Electronic transport and magnetic studies of La1−xCax−0.08Sr0.04Ba0.04MnO3

Lanthanum based mixed valence manganite system La_1−xCa_x−0.08Sr_0.04Ba_0.04MnO₃ (LCSBMO; x=0.15, 0.24 and 0.33) synthesized through the sol-gel route is systematically investigated in this paper. The electronic transport and magnetic susceptibility properties are analyzed and compared, apart from the study of unit cell structure, microstructure and composition. Second order phase transition is observed in all the samples and significant difference is observed between the insulator to metal transition temperature (T_MI) and paramagnetic (PM) to ferromagnetic (FM) transition temperature (T_C). In contrast to the insulating FM behaviour usually observed in La_1−xCa_xMnO₃ (LCMO) for x=0.15, a clear insulator to metal transition is observed for LCSBMO for the same percentage of lanthanum. The temperature dependent resistivity of polycrystalline pellets, when obeying the well studied law ρ=ρ_o+ρ₂T² for T<T_MI, is observed to differ significantly in the values of ρ_o and ρ₂, with the electrical conductivity increasing with x. The variable range hopping model has been found to fit resistivity data better than the small polaron model for T>T_MI. AC magnetic susceptibility study of the polycrystalline powders of the manganite system shows the highest PM to FM transition of 285 K for x=0.33.     

Magnetic properties of compounds in the system CaBaCo4−x−yZnxAlyO7 (x=0,1,2, y=0,1)

The magnetic properties of four compounds in the series CaBaCo_4−x−yZn_xAl_yO₇ (x=0,1,2, y=0,1) were investigated. Using AC-susceptibility and DC-magnetometry, magnetic transitions (T_fs) were found for all four compositions in the range 50-3 K. The data from the AC measurements proved to be frequency dependent: T_f increases with higher frequencies. An energy-loss in the magnetic coupling, indicated as contributions in the imaginary part of the magnetic susceptibility (χ″), was seen for every compound and its maximum appeared just below the maximum χ′. Modelling the data with Arrhenius-, Vogel-Fulcher-, and the power-law made it possible to relate the four compounds to spin-glass materials. The Casimir-du Pré relation was used to extract average relaxation times at T_f. The DC magnetisations clearly show differences between field-cooled and zero-field-cooled measurements. None of the compounds exhibit any metamagnetic properties up to 8 T. A new method is presented to calculate the saturation fields using DC data. Relaxation measurements on three compounds indicate that the systems relax very fast, in contrast to spin-glasses. Aging does not affect the fast relaxations. The compounds are interpreted as disordered anti-ferromagnets with spin-glass features.     

EPR investigations of oxidation effects in (V1−xMox)2−δO3

Electron paramagnetic resonance (EPR) investigations has been carried out on the new family of molybdenum doped vanadium sesquioxides (V_1−xMo_x)_2−δO₃. The oxidation effects were monitored from the rate of paramagnetic V⁴⁺ created when the sample is exposed to the air. The effects of the oxidation time, sample temperature, and annealing at 1000 °C under a diluted hydrogen atmosphere on the EPR signal features are analyzed. The V⁴⁺ concentration in the oxidized samples is determined and the relaxation effects driven by the conduction electrons are pointed out from the thermal behaviour of the EPR line features. EPR spectra of all the oxidized samples also reveal a small ferromagnetic contribution strongly correlated with the V⁴⁺ content.     

Structure and magnetic properties of perovskite Sr2CuNbO6−δ

Single phase perovskite Sr₂CuNbO_6−δ with a high proportion of Cu¹⁺ ions and oxygen vacancies was synthesized by solid-state reaction. The structure was determined by Rietveld method with space group Pm3m. Isotropic g value was evaluated from electron spin resonance (ESR) measurements. The ESR result is consistent with that of magnetic susceptibility.     

Effect of chromium substitution in Ca4Mn3O10

Ca₄Mn_3−xCr_xO₁₀ compounds were synthesized in order to investigate the role of an isoelectronic substitution in the layered manganite. Induced structural changes are mainly described as a distortion of the two types of octahedra in the n=3 RP structure. The results indicate that Cr³⁺ is not the only significant valence state for chromium ions. Electrical and magnetic characterization allow to conclude that chromium does not favour the double exchange mechanism in these compounds.     

Magnetic properties and magnetic structures of Sr3−xCaxRu2O7

We have measured magnetization curves and powder neutron diffraction of double-layered Ruddlesden-Popper type ruthenate Sr_3−xCa_xRu₂O₇ (x=1.5, 2.0 and 3.0). The field dependence of the magnetization revealed that the transition field of metamagnetic transition along the b-axis shifted to lower fields and that the transition became broad with increasing Sr content. The slope of the magnetization curve also increased with increasing Sr content below the metamagnetic transition. These results indicate that an itinerant component is partly introduced by the Sr substitution. From the magnetic reflection, on cooling below T_N, an additional reflection was observed at (0 0 1) for each x, and the amplitude increased with decreasing temperature. The observed diffraction patterns are very similar to those of Ca₃Ru₂O₇. We conclude that the magnetic structure of the antiferromagnetic ordered phase is basically the same structure with that of Ca₃Ru₂O₇.     

Synthesis and crystallographic studies of garnet-type AgCa2Mn2V3O12 and NaPb2Mn2V3O12

High-purity powder specimens of AgCa₂Mn₂V₃O₁₂ and NaPb₂Mn₂V₃O₁₂ have been successfully synthesized by solid-state chemical reaction. The Rietveld refinements from X-ray powder diffraction data verified that these compounds have the garnet-type structure (space group , No. 230) with the lattice constant of a=12.596(2) Å for AgCa₂Mn₂V₃O₁₂ and a=12.876(2) Å for NaPb₂Mn₂V₃O₁₂. Calculation of the bond valence sum supported that Mn is divalent and V is pentavalent in these garnets. Estimation of the quadratic elongation and the bond angle variance showed that the distortions of the MnO₆ octahedra and the VO₄ tetrahedra are significantly suppressed. Our new results of AgCa₂Mn₂V₃O₁₂ and NaPb₂Mn₂V₃O₁₂ are compared to those of AgCa₂M₂V₃O₁₂ and NaPb₂M₂V₃O₁₂ (M=Mg, Co, Ni, Zn).     

Electrical and electrochemical properties of molten salt-synthesized Li4Ti5−xSnxO12 (x=0.0, 0.05 and 0.1) as anodes for Li-ion batteries

Submicron-sized polyhedral Li₄Ti_5−xSn_xO₁₂ (x=0.0, 0.05, and 0.1) materials were successfully prepared by a single-step molten salt method. The structural, morphological, transport and electrochemical properties of the Li₄Ti_5−xSn_xO₁₂ were studied. X-ray diffraction patterns showed the formation of a cubic structure with a lattice constant of 8.31 Å, and the addition of dopants follows Vegard's law. Furthermore, FT-IR spectra revealed symmetric stretching vibrations of octahedral groups of MO₆ lattice in Li₄Ti₅O₁₂. The formation of polyhedral submicron Li₄Ti_5−xSn_xO₁₂ particles was inferred from FE-SEM images, and a particle size reduction was observed for Sn-doped Li₄Ti₅O₁₂. The chemical composition of Ti, O and Sn was verified by EDAX. The DC electrical conductivity was found to increase with increasing temperature, and a maximum conductivity of 8.96×10⁻⁶ S cm⁻¹ was observed at 200 °C for Li₄Ti₅O₁₂. The galvanostatic charge–discharge behavior indicates that the Sn-doped Li₄Ti₅O₁₂ could be used as an anode for Li-ion batteries due to its enhanced electrochemical properties.     

Characterization of multiferroic LiTMxCu2−xO2 (TM=Ni and Zn) single crystals

We investigate a series of single crystalline samples of LiT_MxCu_2−xO₂ (T_M=Ni and Zn) grown by floating-zone technique. As-grown crystals showed a clear X-ray diffraction pattern of pure(single) phase. These crystals exhibit varied magnetic and electrical properties owing to successful incorporation of Ni and Zn dopants.     

Synthesis, structural and magnetic properties of spin ladder compound Ca1−xCoxCu2O3

MCu₂O₃ (M=Ca and Co) system has two-leg spin ladder structure similar to that of the prototype SrCu₂O₃ system except that the rungs are buckled with an angle of 123° and 105° for CaCu₂O₃ and CoCu₂O₃ compounds, respectively. We have synthesized powder samples of (Ca_1−xCo_x)Cu₂O₃ (x=0.00-1.00) by the solid state reaction method and their structural and magnetic properties have been investigated. All the synthesized compounds crystallize in orthorhombic structure with space group Pmmn. Lattice parameters of (Ca_1−xCo_x)Cu₂O₃ decrease with the increase in Co content. DC magnetic susceptibility χ(T) results of the end products CaCu₂O₃ and CoCu₂O₃ show antiferromagnetic transition (T_N) at 27 and 215 K, respectively. Co doping into (Ca_1−xCo_x)Cu₂O₃ enhances its T_N systematically with increasing Co concentration. The χ(T) of CoCu₂O₃ shows a broad transition with the peak temperature around 215 K and it was found to be field independent up to 90 kOe. The ambiguity concerning the transition was ruled out by recording the temperature dependent X-ray diffraction pattern on CoCu₂O₃ system, which indicated that there is no structural transition in the investigated temperature range of 115-300 K. Further, specific heat measurement on CoCu₂O₃ confirms the magnetic phase transition by the appearance of a sharp peak at 215 K.     

Growth and X-ray studies of (NaCl)x(KCl)y−x(KBr)1−y single crystals

Ternary mixed crystals of NaCl, KCl and KBr were grown by the melt method for the first time. Densities and refractive indices of all the grown crystals were determined and also used for the estimation of the bulk composition in the crystal. Lattice parameters and thermal parameters like Debye-Waller factor, mean square amplitude of vibration, Debye temperature and Debye frequency were determined from the X-ray powder diffraction data. The observed lattice parameters showed the existence of two phases in crystals with NaCl content greater than 0.1 mole fraction. The thermal parameters show a highly non-linear composition dependence. The results are reported.     

Magnetoelectric characterization of xNi0.75Co0.25Fe2O4-(1−x)BiFeO3 nanocomposites

Magnetoelectric (ME) nanocomposites containing Ni_0.75Co_0.25Fe₂O₄-BiFeO₃ phases were prepared by citrate sol-gel process. X-ray diffraction (XRD) analysis showed phase formation of xNi_0.75Co_0.25Fe₂O₄-(1−x)BiFeO₃ (x=0.1, 0.2, 0.3 and 0.4) composites on heating at 700 °C. Transmission electron microscopy revealed the formation of powders of nano order size and the crystal size was found to vary from 30 to 85 nm. Dispersion in dielectric constant (ε) and dielectric loss (tan δ) in the low-frequency range have been observed. It is seen that nanocomposites exhibit strong magnetic properties and a large ME effect. On increasing Ni_0.75Co_0.25Fe₂O₄ contents in the nanocomposites, the saturation magnetization (M_S) and coercivity (H_C) increased after annealing at 700 °C. The large ME output in the nanocomposites exhibits strong dependence on magnetic bias and magnetic field frequency. The large value of ME output can be attributed to small grain size of ferrite phase of nanocomposite being prepared by citrate precursor process.     

Hydrothermal synthesis and structural characterization of (1−x)α-Fe2O3-xSnO2 nanoparticles

Structural and morphological characteristics of (1−x)α-Fe₂O₃-xSnO₂ (x=0.0-1.0) nanoparticles obtained under hydrothermal conditions have been investigated by X-ray diffraction (XRD), transmission Mössbauer spectroscopy, scanning and transmission electron microscopy as well as energy dispersive X-ray analysis. On the basis of the Rietveld structure refinements of the XRD spectra at low tin concentrations, it was found that Sn⁴⁺ ions partially substitute for Fe³⁺ at the octahedral sites and also occupy the interstitial octahedral sites which are vacant in α-Fe₂O₃ corundum structure. A phase separation of α-Fe₂O₃ and SnO₂ was observed for x≥0.4: the α-Fe₂O₃ structure containing tin decreases simultaneously with the increase of the SnO₂ phase containing substitutional iron ions. The mean particle dimension decreases from 70 to 6 nm, as the molar fraction x increases up to x=1.0. The estimated solubility limits in the nanoparticle system (1−x)α-Fe₂O₃-xSnO₂ synthesized under hydrothermal conditions are: x≤0.2 for Sn⁴⁺ in α-Fe₂O₃ and x≥0.7 for Fe³⁺ in SnO₂.     

Influence of lithium phosphate concentration and temperature on the electrical conduction of (76V2O5-24P2O5)1−X (Li3PO4)X glasses

Characterization of the (76V₂O₅-24P₂O₅)_1−X (Li₃PO₅)_X, where X=0.0,0.01,0.02,0.10 and 0.15, glass has been done using X-ray diffraction and differential thermal analysis (DTA). The dc conductivity of the glass samples was studied over a temperature range from 300 to 593 K. The temperature dependence of dc conductivity shows two regions. One at relatively high temperature range, above θ_D/2, and the other at relatively low temperature range, below θ_D/2. The I-V characteristics of the glasses have been studied as a function of both temperature and Li₃PO₄ content. The I-V characteristics exhibits threshold switching with differential negative resistance. It's found that both the threshold voltage (V_th) and threshold current (I_th) are dependent on the temperature and lithium phosphate concentration.     

Synthesis, structure and magnetic properties in the Nd2O3-Gd2O3 mixed system synthesized at 1200 °C

Phase relation studies in the Gd₂O₃-Nd₂O₃ system have been performed on (Gd_1−xNd_x)₂O₃ samples (0?x?1) with the purpose of performing a systematic study of the composition effects on their structural and magnetic properties. All the samples were synthesized by calcination of the related oxalates at 1200 °C in order to ensure the complete decomposition of the oxalates. Five phase regions, namely an A-type hexagonal, a B-type monoclinic, a C-type cubic solid solution and two biphasic mixtures of the former three phase fields were detected in this system. The magnetic susceptibility measurements showed the presence of antiferromagnetic interactions in all samples. The Curie-Weiss temperature shows a nonlinear dependence on concentration. Deduced effective magnetic moments are close to the free ion values.     

Electrical conduction and magnetoelectric effect of (x) BaTiO3 + (1−x) Ni0.92Co0.03Cu0.05Fe2O4 composites in ferroelectric rich region

Particulate composites with composition (x)BaTiO₃+(1−x)Ni_0.92Co_0.03Cu_0.05Fe₂O₄ in which x varies as 1, 0.85, 0.70, 0.55 and 0 (in mol%) were prepared by the conventional double sintering ceramic technique. The presence of two phases viz. ferromagnetic (Ni_0.92Co_0.03Cu_0.05Fe₂O₄) and ferroelectric (BaTiO₃) was confirmed by X-ray diffraction analysis. The dc resistivity and thermo-emf measurements were carried out with variation of temperature. The ac conductivity (σ_ac) measurements investigated in the frequency range 100 Hz to 1 MHz conclude that the conduction in these composites is due to small polarons. The variation of dielectric constant and loss tangent with frequency (20 Hz to 1 MHz) was studied. The static magnetoelectric conversion factor, i.e. dc (dE/dH)_H was measured as a function of intensity of applied magnetic field. The changes were observed in electrical properties as well as in magnetoelectric voltage coefficient as the molar ratio of the constituent phases was varied. A maximum value of magnetoelectric conversion factor of 536.06 μV/cm Oe was observed for the composite with 70% BaTiO₃+30% Ni_0.92Co_0.03Cu_0.05Fe₂O₄ at a dc magnetic field of 2.3 K Oe. The maximum magnetoelectric conversion output has been explained in terms of ferrite-ferroelectric content, applied static magnetic field and resistivity.     

Critical behaviour and magnetic properties of A-B spinel ZnxCu1−xFe2O4

Polycrystalline Zn_0.6Cu_0.4Fe₂O₄ ferrites have been prepared using a solid-state reaction technique. Their structural and magnetic properties have been studied, using X-ray diffraction and Mössbauer and magnetic measurements. These results have been compared to a more general theoretical study, on Zn_xCu_1−xFe₂O₄, based on mean field theory and high-temperature series expansions (HTSE), and extrapolated with the Padé approximant method. The nearest neighbour super-exchange interactions for the intra-site and the inter-site of Zn_xCu_1−xFe₂O₄ spinel ferrites, in the range 0≤x≤1, have been computed using the probability approach, based on Mössbauer data. The Curie temperature T_C is calculated as a function of Zn concentration. The theoretical results obtained are in good agreement with the experimental results obtained by magnetic measurements.     

Low field AC susceptibility study of intergranular critical current density in Mg-substituted CuBa2Ca3Cu4O12−y high temperature superconductors

Measurements of the a.c.susceptibility (χ=χ′+iχ″) have been made on the Mg substituted high T_C superconducting system, CuBa₂(Mg_xCa_1−x)₃Cu₄O_12−y (Cu-1234) with x=0, 0.10 & 0.20, at different values of the a.c.field amplitude. Estimates of the intergranular critical current density(J_C) made from the field dependent χ″-T curves show an improvement in the Mg-substituted Cu-1234 system. Results have been analysed in the light of the crystal structure and the superconducting anisotropy factor (γ=ξ_ab/ξ_c) of the Cu-1234 system. Lower superconducting anisotropy emanating from Mg substitution has been found to be significant, resulting in better superconducting properties.     

Simulation of Al/Fe disorder in Ca2FexAl2−xO5

A method is described which is capable of simulating disorder across a range of composition. It is used to study the disorder of iron and aluminium on the octahedral and tetrahedral cation sites in the Brownmillerite phase Ca₂Fe_xAl_2−xO₅. The arrangements with the lowest lattice energies are those that maximize the number of Fe³⁺ on octahedral sites throughout the composition range. An exchange of one Fe³⁺ with one Al³⁺, which results in a decrease in the number of iron ions on octahedral sites, increases the lattice energy by an amount that is dependent on x but is independent of the number of exchanges. The exchange of trivalent cations also results in an expansion in the b lattice direction, accompanied by a decrease in the a and c directions across the full range of composition.