Preparation and low field magnetoresistance of double layered manganite La1.4Ca1.6Mn2O7

Double layered manganite of La_1.4Ca_1.6Mn₂O₇ (DLCMO) was prepared using solid state reaction method and had a metal-insulator transition temperature (T_MI) of 125 K. The short range 2D-feerromagnetic ordering (T_C2) starts growing when T<168 K and it gets converted into 3D-ferromagnetic ordering (T_C1) at 114 K. Low field magnetoresistance (MR) behaviour of the DLCMO was investigated and compared with an infinite layered manganite La_0.7Ca_0.3MnO₃ (LCMO). For DLCMO, in the temperature range between T_C1 and T_C2, the MR showed a gradual increase with the magnetic field. The observed MR and R-T behaviour of double layered manganite for T_C1<T<T_C2 has been explained in the frame work of the two phase model [ferromagnetic (FM) domains and paramagnetic (PM) regions] and percolative behaviour of transport in FM-PM mixture.     

Low field magnetoresistance and conduction noise in layered manganite La1.4Ca1.6Mn2O7

Temperature dependence of conduction noise and low field magnetoresistance of layered manganite La_1.4Ca_1.6Mn₂O₇ (DLCMO) are reported and compared with the infinite layered manganite La_0.7Ca_0.3MnO₃ (LCMO). The double layered manganite was prepared using standard solid state reaction method and had a metal-insulator transition temperature (T_M-I) of 155 K. The temperature dependence of susceptibility showed evolution of ferromagnetic ordering at 168 K. The observed voltage noise spectral density (S_V) shows 1/f^α type of behaviour at all temperatures from 77 K to 300 K. In the ferromagnetic region (T<168 K), S_V/V² shows two peaks at 164 K and 114 K. The observed two peaks in normalised conduction noise of DLCMO is attributed to the excess noise generated due to setting up of short range 2D-ferromagnetic ordering and long range 3D-ferromagnetic ordering at two different temperatures T_C2 and T_C1. In temperature range between T_C1 and T_C2, the magnetoresistance (MR) showed a gradual increase with the magnetic field. The observed MR has been explained in the framework of the two phase model [ferromagnetic (FM) domains and paramagnetic (PM) regions].     

Magnetic phase diagram of double-layered La2−2xCa1+2xMn2O7 manganite

Double-layered manganite La_2−2xCa_1+2xMn₂O₇ have been synthesized for compositions ‘x’=0.0, 0.1, 0.2, 0.3, 0.4 and 0.5 by solid state reaction method. From X-ray diffraction study, their crystal structures were found to be tetragonal perovskite with lattice parameters decreasing with increasing ‘x’. The decreasing lattice parameters affect the balance between in-plane, intra-bilayer and inter-bilayer exchange interactions, which is reflected on magnetotransport properties. The metal-to-insulator transition temperature is found to vary with composition and peaked around ‘x’=0.3. From ac-susceptibility study, 2D-ferromagnetic ordering was observed at higher temperatures for all compositions whereas 3D-ferromagnetic ordering was observed at quite low temperatures. In low-temperature region, decreasing susceptibility shows antiferromagnetic state for all compositions. On the basis of electrical and magnetic properties, a magnetic phase diagram is given.     

Effect of Cr doping in the bilayer manganite La1.4Sr1.6Mn2O7

The effect of Cr doping on magnetic and electrical properties in the bilayer manganites La_1.4Sr_1.6(Mn_1−yCr_y)₂O₇ (y=0-0.1) has been investigated. When y≤0.025, Cr doping enhances the three-dimensional magnetic transition temperature T_C and the insulator-metal transition temperature T_IM as well as decreases the peak resistivity at T_IM, and the saturated magnetization decreases slightly. When y≥0.035, T_IM decreases gradually accompanied by the increase of peak resistivity, but T_C remains nearly constant, and the saturated magnetization decreases heavily. In the whole doping region, the two-dimensional magnetic transition temperature T^? monotonously decreases with an increasing of Cr doping level. These results can be explained by considering different magnetic (including ferromagnetic and antiferromagnetic) interactions between Mn ions and Cr ions.     

Preparation and study of silver added La0.67Ca0.33MnO3 film

La_0.67Ca_0.33MnO₃ (LCMO) and Ag admixed La_0.67Ca_0.33MnO₃ (Ag-LCMO) polycrystalline films have been prepared on SrTiO₃ single crystal (100) substrates by ultrasonic spray pyrolysis technique. These films are characterized using XRD, SEM, and temperature dependence of resistivity (ρ-T) and ac susceptibility (χ-T). The films are having cubic structure with lattice parameters as 3.890 and 3.885 Å for LCMO and Ag-LCMO films, respectively. The peek in ρ-T curve (T_p) and the ferromagnetic transition temperature (T_C) for the Ag-LCMO film is higher than that of LCMO film. The stability of both the films was tested by repeated measurements of its characteristics over a period of one week after several thermal cycling from room temperature to 77 K. In the LCMO film, the peak in the ρ-T curve (T_p) is found to shift towards lower value and conduction noise of the film increases in the subsequent measurements. In the case of Ag-LCMO the value of T_p, T_C and conduction noise of the film did not change even after several measurements. Silver segregating at the grain boundaries in Ag-LCMO polycrystalline film seems to be responsible for improving the characteristics of Ag-LCMO films.     

Study of the structural, dielectric and magnetic properties of Bi2O3 and PbO addition on BiFeO3 ceramic matrix

In this paper we studied the effects of Bi₂O₃ and PbO addition on BiFeO₃ (BFO) ceramic matrix. The structural, dielectric and magnetic properties of fifteen BFO samples were discussed in view of possible applications in RF and microwave devices. The present work also reports the preparation of the samples. Polyvinyl alcohol (PVA) and tetraethyl orthosilicate (TEOS) were also added as a binder in the fabrication procedure. The samples have been studied by X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and magnetic hysteresis measurements. Further, a study based on impedance spectroscopy also has been done. Dielectric permittivity (ε′) and dielectric loss (tan δ) were measured at room temperature in the frequency range 100 Hz-10 MHz, as well as a.c. conductivity. The -Im[Z(f)] versus Re[Z(f)] plot has been obtained. The samples were investigated in view of possible applications like miniaturized filters, diplexers and dielectric resonator antennas (DRA). In the RF and MW frequency region, the application of magneto-dielectric and multiferroic perovskite composite materials is desirable for the miniaturization of components.     

Temperature dependence of electroresistance for La0.67Ba0.33MnO3 manganite

The influence of dc biasing current on temperature dependence of resistance of La_0.67Ba_0.33MnO₃ bulk sample is reported. A decrease in the resistance (electroresistance) on the application of higher bias current is observed. The electroresistance is maximum at metal-insulator transition temperature (T_MI) and decreases when the temperature is either increased or decreased from T_MI. A two-phase model is proposed to explain the occurrence of electroresistance. The higher bias current leads to an increase in alignment of spins and thus, in turn, leads to an increase in spin stiffness coefficient and decrease in the resistance at T_MI.     

Phase stability in the LnCa2Mn2O7 (Ln=Pr, Nd, Sm and Gd) Ruddlesden-Popper series

The synthesis of the Ruddlesden-Popper series, LnCa₂Mn₂O₇, (Ln=Pr, Nd, Sm and Gd) is described and their structure and electronic properties investigated. The reduction in size of the A-site cation causes an increase in the distortion of their orthorhombic structures (space group Cmcm). All of these compounds form with a perovskite impurity, the amount of which increases on reduction of the cation size. The synthesis temperature also alters the amount of perovskite impurity in the phase, but only to a lower limit, implying the perovskite phase is intrinsic to the material and that a phase equilibrium exists between the layered Ruddlesden-Popper and perovskite phases, which is controlled by the cation size. The magnetic susceptibility show transitions characteristic of the perovskite phase, therefore little direct information can be obtained about the Ruddlesden-Popper phases, except that ferromagnetism is not observed in any of these materials.     

Magnetic susceptibility of vanadium garnets NaPb2Co2V3O12 and NaPb2Ni2V3O12

Vanadium garnets NaPb₂Co₂V₃O₁₂ and NaPb₂Ni₂V₃O₁₂ have been successfully synthesized. The X-ray diffraction experiments indicate that these compounds have the garnet structure of cubic symmetry of space group with the lattice constant of 12.742 Å (NaPb₂Co₂V₃O₁₂) and 12.666 Å (NaPb₂Ni₂V₃O₁₂), respectively. The magnetic susceptibility of NaPb₂Ni₂V₃O₁₂ shows the Curie-Weiss paramagnetic behavior between 4.2 and 350 K. The effective magnetic moment μ_eff of NaPb₂Ni₂V₃O₁₂ is 3.14 μ_B due to Ni²⁺ ion at A-site and the Weiss constant is −3.67 K (antiferromagnetic sign). For NaPb₂Co₂V₃O₁₂, the simple Curie-Weiss law cannot be applicable. The ground state is the spin doublet and the first excited state is spin quartet , according to Tanabe-Sugano energy diagram on the basis of octahedral crystalline symmetry. This excited spin quartet state just a bit higher than ground state influences strongly the complex temperature dependence of magnetic susceptibility for NaPb₂Co₂V₃O₁₂.     

Chemical pressure effect on the magnetic order of the La1.4Sr1.6Mn2O7 bilayered manganite

In this paper we have studied, by means of high-resolution neutron powder diffraction, the structural and magnetic feature of pure La_1.4Sr_1.6Mn₂O₇ and Sr-doped (25%) La_1.4Sr_1.6Mn₂O₇.Our data reveal the stabilization of the A-type AFM long-range order for the La_1.4Sr_1.6Mn₂O₇ bilayered manganites induced by the partial replacement of the Sr with the smaller Ca, keeping constant the hole doping. This can be in turn due to the change in the orbital character of the e_g electrons as a function of Ca-doping.     

Antiferromagnetic phase transition in garnet-type AgCa2Co2V3O12 and AgCa2Ni2V3O12

Antiferromagnetic phase transition in two vanadium garnets AgCa₂Co₂V₃O₁₂ and AgCa₂Ni₂V₃O₁₂ has been found and investigated extensively. The heat capacity exhibits sharp peak due to the antiferromagnetic order with the Néel temperature T_N=6.39 K for AgCa₂Co₂V₃O₁₂ and 7.21 K for AgCa₂Ni₂V₃O₁₂, respectively. The magnetic susceptibilities exhibit broad maximum, and these T_N correspond to the inflection points of the magnetic susceptibility χ a little lower than T(χ_max). The magnetic entropy changes from zero to 20 K per mol Co²⁺ and Ni²⁺ ions are 5.31 J K⁻¹ mol-Co²⁺-ion⁻¹ and 6.85 J K⁻¹ mol-Ni²⁺-ion⁻¹, indicating S=1/2 for Co²⁺ ion and S=1 for Ni²⁺ ion. The magnetic susceptibility of AgCa₂Ni₂V₃O₁₂ shows the Curie-Weiss behavior between 20 and 350 K with the effective magnetic moment μ_eff=3.23 μ_B Ni²⁺-ion⁻¹ and the Weiss constant θ=−16.4 K (antiferromagnetic sign). Nevertheless, the simple Curie-Weiss law cannot be applicable for AgCa₂Co₂V₃O₁₂. The complex temperature dependence of magnetic susceptibility has been interpreted within the framework of Tanabe-Sugano energy diagram, which is analyzed on the basis of crystalline electric field. The ground state is the spin doublet state ²E(t₂⁶e) and the first excited state is spin quartet state ⁴T₁(t₂⁵e²) which locates extremely close to the ground state. The low spin state S=1/2 for Co²⁺ ion is verified experimentally at least below 20 K which is in agreement with the result of the heat capacity.     

Syntheses and properties of the Fe-Co/Fe3O4 ferrites

The Fe alloy-ferrite composites Fe-Co/Fe₃O₄ are synthesized by using disproportion of Fe (II) and reduction of Co (II) by Fe⁰ in a concentrated and boiling KOH solution. The Fe alloy and ferrites are prepared in aqueous solution without any templet and surfactants at low temperature. Their structures and magnetic properties are investigated by X-ray diffractometer (XRD) and vibrating sample magnetometer (VSM). From the results of XRD, it is shown that the samples have b.c.c and f.c.c structure of Fe, and the spinel structures of the ferrite before calcinations; the samples have b.c.c and spinel structures after calcinations at 300 °C; and the samples have only f.c.c structure and the spinel structures calcined at 500 °C.     

Magnetic properties for the Cu2MnSnSe4 and Cu2FeSnSe4 compounds

Magnetic susceptibility χ measurements in the range from 2 to 300 K were carried out on samples of the Cu₂FeSnSe₄ and Cu₂MnSnSe₄ compounds. It was found that Cu₂FeSnSe₄ was antiferromagnetic showing ideal Curie-Weiss behavior with a Néel temperature T_N of about 19 K and Curie-Weiss temperature θ=−200 K, while for Cu₂MnSnSe₄ the behavior was spin-glass with a freezing temperature T_f of about 22 K and Curie-Weiss temperature θ=−25 K. The spin-glass order parameter q(T), determined from the susceptibility data, was found to be in agreement with the prediction of conventional spin-glass theory.     

Impurity-induced magnetic order in the mixture of two spin gap systems (CH3)2CHNH3CuCl3 and (CH3)2CHNH3CuBr3

The ground state of the solid solution of the two spin gap systems (CH₃)₂CHNH₃CuCl₃ and (CH₃)₂CHNH₃CuBr₃ has been investigated by ¹H NMR. The existence of a magnetic ordering in the sample with the Cl-content x=0.85 was clearly demonstrated by a drastic splitting in a resonance line at low temperatures below T_N=13.5 K. The observed NMR spectra in the ordered state was qualitatively consistent with the simple antiferromagnetic state.     

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).     

A novel blue-emitting long-lasting proyphosphate phosphor Sr2P2O7:Eu,Y

By introducing the Y³⁺ into Sr₂P₂O₇:Eu²⁺, we successfully prepared a kind of new phosphor with blue long-lasting phosphorescence by the high-temperature solid-state reaction method. In this paper, the properties of Sr₂P₂O₇:Eu²⁺,Y³⁺ were investigated utilizing XRD, photoluminescence, luminescence decay, long-lasting phosphorescence and thermoluminescence (TL) spectra. The phosphor emitted blue light that was related to the 4f⁶5d¹-⁸S_7/2 transition of Eu²⁺. The bright blue phosphorescence could be observed by naked eyes even 8 h after the excitation source was removed. Two TL peaks at 317 and 378 K related to two types of defects appeared in the TL spectrum. By analyzing the TL curve the depths of traps were calculated to be 0.61 and 0.66 eV. Also, the mechanism of LLP was discussed in this report.     

Photoluminescence and gamma-ray irradiation of SrAl2O4:Euand Y2O3:Eu phosphors

Y₂O₃:Eu³⁺ phosphor is a very attractive material for use as a red phosphor in many fields. SrAl₂O₄:Eu²⁺ belongs to long lasting phosphor (LLP) and it is a useful bluish-green luminescence material, which can also be a promising candidate as a simple and easy-to-use radiation detection element for visual display of two dimensional radiation distributions. In the present study, both these two kinds of phosphors were synthesized using high temperature solid state reactions. In our work, the influence of gamma-ray irradiation on the properties of these two kinds of phosphors was studied by comparing photoluminescence, brightness and the decay curve of unirradiated and gamma-ray-irradiated samples. Conclusions from the present work can be briefly summarized as follows. In irradiated samples, the brightness is decreased without sensible change in the wavelength distribution of the luminescence spectrum and in the decay kinetic upon gamma exposure. Moreover, the emission due to Eu³⁺→Eu²⁺ conversion in Y₂O₃:Eu³⁺ phosphors was not observed in our sample after irradiation to high exposure. Also the brightness of SrAl₂O₄:Eu²⁺ phosphor turned out to decrease after the exposition to ionizing radiation while the luminescence wavelength distribution remained unchanged. The reason for the effect of gamma-ray irradiation on the properties of phosphors is also discussed in the paper.     

Study of phase transition in ZrO2 doped with Pb3O4

Electrical conductivity of ZrO₂ doped with Pb₃O₄ has been measured at different temperatures for different molar ratios (x=0, 0.01, 0.02, 0.03, 0.04, 0.05 and 0.06). The conductivity increases due to migration of vacancies, created by doping. The conductivity increases with increase in temperature till 180 °C and thereby decreases due to collapse of the fluorite framework. A second rise in conductivity at higher temperatures beyond 500-618 °C is due to phase transition of ZrO₂. DTA and X-ray powder diffraction were carried out for confirming doping effect and transition in ZrO₂.The addition of Pb₃O₄ to ZrO₂ shifted the phase transition of ZrO₂ due to the interaction between Pb₃O₄ and ZrO₂.     

Facile synthesis and electrochemical properties of hierarchical MnO2 submicrospheres and LiMn2O4 microspheres

Hierarchical MnO₂ submicrospheres have been successfully synthesized by a wet chemical method. The as-prepared products were characterized by means of XRD, SEM, FTIR, TG, and TEM. With the as-prepared MnO₂ submicrospheres as precursors, LiMn₂O₄ microspheres were conveniently prepared by a simple solid-state reaction between MnO₂ and LiOH at a temperature as low as 600 °C. Electrochemical properties of the as-prepared MnO₂ submicrospheres and LiMn₂O₄ microspheres as cathode materials in lithium ion cells were investigated by galvanostatic charge/discharge tests.     

Electron hopping mechanism in hematite (α-Fe2O3)

The frequency dependence of the real (?′) and imaginary (?″) parts of the dielectric constant of polycrystalline hematite (α-Fe₂O₃) has been investigated in the frequency range 0-100 kHz and the temperature range 190-350 K, in order to reveal experimentally the electron hopping mechanism that takes place during the Morin transition of spin-flip process. The dielectric behaviour is described well by the Debye-type relaxation (α-dispersion) in the temperature regions T<233 K and T>338 K. In the intermediate temperature range 233 K<T<338 K a charge carrier mechanism takes place (electron jump from the O²⁻ ion into one of the magnetic ions Fe³⁺) which gives rise to the low frequency conductivity and to the Ω-dispersion. The temperature dependence of relaxation time (τ) in the −ln τ vs 10³/T plot shows two linear regions. In the first, T<238 K, τ increases with increasing T implying a negative activation energy −0.01 eV, and in the second region T>318 K τ decreases as the temperature increases implying a positive activation energy 0.12 eV. The total reorganization energy (0.12-0.01) 0.11 eV is in agreement with the adiabatic activation energy 0.11 eV given by an ab initio model in the literature. The temperature dependence of the phase shift in the frequencies 1, 5, 10 kHz applied shows clearly an average Morin temperature T_Mo=284±1 K that is higher than the value of 263 K corresponding to a single crystal due to the size and shape of material grains.