Study of the Magneto-Resistivity and Dependence of Percolation in La0.75Ca0.1Sr0.15Mn1−x Ga x O3 Compounds

We present a study of the effect of Ga-doping on the physical properties of La_0.75Ca_0.1Sr_0.15Mn_1?x Ga _x O₃ (x=0.025, 0.075 and 0.1) compounds prepared by the sol-gel method. The variation of the magnetization (M) vs. temperature (T), under an applied magnetic field of 0.05 T, reveals a ferromagnetic–paramagnetic transition for all samples. The magnetization behavior and the Curie temperature T _C have shown a large dependence on the fractional composition x. In fact, the M(T) curves have revealed the presence of a long-range ferromagnetic state below T _C. The magnetotransport properties have been investigated based on the temperature dependence of the resistivity ρ(T) measurements. We have noted that these samples present an electrical transition from a paramagnetic-semiconductor state to a ferromagnetic-metallic one, when decreasing temperature. We have used the phenomenological equation for conductivity under a percolation approach, which is based upon an approach that the system consists of the phase separated ferromagnetic metallic and paramagnetic insulating regions. We have then fitted the resistivity data measured in the range of 15–300 K and found that the activation barrier decreases with the raising Ga³⁺ ion concentration.     

Lanthanum Deficiency Effect on the Structural, Magnetic, and Transport Properties of the La0.8−x □ x Ca0.2MnO3 Manganites Oxides

The La_0.8?x □ _x Ca_0.2MnO₃ compounds were prepared by the solid-state reaction with 0.00≤x≤ 0.20. Morphologic, X-ray diffraction (XRD), magnetic, and electrical measurements were performed to examine the effect of the lanthanum deficiency on the physical properties of these materials. The scanning electron microscopy shown the existence of a secondary phase attributed to the Mn₃O₄ oxide. The XRD data have been analyzed by the Rietveld refinement technique. We have found that all samples crystallize in an orthorhombic symmetry with Pnma space group and the unit cell volume decreases with lanthanum deficiency. A magnetization versus temperature study has shown that all samples exhibit a magnetic transition from ferromagnetic (FM) to paramagnetic (PM) phase with increasing temperature. The Curie temperature T _C increases with the lanthanum deficiency from 236 K to 247 K for the samples x=0.00 and x=0.20, respectively. Moreover, the temperature dependence of electrical resistivity shows a metal–insulator transition at T _ρ for all samples which increase with vacancy rate. The electrical resistivity is fitted with the phenomenological percolation model, which is based on the phase segregation of ferromagnetic metallic clusters and paramagnetic insulating regions. Thus, show that the conduction mechanist was explained by the electron–electron and electron–phonon scattering process at low temperature and by small polaron hopping at high temperature. Furthermore, we found that the estimated results are in good agreement with experimental data.     

Thermal Properties of La1−x A x CoO3 (A = Ca and Ba)

The thermal properties of La_1?x A _x CoO₃ (A = Ca, Ba and 0.0≤x≤0.3) for temperature (1 K≤T≤300 K) have been investigated using the Modified Rigid ion model (MRIM) with pair wise interionic interaction potential. The temperature dependent (1 K≤T≤300 K) variation of specific heat and thermal expansion for La_1?x A _x CoO₃ (A = Ca, Ba and 0.0≤x≤0.3) has presented proper interpretation of the experimental data. Also the effect of lattice distortions on the thermal properties of these perovskite cobaltates has been studied by an atomistic approach. In addition, the results on the cohesive energy (?), molecular force constant (f), Reststrahlen frequency (υ), Debye temperature (θ _D ) and Gruneisen parameter (γ) are also discussed.     

Electrical and Magnetoresistance Properties of Mg x Mn1−x Fe2O4 Compounds

A systematic study of the electrical properties and magnetoresistivity of Mg _x Mn_1?x Fe₂O₄ (x ranging from 0 to 1.0 in steps of 0.1) compounds is presented. All samples were produced by glycolthermal reaction under a low reaction temperature of 200 ^°C. The X-ray diffraction data indicate formation of a pure cubic spinel phase in all the samples. The effects of Mg concentration and particle size on the properties have been investigated. A decreasing trend in the resistivity of all the compounds with increasing magnetic field has been observed.     

Fe substitution for Mn in Rhombohedral La0.8Sr0.2Mn1−xFexO3 — the Structural,Magnetic, and Magnetocaloric Properties

Journal of Superconductivity and Novel Magnetism - The effect of Fe doping on the structural, magnetic, and magnetocaloric properties was studied on La0.8Sr0.2Mn1?xFexO3...     

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.     

Electrical and Thermal Transport Properties of Transition Metal Substituted in Electron-Doped La0.9Te0.1Mn1?x M x O3 Compounds (M=Cr and Al)

Systematical studies of resistivity ??(T), thermopower S(T), and thermal conduction ??(T) have been performed on the electron-doped manganites La_0.9Te_0.1Mn_1?x Cr(Al) _x O₃ (0.05??x??0.20). The Cr005 and Al005 samples exhibit an insulator-metal transition in the ??(T) curves, however, other samples present insulating behaviors in the whole measured temperature. The S(T) data increases with decreasing temperature and develop a peak at $T_{S}^{P}$ for all samples, with decreasing temperature further, another characteristic temperature $T_{s}^{m}$ is observed. The fits of ??(T) and S(T) data in the high temperature region are presented, and the result shows that the electron conduction is governed by the VRH mechanism for Al-doped samples and the SPC mechanism for Cr-doped ones. As to thermal conduction ??(T), it is suggested that it comes from two sides, one is the phonon-phonon scattering due to disorder and the other is the spin-phonon interaction.     

The T-P-H effects on the magnetoresistance of La0.7Ca0.3Mn1.0O3−δ ceramics and films

Effects of the temperature (T=77–325 K), high hydrostatic pressures (P=0–2.1 GPa), and magnetic fields (H=0–8 kOe) on the electric resistance (R) and magnetoresistance (ΔR/R ₀) were studied in La_0.7Ca_0.3Mn_1.0O_3−δ based ceramics and single crystal films. A significant difference between the magnetoresistance peak temperatures (T _P) observed in the ceramic and film samples is explained by their different deviations from the stoichiometry with respect to oxygen, that is, by a greater concentration of anion vacancies in the film perovskite structure. An increase in the magnetic field strength H and the pressure P leads to a decrease in the electric resistance R. The magnetoresistance grows with increasing field strength H and decreasing pressure P. A growth in the hydrostatic pressure leads to an increase in the T _P value by 12 K for the ceramics and by 40 K for the films.     

Structural Magnetic and Electrical Properties in Nd-Doped Manganites (La0.70−x Nd x )Sr0.30Mn0.70Ti0.30O3 with 0 ≤ x ≤ 0.30

The structural, magnetic and electrical properties of (La_0.70?xNd_x)Sr_0.30Mn_0.70Ti_0.30 O ₃ perovskites (0 ≤ x ≤ 0.30) prepared by the usual ceramic procedure were investigated. Structural Rietveld refinements of the X-ray powder diffraction revealed that for x = 0 the compound crystallizes in a rhombohedral (R \(\bar {{3}}\) c) perovskite structure, while for x = 0.10, 0.20 and 0.30 the structure becomes orthorhombic (Pbnm). The field-cooled, zero-field-cooled and M(H) measurements lead to conclude that the samples with x≥0.10 behave like spin-glass systems. The temperature dependence of the resistivity exhibits two transitions at two temperatures T _p1 and T _p2, with the same width for the two peaks. T _P1 results from the competition between double-exchange and super-exchange mechanisms inside the grain (intrinsic effects) whereas T _P2 arises due to the grain boundary effects (extrinsic effect). The resistivity of this system seems to be related to the competition between antiferromagnetic insulating (AFI) phases, ferromagnetic insulating (FMI) phases and ferromagnetic metal (FMM) phases, which is induced by Nd doping effect.     

Composition range and physicochemical properties of La1 − x Ba x Mn1 − y Fe y O3 solid solutions

We have determined the extent of La_{1 ? x} Ba _x Mn_{1 ? y} Fe _y O₃ solid solutions with orthorhombically and rhombohedrally distorted perovskite structures. A partial phase diagram of the LaMnO_{3 + δ}-BaMnO₃-BaFeO_2.5-LaFeO₃ system in air at a temperature of 1373 K has been proposed for the first time. We have measured the relative length change of La_{1 ? x} Ba _x Mn_{1 ? y} Fe _y O₃ samples and calculated their thermal expansion coefficients.     

Deformation Effects on the Structure and Properties of Y1−x Ca x Ba2Cu3O7−δ (x=0;0.3) Tapes Produced by OPIT Method in the Ag-Tube

Samples of superconducting tapes with two type cores (YBa₂Cu₃O_7−y and Y_0.7Ca_0.3Ba₂Cu₃O_7−δ ), obtained by the OPIT method under two different regime of rolling—cold (20 °C) and hot (825 °C) are investigated. The superconducting core materials were prepared by standard solid-state technology and fill in the high purity (4N) Ag-tubes. The effect of cold and hot rolling deformation on the orientation of supercoducting crystals were studied by XRD analysis. It was found that a higher degree of texturing in the out of plane direction is obtained after hot rolling deformation in tapes with overdoped core. As a result, the highest critical current density (J _c ) at 77 K and zero magnetic field was achieved in them. J _c increases with the time in the superconducting tapes, which is connected probably with the releasing of stress between the superconducting core and Ag sheat.     

The Effect of Zn-Substitution on the Anomalous Resistance Behavior of La1−x Ca x Mn1−y Zn y O3 Compounds

We have studied the effect of Zn substitution on the temperature dependence of the resistivity and the Curie temperature T _p of La_1–x Ca _x Mn_1–y Zn _y O₃ system (with x=0, y=0, and x=0.33, 0.00y0.15) during their pass to the paramagnetic conducting phase. All the specimens show a clear metal-insulator transition passing through the Curie temperature T _p . With increasing Zn concentration, T _p decreases for La/Ca substitution and also for Zn/Mn substitution with fixed 33 at.% of Ca. T _p is found to be slightly decreases for low concentrations of Zn (up to y=0.075) followed by a sharp decrease up to y=0.15. A possible explanation is given. Moreover, the activation energy E _a is calculated in the three different temperature regions characterized in the (-T) curve. Decreasing the temperature below T _p can increase the parallel alignment of the Mn spins and, thus decreases the resistivity of the samples. E _a increases with Zn content (up to y=0.075) followed by a sharp decrease. A strong correlation in the behavior of the normal state and residual resistivity as well as T _p and E _a with Zn content is reported.     

Effect of Composition Variations on Electrical and Optical Properties of La1−x (Ca, Sr) x MnO3 Thin Films

Thin films of La _0.67 Ca _0.33 MnO ₃₊ (d 200 nm) grown at 750° C on MgO(100) substrates by pulsed laser deposition were annealed in various oxygen atmospheres. The LaF ₃ , CaF ₂ and Mn overlayers (d 5-10 nm) have been evaporated onto the films in order to saturate the compound by La, Ca or Mn during subsequent annealing at 900° C in air. Further, the pulsed injection CVD method has been utilized to prepare series of La _1–x Sr _x MnO ₃ films with x = 0÷0.5 on LaAlO ₃ (100). The effect of composition variations on electrical resistivity and magnetoresistance of the films as well as on their optical transmission and reflection spectra in the energy range between 0.2 and 4.0 eV has been investigated.     

Effect of Pr and Ca Substitution on the (La1−x−u Pr x Ca u )(Ba1.5Sr0.5)Cu3O y Superconductors

In the fully oxygenated single phase (La_1–x–u Pr _x Ca _u )(Ba_1.5Sr_0.5)Cu₃O _y (0x1.00, 0u0.20) system, if u=0, the z-coordinate of the Ba site, z(Ba), and d _Ba–bp (distance between the Ba site and the basal plane) are lowered by increasing the amount of Pr in the (La_1–x Pr _x )(Ba_1.5Sr_0.5)Cu₃O _y series. On the contrary, if u0, z(Ba) and d _Ba–bp are raised by increasing the amount of Ca ion in the (La_1–x–u Pr _x Ca _u )(Ba_1.5Sr_0.5)Cu₃O _y (0x0.30, 0u0.20) series. Linear relationships are found between z(Ba), d _Ba–bp and the oxygen stoichiometry (y). T _c and p _sh (the hole concentration in CuO₂ layers) increase with increasing the angle of Cu(2)-Ba-Cu(2). Relative positions of the Cu(2) and Ba sites are important in determining the T _c of these 123 compounds.     

Bond Angle Study of Self-compensating Y1−x Ca x Ba2−x La x Cu3O z System

Single-phase samples of a self-compensating Y_1?x Ca _x Ba_2?x La _x Cu₃O _z system were synthesized through a solid-state reaction method with x<0.4. The structure of all samples were characterized by X-ray diffraction and refined by the Rietveld method. Superconducting properties have been investigated by the DC magnetization measurement. The critical temperature (T _c ) decreases evidently with the increment of x although the carrier concentration remains constant in the samples for different doping level. Careful study of the chemical bonds in the crystalline lattice demonstrates that the T _c is closely correlated to four pairs of bond angles in the unit cell. The analysis indicates that crystalline structure is one of the important factors to high-T _c superconductivity, and its influence is independent of the carrier concentration.