice parameters are a=0.55345(3) nm, b=0.56068(2) nm, c=0.78390(1) nm, β=89.874(2). The divergence between zero-field-cooling and field-cooling M-T curves demonstrates the anisotropic behavior. The Curie temperature measured from C_p-T curve is about 340 K. Isothermal magnetization curve shows that the saturation and spontaneous magnetization are 1.90 and 1.43 μ_B/f.u. at 300 K, respectively. The electrical behavior of the sample shows a semiconductor. The electrical transport behavior can be described by variable range hopping model. Large magnetoresistance, −0.88 and −0.18, can be observed under low magnetic field, 0.5 T, at low and room temperature, respectively.     

Synthesis, crystal structure, dielectric and optical properties of a new rare earth double perovskite: Ca2CeNbO6

In order to develope and understand the phenomena involved in producing advanced materials, a rare earth double perovskite oxide calcium cerium niobate, Ca₂CeNbO₆ (CCN) is synthesized for the first time. The x-ray diffraction pattern of CCN at room temperature (300K) shows orthorhombic perovskite structure, with the lattice parameters, a=9.36Å, b=6.61Å and c=5.88Å and α=β=γ= 90°. A scanning electron micrograph shows the formation of grains with average size ∼2μm. Impedance spectroscopy and Fourier transform infrared spectroscopy are applied to investigate the dielectric and optical properties of CCN. The frequency-dependent electrical data are analyzed in the framework of the conductivity and modulus formalisms. The experimental data of real part of dielectric permittivity (ε′) and imaginary part of electric modulus (M″) are fitted with Davidson-Cole equation to explore the idea of dielectric relaxation (conduction) mechanism in CCN. The frequency-dependent conductivity spectra follow a power law. The scaling behaviour of imaginary electric modulus (M″) suggests that the relaxation describes the same mechanism at various temperatures.     

Preparation and characterisation of the double perovskite Sr2TaMnO6

The presented compound, Sr₂TaMnO₆, has a weak, disordered magnetic structure. The metal oxide was prepared under high isostatic oxygen pressure. The doubling of the perovskite structure was proven with electron diffraction and powder neutron diffraction. Combining neutron- and X-ray diffraction data, the room-temperature structure was modelled with the Rietveld method. Both octahedral positions are partially occupied by Mn and Ta, but with different Mn/Ta ratios. AC- and DC-magnetic measurements indicate a magnetic transition at about 17 K and the AC-magnetic susceptibility, both real and imaginary part, is frequency dependent, suggesting that the material has a spin-glass feature. The magnetic spins freeze during a wide temperature range and a possible explanation is a competative situation between the double exchange (ferromagnetism) and the super-exchange (anti-ferromagnetism).     

Large enhancement of room temperature magnetoresistance in Ag-added La0.67(Ca0.65Ba0.35)0.33MnO3

The electrical and magnetoresistant properties of La_0.67(Ca_0.65Ba_0.35)_0.33MnO₃/Ag_x (abbreviated by LCBMO/Ag_x) have been studied. The results show that Ag addition causes a decrease of resistivity dramatically and especially induces a large enhancement of room temperature magnetoresistance (MR). The room temperature MR ratio for x=0.27 sample in 10 kOe magnetic field is 41%, almost 20 times larger than that for x=0 sample. This enhancement is related to that the Curie temperature (T_c) of the sample is near room temperature, as well as the significant reduction of resistivity. The good fits of experimental results for x=0.27 sample to Brillouin function indicate that the MR behavior in the Ag added LCBMO is induced by the spin-dependent hopping of the electrons between the spin clusters, which is an intrinsic property of the CMR materials.     

Influence of preparation method on SrMoO4 impurity content and magnetotransport properties of double perovskite Sr2FeMoO6 polycrystals

The magnetic and electric properties of the Sr₂FeMoO₆ compound produced under different preparation conditions were studied. Depending on the preparation condition, a strong variation in the nonmagnetic SrMoO₄ impurity content was found, which in turn determined the metallic or semiconducting behavior of the resistivity of the Sr₂FeMoO₆ compound. There was also evidence that SrMoO₄ played a crucial role in modifying the low magnetic field intergrain tunneling magnetoresistance in Sr₂FeMoO₆. In addition, we have established a simple method to prepare the single phase Sr₂FeMoO₆ polycrystals.     

Metastable structure and properties of (La0.73Bi0.27)0.67Ca0.33MnO3

The electrical transport and magnetic properties of high Bi doped (La_0.73Bi_0.27)_0.67Ca_0.33MnO₃ are studied at the temperature and magnetic field ranges from 10 to 300 K and 0 to 3 T. Significant temperature and magnetic field hystereses are observed in both resistivity and magnetization measurements. Meanwhile, an enhanced magnetoresistance effect, within a wide temperature window, is obtained in the (La_0.73Bi_0.27)_0.67Ca_0.33MnO₃. The hysteresis and enhanced magnetoresistance are discussed based on an inhomogeneous metastable structure related to the Bi dopant.     

Magnetic phases in lanthanum-strontium manganite-cobaltite La1.25Sr0.75MnCoO6

Two methods—the solid-phase high-temperature (1300 °C) and the liquid-phase low-temperature (750 °C) routes—were used to synthesize the complex oxide La_1.25Sr_0.75MnCoO₆, which has the structure of rhombohedral perovskite and is characterized by a disordered distribution of Mn and Co in structural sites. It was found by means of X-ray absorption near edge spectroscopy (XANES) at the K-edge that mixed valence states of Co²⁺/Co³⁺ and Mn³⁺/Mn⁴⁺, exist in both phases. Measurements of dc magnetization and real (χ′) and imaginary (χ″) parts of the ac susceptibility showed that the magnetic properties of these oxides are determined by a ferromagnetic transition at T_C=217 K and a frequency-dependent transition at T_g<100 K. The high frequency dependence of T_g is indicative of the cluster-glass behavior of La_1.25Sr_0.75MnCoO₆ (7 5 0) at T<T_C within the ferromagnetic state.     

Crystal and magnetic structure of Ho2NiGe6

The crystal and magnetic structure of Ho₂NiGe₆ was studied by powder neutron diffraction. The paramagnetic neutron diffraction data confirmed the Ce₂CuGe₆-type crystal structure reported earlier for this compound. Below the Néel temperature equal to 11 K the Ho magnetic moments form a uniaxial antiferromagnetic ordering. The Ho magnetic moments equal to 8.16(7)μ_B at 1.5 K are parallel to the b-axis. The data are compared with those published for HoNi_0.46(6)Ge₂.     

Field dependent susceptibility of the 2D ferromagnet (CH3NH3)2CuCl4 in the paramagnetic phase

The temperature dependence of the ac susceptibility (χ) at constant applied magnetic field was investigated in the paramagnetic region of the quasi-2D ferromagnet (CH₃NH₃)₂CuCl₄. Above the Curie temperature (T_C=8.85 K) a maximum in the χ(T,H) curves was observed at T_m(H). The temperature at the maximum increases with increasing applied field. This anomaly is related to short range fluctuations close the order transition. The behavior of T_m(H) is governed by the gap exponent of the scaling function (Δ=γ+β). We found Δ=2.2±0.1 in very good agreement with the previously known values of γ and β.     

Electronic structure and magnetic properties of Laves phase LuFe2

The electronic structure and magnetic properties of the Laves phase of LuFe₂ with C14, C15, and C36 structures has been investigated using the full-potential linearized augmented plane wave method. In order to study the stability of magnetic phases, nonmagnetic and spin-polarized calculations for ferromagnetic ordering were performed. It is found that the ferromagnetic hexagonal C14 phase is the ground-state structure and the C15 phase is an intermediate state between the C14 and C36 structures. There is an increase in the average magnetic moment on the Fe sites in the order of C15 →C14 →C36 structures, whereas the Lu-moment is not significantly different. We also find that there exist both localized and itinerant d electrons, resulting in antiferromagnetic ordering in the three structures. Their density-of-states, equilibrium volumes, and elastic properties are discussed, which is important for the understanding of the physical properties of LuFe₂ and may inspire future experimental research.     

Effects of Ag addition on electrical transport and magnetic properties of La0.67Ba0.33Mn0.88Cr0.12O3

We have prepared a series of polycrystalline manganites with the nominal compositions, La_0.67Ba_0.33Mn_0.88Cr_0.12O₃/Ag_x (LBMCO/Ag_x) (x   is the mole fraction) with x=0$x=0$, 0.05, 0.1, 0.15, 0.2, 0.23, 0.27, 0.3, 0.35. The X-ray diffraction patterns show that the samples with x>0.05$x>0.05$ are two-phase composites. The Ag addition in LBMCO improves the properties of grain surfaces/boundaries and reduces the resistivity of the composites. For x=0.30$x=0.30$ sample, a minimum resistivity is obtained and a maximum room temperature magnetoresistance up to −54.5% was observed at 288 K, 1 T field. The room temperature T_C and the reduced resistivity are responsible for the enhancement of room temperature MR.     

Crystal structure and ferromagnetism of (n-C3H7)4N[CoFe(dto)3] (dto=C2O2S2)

Recently, we have discovered a new type of first order phase transition around 120 K for (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] (dto=C₂O₂S₂), where the charge transfer transition between Fe^II and Fe^III occurs reversibly. In order to elucidate the origin of this peculiar first order phase transition. Detailed information about the crystal structure is indispensable. We have synthesized the single crystal of (n-C₃H₇)₄N[Co^IIFe^III(dto)₃] whose crystal structure is isomorphous to that of (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃], and determined its detailed crystal structure. Crystal data: space group P6₃, a=b=10.044(2) Å, c=15.960(6) Å, α=β=90°, γ=120°, Z=2 (C₁₈H₂₈NS₆O₆FeCo). In this complex, we found a ferromagnetic transition at T_c=3.5 K. Moreover, on the basis of the crystal data of (n-C₃H₇)₄N[Co^IIFe^III(dto)₃], we determined the crystal structure of (n-C₃H₇)₄N[Fe^IIFe^III(dto)₃] by simulation of powder X-ray diffraction results.     

Sterically stabilized water based magnetic fluids: Synthesis,structure and properties

Magnetic fluids (MFs), prepared by chemical co-precipitation followed by double layer steric and electrostatic (combined) stabilization of magnetite nanoparticles dispersed in water, are presented. Several combinations of surfactants with different chain lengths (lauric acid (LA), myristic acid (MA), oleic acid (OA) and dodecyl-benzene-sulphonic acid (DBS)) were used, such as LA+LA, MA+MA, LA+DBS, MA+DBS, OA+DBS, OA+OA and DBS+DBS. Static light scattering, transmission electron microscopy, small angle neutron scattering, magnetic and magneto-rheological measurements revealed that MFs with MA+MA or LA+LA biocompatible double layer covered magnetite nanoparticles are the most stable colloidal systems among the investigated samples, and thus suitable for biomedical applications.     

Magnetic and structural studies of the double perovskites Ba2REMoO6

A magnetic, electronic and structural study of the double perovskites Ba₂REMoO₆ (RE=Sm, Eu, Gd, Dy) has been performed. All materials crystallise in the cubic symmetry space group and the cell volume decreases as RE varies from Sm to Dy in accordance with Vegard's law. An antiferromagnetic transition is observed below T_N=130 and 112 K for RE=Sm and Eu, respectively. The Néel temperatures of these ordered rare earth molybdenum double perovskites are much higher than previously observed in double perovskites containing Eu or Sm and a 4d or 5d transition metal arranged in an ordered rock salt configuration. The high Néel temperatures arise due to a strong superexchange magnetic interaction via the Mo-O-RE-O-Mo pathway. All of the phases are electronically insulating and there is no evidence of magnetoresistance at any temperature.     

Influences of Bi2O3/V2O5 Additives on the Microstructure and Magnetic Properties of Lithium Ferrite

Lithium ferrite materials with different concentrations of Bi2O3 and V2O5 additives are prepared by the conventional ceramic technique. The x-ray diffraction analysis proves that the additives do not affect the final crystal phase of the lithium ferrite in our testing range. Both Bi2O3 and V2O5 additives could promote densification and lower sintering temperature of the lithium ferrite. The average grain size first increases, and then gradually decreases with the Bi2O3 content. The maximal grain size appears with 0.25 wt% Bi2O3. The average grain size first increases, and then is kept almost unchanged with the V2O5 content. The maximal average grain size of the samples with V2O5 additive is much smaller than that of the samples with Bi2O3 additive. Furthermore, the V2O5 additive more easily enters the crystal lattice of the lithium ferrite than the Bi2O3 additive. These characteristics evidently affect the magnetic properties, such as saturation flux density, ratio of remanence Br to saturation flux density Bs, and coercive force of the lithium ferrite. The mechanisms involved are discussed.     

Effects of precursor types of Fe and Ni components on the properties of NiFe2O4 powders prepared by spray pyrolysis

The effects of the precursor types of Ni and Fe components on the morphology, mean size, and magnetic property of NiFe₂O₄ powders prepared by spray pyrolysis from the spray solution, with citric acid were studied. The precursor powders with hollow and thin wall structure turned to the nano-sized NiFe₂O₄ powders after post-treatment at a temperature of 800 °C. The nickel ferrite powders obtained from the spray solution with ferric chloride had nanometer sizes and narrow size distributions irrespective of the types of nickel precursor. The nickel ferrite powders obtained from the spray solution with ferric nitrate and nickel chloride also had nanometer size and narrow size distribution. The saturation magnetizations of the NiFe₂O₄ powders changed from 37 to 42 emu/g according to the types of the Fe and Ni precursors. The saturation magnetizations of the NiFe₂O₄ powders increased with increasing the Brunauer-Emmett-Teller (BET) surface areas of the powders.     

Synthesis and magnetic properties of antiferromagnetic Co3O4 nanoparticles

Antiferromagnetic Co₃O₄ nanoparticles with diameter around 30 nm have been synthesized by a solution-based method. The phase identification by the wide-angle X-ray powder diffraction indicates that the Co₃O₄ nanoparticle has a cubic spinel structure with a lattice constant of 0.80843(2) nm. The image of field emission scanning electron microscope shows that the nanoparticles are assembled together to form nanorods. The magnetic properties of Co₃O₄ fine particles have been measured by a superconducting quantum interference device magnetometer. A deviation of the Néel temperature from the bulk is observed, which can be well described by the theory of finite-size scaling. An enhanced coercivity as well as a loop shift are observed in the field-cooled hysteresis loop. The exchange bias field decreases with increasing temperature and diminishes at the Néel temperature. The training effect and the opening of the loop reveal the existence of the spin-glass-like surface spins.     

Phase boundary between uniaxial and planar ferromagnets in layered perovskite manganite La2−2xSr1+2xMn2O7 (0.313⩽x⩽0.350)

We have examined magnetizations as a function of temperature and magnetic field in layered perovskite manganites La_2−2xSr_1+2xMn₂O₇ single crystals (x=0.313, 0.315, 0.318, 0.320 and 0.350) in order to determine the phase boundary between two ferromagnets (one is an uniaxial ferromagnet whose easy axis is parallel to the c-axis and the other is a planar ferromagnet whose easy axis is within the ab-plane) and following results are obtained: (i) all the present manganites exhibit magnetic transitions from a ferromagnet to a paramagnet at 76, 107, 116, 120 and 125 K for x=0.313, 0.315, 0.318, 0.320 and 0.350, respectively; (ii) for x=0.318, 0.320 and 0.350, the magnetic structure is a planar ferromagnet below Curie temperature; (iii) for x=0.313 and 0.315, the magnetic structure changes from an uniaxial to a planar ferromagnet at 66 and 85 K, respectively. From the results described above we have constructed the magnetic phase diagram of layered perovskite manganite La_2−2xSr_1+2xMn₂O₇ (0.313?x?0.350).     

Crystal structure and the magnetic properties of CeTiGe3

The crystal structure of the RTiGe₃ compounds (R=La, Ce and Pr) has been studied by X-ray powder diffraction methods; Rietveld refinement has been carried out on the La homologue. These compounds crystallise in the BaNiO₃ prototype structure, hP10-P6₃/mmc, also called the hexagonal perovskite (a=6.300(1), c=5.915(1) Å for LaTiGe₃). This seems to be the first example in which an intermetallic phase adopts such a structure type which can be considered as derived from the Ni₃Sn type (anti) by a distortion of the lattice and the occupation of the quasi-octahedral 2a site at the origin of the cell (0, 0, 0) by Ti. The composition, also confirmed by microprobe analyses, was found to be strictly 1:1:3 indicating that these are line compounds, forming very likely by a peritectoid reaction. The existence of homologous compounds has been established for the lighter rare earths La, Ce and Pr. Heat capacity and magnetisation data show that CeTiGe₃ orders ferromagnetically with a Curie temperature of nearly 14 K. On the other hand a−ln T variation of the magnetic part of the resistivity below 300 K is consistent with that expected for single impurity Kondo behaviour. CeTiGe₃ is thus an uncommon example of a ferromagnetic dense Kondo lattice.