Spin-Reorientation and M?ssbauer Study of Orthoferrite TbFe0.75Mn0.25O3

The crystallographic and magnetic properties of TbFe_0.75Mn_0.25O₃ powder were characterized by x-ray diffraction (XRD), Mössbauer spectroscopy, and vibrating-sample magnetometry (VSM). The crystal structure was found to be orthorhombic (space group Pbnm) with lattice constants a ₀=5.317 Å, b ₀=5.604 Å, and c ₀=7.598 Å, respectively. Mössbauer spectra of TbFe_0.75Mn_0.25O₃ have been taken at various temperatures ranging from 4.2 to 550 K. For Mössbauer spectra, we have fitted the spectra to a model based on a random distribution of Fe and Mn ions on the octahedral sites. The magnetic hyperfine fields of the four pattern (B ₀,B ₁,B ₂,B ₃) at 4.2 K are found to be H _hf=553, 544, 535, and 527 kOe, respectively. Isomer shift at room temperature is 0.25?C0.26 mm/s, which means that the valence state of Fe ions is ferric (Fe³⁺). A sudden change in both the magnitude of magnetic hyperfine field and its slope between 150 and 220 K suggests that magnetic phase transition related to the spin ordering takes place abruptly. The Néel temperature was determined to be T _N=550±5 K. The inflection points arising from a spin reorientation in the temperature dependence of the magnetic moment is observed. Its spin-reorientation transition is 70 K lower than that of 250 K for pure TbFeO₃.     

High-pressure Synthesis and Physical Characterization of Y-based 1222-type Niobio–Cuprate NbSr2(Y1.5Ce0.5)Cu2O10

We have successfully synthesized a new 1222-type Y-based niobio-cuprate for the first time with the chemical composition, NbSr₂(Y_1.5Ce_0.5)Cu₂O₁₀ by a high-pressure and high-temperature (HPHT) technique under different synthesis conditions. Good quality (nearly single-phase) Nb1222Y polycrystalline sample was synthesized at 1,450^?○C and 6 GPa for 2 h. It crystallizes in a tetragonal symmetry of space group I4/mmm with lattice parameters a=b=3.863(2) Å and c=28.687(5) Å, which is isostructural with the corresponding Ru1222Y (RuSr₂(Y_1.5Ce_0.5)Cu₂O₁₀) phase. The dc-magnetic susceptibility measurements show a mixed magnetic state at ～29 K, which is apparently associated with magneto-superconductivity. The magnetic moment (M) versus field (H) curve of Nb1222Y sample measured at 1.8 K looks like a hybrid and it normally ensembles for both superconducting and a ferromagnetic curve. As the field increases, a low field diamagnetic signal emerges. However, temperature dependence of electrical resistivity of Nb1222Y did not show superconductivity, but semiconducting nature with a relatively large magnetoresistance (～6% at almost entire region). No specific heat anomaly was observed at the vicinity of the transition temperature.     

Heat capacity, thermopower and magnetoresistance effects in multiferroic La0.5Bi0.5Mn0.5Fe0.5O3

We have carried out systematic investigations in perovskite multiferroic La_0.5Bi_0.5Mn_0.5Fe_0.5O₃ by means of X-ray diffraction, magnetisation, electrical resistivity, thermoelectric and heat capacity measurements. The magnetic behaviour of this composition is rather complex, though the magnetisation curve seems to be like a weak ferromagnetic material. However, there is no clear evidence of λ-anomaly in the heat capacity data down to 2 K, yet this behaviour corroborate the trends of semiconducting silicon below room temperature. The sensitivity of magnetic behaviour to the iron-manganese ratio is also demonstrated. In presence of an external field of 7 T, it exhibits a magnetoresistance of ?5 % at 130 K. The thermoelectric value increases linearly with decreasing temperature, and at room temperature the value is +85 μV/K, which is associated with the p-type polaronic conductivity.     

Cooperative Jahn–Teller transition in Li[Li x Mn2–x ]O4: a muon-spin rotaion/relaxation (μSR) view

The magnetic nature of the spinel antiferromagnet Li[Li _x Mn_2–x ]O₄ with x = 0?0.15 has been studied with muon-spin-rotation and relaxation (μ⁺SR) spectroscopy. Both weak transverse field and zero field μ⁺SR measurements indicate that the whole sample enters into a static disordered magnetic phase below T _N for all the samples measured; T _N = 61 K for LiMn₂O₄, whereas 27–23 K for the x = 0.05?0.15 samples. It was also clarified that both the field distribution width and the field fluctuation rate show a clear change at the Jahn–Teller transition temperature (T _JT = 280 K) for LiMn₂O₄, and a short-range cooperative JT distortion appears below 280 K even for Li[Li_0.15Mn_1.85]O₄.     

Synthesis and Superconductivity of New BiS2 Based Superconductor PrO0.5F0.5BiS2

We report synthesis and superconductivity at 3.7 K in PrO_0.5F_0.5BiS₂. The newly discovered material belongs to the layered sulfide based REO_0.5F_0.5BiS₂ compounds having a ZrCuSiAs-type structure. The bulk polycrystalline compound is synthesized by the vacuum encapsulation technique at 780 ^°C in a single step. Detailed structural analysis has shown that the as synthesized PrO_0.5F_0.5BiS₂ is crystallized in a tetragonal P4/nmm space group with lattice parameters a=4.015(5) Å, c=13.362(4) Å. Bulk superconductivity is observed in PrO_0.5F_0.5BiS₂ below 4 K from magnetic and transport measurements. Electrical transport measurements showed superconducting transition temperature (T _c ) onset at 3.7 K and T _c (ρ=0) at 3.1 K. The hump at T _c related to the superconducting transition is not observed in the heat capacity measurement and rather a Schottky-type anomaly is observed at below ～6 K. The compound is slightly semiconducting in a normal state. Isothermal magnetization (MH) exhibited typical type II behavior with a lower critical field (H _c1) of around 8 Oe.     

Structure and dielectric properties of Nd substituted Bi1.5MgNb1.5O7 ceramics

New pyrochlore ceramics Bi_(1.5?x)Nd _x MgNb_1.5O₇ with x = 0–0.6 were synthesized by doping Nd into the A site in the Bi_1.5MgNb_1.5O₇ pyrochlore. The Nd substituted Bi_1.5MgNb_1.5O₇ ceramics were prepared utilizing solid-state reaction and investigated by X-ray diffraction, scanning electron microscope and dielectric measurements. The crystal structure was characterized as single cubic pyrochlore phase for x ≤ 0.5 while the second phase appeared at x = 0.6. With increasing Nd content from 0 to 0.5, the lattice parameters decreased slightly and the average grain size pronouncedly reduced from ~7.5 to ~5.0 μm. The Nd incorporation resulted in attractive enhancement of the dielectric properties including the temperature coefficient of dielectric constant that was remarkably optimized with the lowest value of ?16 ppm/°C at x = 0.5. A good combination of low dielectric loss, superior temperature coefficient and high dielectric permittivity can be achieved for the composition of x = 0.5, which seems to be very appealing for the practical use in high-frequency multilayer ceramic capacitors.     

Electrochemical performance and thermal stability of GaF3-coated LiNi0.5Mn1.5O4 as 5 V cathode materials for lithium ion batteries

LiNi_0.5Mn_1.5O₄ coated with various amounts of GaF₃ were prepared and investigated as cathode materials for lithium ion batteries. The sample was characterized by X-ray diffraction, transmission electron microscopy, and energy-dispersive X-ray spectroscopy (EDX). The results indicated that the electrochemical performance of LiNi_0.5Mn_1.5O₄ was effectively improved by the GaF₃ coating. The 0.5 wt% GaF₃-coated LiNi_0.5Mn_1.5O₄ delivered a discharge capacity of 97 mAh g^?1 at 20 C (3000 mA g^?1), while the pristine sample only yielded 80 mAh g^?1 at 10 C. Meanwhile, the 0.5 wt% GaF₃-coated LiNi_0.5Mn_1.5O₄ exhibited an obviously better cycle life than the bare sample at 60 °C, delivering a discharge capacity of 120.4 mAh g^?1 after 300 cycles, 82.9 % of its initial discharge capacity, while the pristine only gave 75 mAh g^?1. At 0.1 C, the self-discharge of 0.5 wt% GaF₃-coated LiNi_0.5Mn_1.5O₄ is about 3.4 %, while the pristine is about 10.2 % after a 5-day rest at room temperature. Furthermore, GaF₃ coating greatly reduced the self-heating rate and improved the thermal stability of LiNi_0.5Mn_1.5O₄. These improvements were attributed to the GaF₃ layer not only increasing the electronic conductivity of the LiNi_0.5Mn_1.5O₄ but also effectively suppressing the reaction between the LiNi_0.5Mn_1.5O₄ and the electrolytes, which reduced the charge-transfer impedance and the dissolution of Ni and Mn during cycling.     

Magnetization reversal and tunable exchange bias behavior in Mn-substituted NiCr<Subscript>2</Subscript>O<Subscript>4</Subscript>

Single phase samples of Ni(Cr_1?xMn _x )₂O₄ (x = 0–0.50) were synthesized by using sol–gel route. Investigation of structural, magnetic, exchange bias and magnetization reversal properties was carried out in the bulk samples of Ni(Cr_1?xMn _x )₂O₄. Rietveld refinement of the X-ray diffraction patterns recorded at room temperature reveals the tetragonal structure for x = 0 sample with I4₁/amd space group and cubic structure for x ≥ 0.05 samples with \( {\text{Fd}\bar{3}\text{m}} \) space group. Magnetization measurements show that all samples exhibit ferrimagnetic behavior, and the transition temperature (T_C) is found to increase from 73 K for x = 0 to 138 K for x = 0.50. Mn substitution induces magnetization reversal behavior especially for 30 at% of Mn in NiCr₂O₄ system with a magnetic compensation temperature of 45 K. This magnetization reversal is explained in terms of different site occupation of Mn ions and the different temperature dependence of the magnetic moments of different sublattices. Study of exchange bias behavior in x = 0.10 and 0.30 samples reveals that they exhibit negative and tunable positive and negative exchange bias behavior, respectively. The magnitudes of maximum exchange bias field of these samples are found to be 640 and 5306 Oe, respectively. Exchange bias in x = 0.10 sample originates from the anisotropic exchange interaction between the ferrimagnetic and the antiferromagnetic components of magnetic moment. The tunable exchange bias behavior in x = 0.30 sample is explained in terms of change in domination of one sublattice moment over the other as the temperature is varied.     

MgAl2O4–LaCr0.5Mn0.5O3 composite ceramics for high temperature NTC thermistors

New negative temperature coefficient (NTC) ceramics based on xMgAl₂O₄–(1 ? x)LaCr_0.5Mn_0.5O₃ (x = 0, 0.4, 0.6, 0.8) composition were investigated. The composite ceramics (x = 0.4, 0.6, 0.8) consisted of two phases: a cubic spinel MgAl₂O₄ phase and an orthorhombic perovskite LaCr_0.5Mn_0.5O₃ phase isomorphic to LaCrO₃. The brighter regions were the LaCr_0.5Mn_0.5O₃ and the darker was the MgAl₂O_4. There was ion diffusion between these two phases. X-ray photoelectron spectroscopy analysis corroborated the presence of Cr³⁺, Cr⁴⁺, Mn³⁺ and Mn⁴⁺ ions on lattice sites, which may result in the hopping conduction. The ρ ₃₀₀ and B _400/800 constant increased with increasing MgAl₂O₄ content. The values of ρ ₃₀₀, B _400/800 and activation energy of NTC thermistors were in the range 1.76–1.22 × 10⁸ Ωcm, 2,646–8,711 K, 0.228–0.746 eV, respectively. This means the electrical properties can be adjusted to desired values, depending on the MgAl₂O₄ content.     

Magnetic State and Magnetocaloric Effect of SmMnO3

We present a study of magnetization and magnetocaloric effect for the SmMnO₃ compound. This compound was synthesized by combustion reaction and its magnetic and structural properties studied by X-ray powder diffraction (XRD) and magnetization (M) measurements as a function of temperature and under magnetic fields. The XRD pattern at room temperature confirmed the presence of a single phase with orthorhombic structure. From magnetization versus temperature, we observe two magnetic orderings, the first one at 6 K due to Sm³⁺, and the other one at T _N =57(2) K is the anti-ferromagnetic long-range ordering. The magnetic entropy change, ΔS _M , was obtained from magnetization isotherms close to T _N where it reaches a maximum value of about 8.0 J/kg K for an applied field of 7 T.     

Investigation of Anomalous Magnetic Transition in Pr0.5Sr0.5CoO3

Polycrystalline perovskite cobalt oxides Pr_0.5Sr_0.5CoO₃ were prepared by the sol-gel method. We mainly study the anomalous magnetic transition of Pr_0.5Sr_0.5CoO₃. We report the investigations of polycrystalline samples of the metallic ferromagnetic material Pr_0.5Sr_0.5CoO₃ through measurements of X-ray diffraction, the magnetization, and the resistivity. We found an unusual anomaly around T _A=120 K, much below the ferromagnetic transition (T _C=228 K). Further using the variable temperature X-ray diffraction and electron spin resonance (ESR) measurements, we found that as the temperature goes down, the crystal structure changes obviously at T _A=120 K. We show that this actually results in anomalous magnetic transition.     

Structural and magnetic properties of CaAl4Fe8O19

A new compound, CaAl₄Fe₈O₁₉, was synthesized for the first time and characterized by X-ray diffraction. It was found to have a hexagonal magnetoplumbite structure with lattice parametersa=5·83 Å andc=22·14 Å. The electrical studies showed that the compound was a semiconductor with energy of activationq=0·86 eV. The magnetic susceptibility was studied in the temperature range 300 K to 850 K, in which the compound was paramagnetic with a Curie molar constant of 31·03.     

The chemical control of colossal magnetoresistance (CMR) in the new two-dimensional La1.2(Sr1.8−xCax)Mn2O7 system

The magnetic and electrical properties of the colossal magnetoresistance (CMR) La_1.2(Sr_1.8−xCa_x)Mn₂O₇ materials have been investigated. Based on the chemical titration method, the valence of Mn was determined to be 3.39±0.04 for all measured La_1.2(Sr_1.8−xCa_x)Mn₂O₇. The magnetoresistance ratio [ρ(0)/ρ(H)] was efficiently increased from ∼192% (135 K, 1.5 T) for x=0 to 208% (102 K, 1.5 T) for x=0.4 in La_1.2(Sr_1.8−xCa_x)Mn₂O₇. Moreover, the Curie temperature (T_c) decreased with increasing x and was strongly dependent on the apical bond lengths of Mn–O(2).     

Effect of Ni Doping on the Structural, Magnetic and Magnetocaloric Properties of Pr0.7Ca0.3Mn1−y Ni y O3 Manganites

In this paper, we report the structural, magnetic, and magnetocaloric properties of Ni-doped Pr_0.7Ca_0.3Mn_1?y Ni _y O₃ (0≤y≤0.1) powder samples. Our compounds were synthesized using the solid state reaction method at high temperature. X-ray diffraction analysis using Rietveld refinement show that all our samples are single phase and crystallize in the orthorhombic structure with the Pnma space group. Magnetization measurements versus temperature in a magnetic applied field of 50 mT reveal that Ni doping leads to a paramagnetic-ferromagnetic transition with a Curie temperature increasing from 106 K for y=0.02 to 118.5 K for y=0.1. A large magnetic entropy change |ΔS _M| has been observed in our doped samples with maximum values around their respective Curie temperatures T _C. $|\Delta S_{\mathrm{M}}^{\mathrm{max}}|$ is found to be 2 J?kg^?1?K^?1, 2.96 J?kg^?1?K^?1 and 2.94 J?kg^?1?K^?1 for y=0.02, 0.05, and 0.1, respectively, in a magnetic field change of 5 T. Large relative cooling power (RCP) value of 352.24 J?kg^?1 is obtained for Pr_0.7Ca_0.3Mn_0.9Ni_0.05O₃ sample under 5 T.     

High Field and Low Temperature Magnetization of ErCu2Si2 Single Crystal

The magnetic measurements have been performed for high fields up to 18 T and low temperatures down to 0.5 K on the single crystal compound ErCu₂Si₂. In the temperature dependence of magnetic susceptibility, two anomalies are seen at T _N =1.51 K and T _t =0.97 K which is corresponding to an antiferromagnetic ordering temperature and may come from a change of magnetic structure. Low temperature magnetization shows a metamagnetic transition around 0.4 T along the easy magnetization direction of [001]. The magnetization is almost saturated above 3 T and reaches 8.8μ _B /f.u. at 18 T. A metamagnetic transition appears around 0.5 T in the basal plane magnetization processes as well. The anisotropy within the basal plane is fairly large for high fields. These behaviors have been explained from crystalline electric field effects and magnetic interaction in terms of molecular field.     

Effects of rare-earth Sm<Subscript>2</Subscript>O<Subscript>3</Subscript> addition on relaxation behavior and electric properties of 0.5PNN-0.5PZT ceramics

Rare-earth Sm₂O₃-doped Pb(Ni_1/3Nb_2/3)_0.5(Zr_0.3Ti_0.7)_0.5O₃ (0.5PNN-0.5PZT + xSm) piezoelectric ceramics were prepared by a two-step solid-state reaction method. The effects of Sm₂O₃ doping on phase structure, electric properties and dielectric relaxation of 0.5PNN-0.5PZT ceramics were investigated. The results showed that Sm³⁺ tended to occupy the B-site of Zr⁴⁺. The XRD patterns showed that all ceramics had a pure perovskite phase structure. With the increasing of Sm₂O₃ addition, Curie temperature moved towards low temperatures obviously and dispersion coefficient γ first increased and then decreased gradually. When x = 0.2 wt%, the γ reaches at maximum (1.998), it was indicated that a nearly complete diffuse phase transition existed and relaxation behaviors enhanced. But the ceramics with x = 0.2 wt% exhibited terrible electric properties: d₃₃ = 605 pC/N, k_p = 0.55, ε_r ~ 5020, tanδ ~ 2.20 %. It showed that the relaxation behaviors have relationships with electric properties to some extent.     

Tuning the microwave dielectric properties of La0.97Sm0.03(Mg0.5Sn0.5)O3 by adding Ca0.8Sm0.4/3TiO3

The influence of sintering temperature on the microwave dielectric properties and microstructure of the (1 ? y)La_0.97Sm_0.03(Mg_0.5Sn_0.5)O₃–yCa_0.8Sm_0.4/3TiO₃ ceramic system were investigated with a view to their application in microwave devices. The (1 ? y)La_0.97Sm_0.03(Mg_0.5Sn_0.5)O₃–yCa_0.8Sm_0.4/3TiO₃ ceramic systems were prepared using the conventional solid-state method. The X-ray diffraction (XRD) patterns of the (1 ? y)La_0.97Sm_0.03(Mg_0.5Sn_0.5)O₃–yCa_0.8Sm_0.4/3TiO₃ ceramic system did not significantly vary with sintering temperature. The XRD patterns of the (1 ? y)La_0.97Sm_0.03(Mg_0.5Sn_0.5)O₃–yCa_0.8Sm_0.4/3TiO₃ ceramic system shifted to higher angle as y increased. A dielectric constant of 37.5, a quality factor (Q × f) of 40,300 GHz, and a temperature coefficient of resonant frequency of 2.4 ppm/°C were obtained when the 0.425La_0.97Sm_0.03(Mg_0.5Sn_0.5)O₃–0.575Ca_0.8Sm_0.4/3TiO₃ ceramic system was sintered at 1,600 °C for 4 h.     

A Study of Structural and Physical Properties of Heavily Co-doped LaSr2Mn2O7 Bi-layered Manganite

In this paper, we report the influence of cobalt doping on the structure and magnetic properties of bilayered-perovskite LaSr₂Mn_2?x Co _x O₇ (x=0.25,0.3). Parent and doped samples were synthesized through the sol-gel method under optimized conditions. X-ray diffraction studies coupled with the Rietveld refinement of xrd data, show that both doped and parent samples crystallize in a Sr₃Ti₂O₇-type layered perovskite structure. Temperature dependent magnetic behavior of the parent sample exhibits an antiferromagnetic and a charge-ordering state around 220 K. Magnetic field dependence of magnetization M(H) for doped samples at 20 K show well-defined M–H loops confirming the formation of intrinsic ferromagnetic ordering due to the Co doping. Additionally, we studied the ac–magnetic susceptibility and magnetization relaxation process in our samples. Dynamic scaling analysis hints an existence of glassy magnetic states in doped samples. Electrical and magnetic properties of our samples further demonstrate that Co doping suppresses charge ordering in bilayered LaSr₂Mn₂O₇.     

Enhanced capacity for lithium–air batteries using LaFe0.5Mn0.5O3–CeO2 composite catalyst

LaFe_0.5Mn_0.5O₃ and Ce-incorporated LaFe_0.5Mn_0.5O₃ catalysts for Li–air batteries were synthesized by co-precipitation (CP) and micro-emulsion methods with the increasing Ce/(La+Ce) ratios from 0 to 0.5. Ce has a low solubility in LaFe_0.5Mn_0.5O₃ perovskite lattices. Instead of forming single-phase La_1?x Ce _x Fe_0.5Mn_0.5O₃ perovskite, a multi-phase LaFe_0.5Mn_0.5O₃–CeO₂ composite was obtained even for Ce/(La+Ce) = 0.05. Such catalysts were used in the cathode of Li–air batteries and the discharge test showed that LaFe_0.5Mn_0.5O₃–CeO₂ composite catalyst can effectively improve the specific capacity with the highest capacity of ~4700 mAh/g for Ce/(La+Ce) = 0.05 (by CP). There is also a 0.05 V increase in discharge voltage compared with the reference cell without catalyst, with the discharge voltage plateau at ~2.75 V. The overall ranking in terms of capacity was Ce/(La+Ce) = 0.05 > Ce/(La+Ce) = 0.1 > Ce/(La+Ce) = 0.5 > Ce/(La+Ce) = 0. The capacity increase for Ce/(La+Ce) = 0.05 and 0.1 samples is attributed to the enhanced oxygen storage/release capability and the increased conductivity with the incorporation of CeO₂.     

Effect of Sm3+ on dielectric and magnetic properties of Y3Fe5O12 nanoparticles

The Sm³⁺ doped Y_3?xSm_xFe₅O₁₂ (x = 0–3) nanopowders were prepared using modified sol–gel route. The crystalline structure and morphology was confirmed by X-ray diffraction and atomic force microscopy. The nanopowders were sintered at 950 °C/90 min using microwave sintering method. The lattice parameters and density of the samples were increased with an increase of Sm³⁺ concentration. The room temperature dielectric (ε′ and ε″) and magnetic (μ′ and μ″) properties were measured in the frequency range up to 20 GHz. The room temperature magnetization studies were carried out using Vibrating sample magnetometer using filed of 1.5 T. Results of VSM show that the saturation and remnant magnetization of Y_3?xSm_xFe₅O₁₂ (0–3) decreases on increasing the Sm concentration (x). The low values of magnetic (μ′ and μ″) properties makes them a good candidates for microwave devices, which can be operated in the high frequency range.