Structural,electrical and magnetic properties of polycrystalline La0.67(Ca1−x Sr x )0.33MnO3 manganites

Polycrystalline La_0.67(Ca_1?x Sr _x )_0.33MnO₃ with different substitution level of strontium element, were synthesized via solid state reaction. Structure of samples was characterized by X-ray diffraction (XRD). XRD patterns reveal that La_0.67Ca_0.33MnO₃ exhibits orthorhombic structure with space group Pnma. Phase transitions from orthorhombic to rhombohedral take place as Ca ions were gradually substituted by Sr ions. The XRD data were further analyzed by Rietveld refinement technique. The data show that Mn–O–Mn bond angle increases as x increases. Microstructures obtained from SEM show that substitution of Sr ions has demoted the grain growth and densification process during sintering. The substitution of Sr ions has greatly influenced the hopping integral of electron via double exchange interaction, thus affecting the electrical properties and magnetic properties as well. The resistivity decreases and the metal–insulator transition temperature (T _p ) shifts to higher temperature as x increases. The magnetoresistance (MR) effect gradually decreases and MR peak shifts to higher temperature as x increases. The magnetization measured at room temperature is found to be increasing as x increases.     

Specific heat and magnetocaloric effect of Pr1−x Ag x MnO3 manganites

This paper presents experimental results of the specific heat and magnetocaloric effect in Pr_1?x Ag _x MnO₃ (x = 0.05–0.25) manganites. A maximum value of magnetic entropy change for the sample with x = 0.15 was observed in magnetic field of 18 kOe at T _C = 125 K and was ΔS _max ≈ 2.96 J/kg K. In Pr_0.95Ag_0.05MnO₃ sample, a sign inversion of the magnetocaloric effect was observed at low temperatures (~80 K) in low magnetic fields of 750 Oe. The coexistence of ferromagnetic and canted antiferromagnetic phases with closely spaced critical temperatures was found to force the magnetocaloric effect into abrupt sign inversion.     

Mixed-valence La0.80(Ag1−x Sr x )0.20MnO3 manganites with magnetocaloric effect

We report the effect of the simultaneous inclusion of Ag¹⁺ and Sr²⁺ cations on the crystal structure and the magnetocaloric effect (MCE) of La_0.80(Ag_1?x Sr _x )_0.20MnO₃ manganites (x = 0.0, 0.25, 0.50, 0.75, 1.0) synthesized by the solid-state reaction method. X-ray photoelectron spectroscopy was used to verify chemical composition and the amount of Mn³⁺–Mn⁴⁺ cations, while the magnetic performance was evaluated by means of a Physical Property Measuring System. The progressive substitution of Ag¹⁺ by Sr²⁺ causes the increase of the number of Mn⁴⁺ cations, together with increasing ionic radii for the A-site contents up to x = 0.75, both favoring the enhancement of the double-exchange interaction and hence, the MCE. Excellent values of magnetic entropy change (?4.6 J/kg K, μ _oΔH = 5.0 T) were observed for the x = 0.25 manganite, while for the x = 0.50 sample interesting refrigerant capacity (129 J/kg, μ _oΔH = 2.0 T) and a wide interval of temperature at the full-width at half maximum δT _FWHM of the magnetic entropy change curve were recorded (101 K, μ _oΔH = 2.0 T) caused by two successive magnetic transitions. Magnetic performance was explained in terms of the effect of the cation substitution on the double-exchange interaction and the tolerance factor.     

Structural, transport, magnetic, and dielectric properties of La1−x Te x MnO3 (x = 0.10 and 0.15)

In this study, we have investigated the structure, temperature-dependent resistivity, magnetization, and dielectric properties of La_1?x Te _x MnO_3±δ (x = 0.10 and 0.15). X-ray diffraction analysis confirms the rhombohedral crystal symmetry with space group R $ \overline{3} $ c. For both the samples, the temperature dependence of magnetization plots show paramagnetic-to-ferromagnetic phase transition. The Curie temperature (T _c) and magnitude of magnetization increase with the Te concentration. Field-dependent magnetization produces the asymmetric hysteresis loop that has been attributed to the magneto crystalline anisotropy induced by lattice distortion and the rare earth spin coupling at room temperature. Temperature-dependent resistivity plots exhibit metal–insulator transition (MIT) and charge-ordering state. These plots have been fitted using variable range hopping model, and the density of states [N(E_F)] has been estimated. Magnetoresistance is measured as a function of temperature in the field of 1T, 5T, and 8T. The dielectric constant shows an anomaly near MIT. The dielectric constant exhibits a peaking behavior with the applied frequency and the temperature dependence of dielectric constant attains colossal values at high temperatures.     

Structural, magnetic, infrared and Raman studies of La0.8Sr x Ca0.2-x MnO3 (0 ≤ x ≤ 0.2)

We report the results of magnetic, X-ray powder diffraction, infrared and temperature dependent Raman spectra of the La_0.8Sr _x Ca_0.2-x MnO₃ (0 ≤ x ≤ 0.2) manganite. The structure refinement using the Rietveld method indicates that the partial substitution of strontium for calcium (for x ≥ 0.15) modifies the orthorhombic structure of the CaMnO₃ perovskite towards a rhombohedral phase. Magnetic measurement confirms the increase in the Curie temperature from 180 K for La_0.8Ca_0.2MnO₃ to 307 K for La_0.8Sr_0.2MnO₃, respectively. It is argued that the substitution with the larger Sr²⁺ ion strengthens the double-exchange interaction and gives rise to the observed increase of transition temperatures. All manganites show two IR active vibrational modes around 400 and 600 cm^?1. Moreover, when x ≤ 0.1, the absorption band around 400 cm^?1 splits into two peaks. In addition, we have analyzed the frequencies and widths of the observed Raman modes as a function of temperature for all samples with various Sr content. The mode splitting is attributed to both magnetic ordering and large orthorhombic distortion in doped rhombohedral manganites.     

Magnetocaloric study of monovalent-doped manganites Pr0.6Sr0.4−x Na x MnO3 (x = 0–0.2)

A systematic investigation of structural, magnetic, and magnetocaloric properties is reported for a series of monovalent sodium-doped manganites Pr_0.6Sr_0.4?x Na _x MnO₃ (x = 0, 0.05, 0.1, 0.15, and 0.2). Rietveld refinements of the X-ray diffraction patterns show that all powder samples are single-phased and crystallized in the orthorhombic structure with Pnma space group. Magnetic characterization and Arrott plot confirm the second-order phase transition at Curie temperature T _C decreasing from 310 K for x = 0 down to 272 K for x = 0.2. Magnetic entropy change is largest for x = 0 reaching 1.95 J kg^?1 K^?1 at 2 T field. This corresponds to a large relative cooling power of 102 J kg^?1 . Magnetic field sensitivity of magnetic entropy change and relative cooling power are analyzed and discussed.     

The investigation of the effect of K doping on the structural,magnetic, and magnetocaloric properties of La1.4−xKxCa1.6Mn2O7 (0.0 ≤ x ≤ 0.4) double perovskite manganite

Journal of Materials Science: Materials in Electronics - In this study, the effect of potassium doping on the morphological, crystallographic, magnetic and magnetocaloric properties of...     

Enhanced Magnetoresistance Effect and Magnetic Properties of Double Perovskite La2−xSrxMnRuO6 (0 ≤ x ≤ 1.0)

Journal of Superconductivity and Novel Magnetism - Double perovskite La2?xSrxMnRuO6 (x?=?0, 0.3, 0.5, 0.8, 1.0) samples were synthesized by the standard solid-state reaction...     

Improving the physical properties of polycrystalline-deficient Pr0.8Sr0.2−x□xMnO3 (0 ≤ x ≤ 0.2) compounds for electronic devices

Deficient strontium effects upon structural and spectroscopic properties of Pr_0.8Sr_0.2MnO₃ have been investigated. Pr_0.8Sr_0.2?x□_xMnO₃ (0?≤?x?≤?0.2) powder samples have been elaborated by the conventional ceramic method. All the samples crystallized in the orthorhombic perovskite system with Pnma (for x?=?0 and 0.05) and Pbnm (for x?=?0.1 and 0.2) space groups. The impedance spectroscopy measurements, the AC conductivity was investigated in the frequency range 40 to 5?×?10⁶ Hz and in the temperature range 80–500 K. DC measurements show that the studied compounds exhibit a semiconductor behavior in the temperature range studied. We have demonstrated that the conduction mechanism is governed by the hopping process at high temperatures as observed for doped perovskite materials. The AC conductivity results were well described by the Jonscher power law. The evolution of the frequency exponent s was determined in order to investigate the conduction mechanism in deficient structures. A discrepancy between the results obtained with the parent sample and deficient structures was observed. Analysis of the complex impedance shows that our structures obey Cole–Cole model. Nyquist plots were obtained using the Maxwell–Wagner equivalent circuit model. The evolution of the real and imaginary parts ε′ and ε″ of the permittivity was studied as a function of frequency, temperature, and strontium deficiency. Both ε′ and ε″ exhibit a strong dependence on these parameters. A step-like decrease in the dielectric constant ε′ was observed which may have originated from the space charge polarization and blocking electrode effect. The dielectric loss ε″ decreased with increasing frequency but increased with temperature for all studied samples. This result was correlated with the conduction mechanism which is governed by grain boundaries at low frequencies and grains at high frequencies. In addition, it affected the capacitance values in each frequency interval.

     

Structural,electrical, and magnetic properties of La0.7Ca0.3 − x Na x MnO3 ± γ solid solutions

La_0.7Ca_{0.3 − x} Na _x MnO_{3 ± γ} (LCNM) solid solutions with x = 0, 0.04, 0.06, 0.08, and 0.10 have been synthesized (rhombohedral structure, sp. gr. R c). Sodium volatility during sintering is shown to lead to the formation of vacancies on the lanthanum and oxygen sites. Ca²⁺ → Na⁺ substitution does not increase the fraction of Mn⁴⁺ in LCNM, but the increase in the concentration of lanthanum vacancies with increasing sodium content leads to an increase in ferromagnetic ordering temperature T _C and magnetoresistance, which depend, in addition, on heat-treatment conditions on account of the sodium volatility. Original Russian Text ? O.Z. Yanchevskii, A.I. Tovstolytkin, O.I. V’yunov, A.G. Belous, 2008, published in Neorganicheskie Materialy, 2008, Vol. 44, No. 2, pp. 226–233.     

Electrical transport properties of Nd0.67−x Eu x Sr0.33MnO3 (0 ≤ x ≤ 0.67) manganites

A systematic investigation of electrical transport properties viz., electrical conductivity and thermopower of Eu-doped Neodymium-based colossal magnetoresistive manganites with compositional formula, Nd_0.67−x Eu _x Sr_0.33MnO₃ (x = 0–0.67) has been undertaken. These materials were prepared by citrate gel route and characterized by X-ray diffraction, scanning electron microscopy, AC susceptibility, and electrical resistivity measurements. With a view to understand the complex conduction mechanism of these materials, electrical resistivity and thermoelectric power (TEP) data have been analyzed using various theoretical models. It has been concluded that the ferromagnetic metallic part of the conduction mechanism is explained by grain/domain boundary, electron–electron, and magnon scattering mechanisms, while that of high temperature paramagnetic insulating region might be due to small polaron hopping mechanism. The sign change of charge carriers observed in TEP measurements is attributed to the oxygen deficiency of the samples.     

Synthesis, structure, and magnetic properties of nanocrystalline Y3 − x La x Fe5O12 (0 ≤ x ≤ 0.6)

Y_{3 − x} La _x Fe₅O₁₂(x = 0, 0.2, 0.4, 0.6) nanocrystals have been prepared through hydroxide coprecipitation followed by dehydration. Increasing x from 0 to 0.6 reduces the particle size of the solid solution (from 54 to 43 nm), increases its unit-cell parameter, and influences its magnetic properties.     

Appearance of Griffiths Phase in La0.7−x Pr x Ba0.3MnO3 (0 ≤x≤ 0.2)

Polycrystalline La_0.7?x Pr _x Ba_0.3MnO₃ (0 ≤x≤ 0.2) samples were prepared using solid-state reaction and checked by X-ray diffraction. Magnetization measurements versus temperature and applied magnetic field were used to investigate their magnetic properties. For samples with x= 0.2, the Griffiths phase is observed when the inverse of susceptibility (1/ χ vs. T) is analyzed.     

Electrical properties of (Ni1−x )Li x )O (x=0.1 and 0.2) under various relative humidities

Rocksalt-type (Ni_1–x Li _x )O (x=0.1 and 0.2) was synthesized at 1350° C in air and its electrical resistivity (R) was measured under various relative humidities (H). R increases with increasing H in the range 0H79%, reaches a maximum value, then decreases in the range 79%<H100%. The increase in R is explained by an electron boundary layer model. On the other hand, the decrease in R is explained by ionic conductivity.     

SrTi1−x Fe x O3 nanoparticle: a study of structural,optical, impedance and magnetic properties

We investigate the effects of Fe-dopant concentration on the structure, as well as optical, electrical transport and magnetic properties of SrTi_{1? x} Fe _x O₃ (x?=?0.00, 0.10, 0.20, 0.30, 0.40 and 0.50) nanoparticles prepared by sol–gel method. The dopant-induced changes are studied by X-ray diffraction (XRD), Raman, optical absorption, impedance and magnetic measurements. The results show an average particle size of about 15–30?nm, depending on the Fe-doped concentration. The decrease in lattice parameters and the change of phonon modes related to structural changes, decreasing gap with increasing dopant concentration in conjunction with increasing grain boundary contribution to the impedance. The Fe-doped content has affected the structure, absorption and Raman spectroscopy of the samples. These indicate that the Fe ion has replaced the site of Ti in unit cell. By this method, we have decreased the annealing temperature considerably than that in the conventional solid-state reaction.     

Emission (Gd3+ and Sm3+) and ESR (Gd3+) studies of La1−xLnxB3O6 (Ln = Gd,Sm; 0 ≤ x ≤ 0.2 for Gd; 0 ≤ x ≤ 0.1 for Sm) phosphors

Polycrystalline powders of rare-earth doped La_1?xGd_xB₃O₆ (0?≤?x?≤?0.2) and La_1?xSm_xB₃O₆ (0.0?≤?x?≤?0.1) phosphors were successfully prepared by a B₂O₃ flux method. All the phosphor samples are well characterized by powder X-ray diffraction (XRD), infrared (IR), scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) methods and fluorescence lifetime of Sm³⁺ ion. The XRD patterns show that La_1?xM_xB₃O₆ (M?=?Gd and Sm) adopt monoclinic with the I2/a space group. The SEM–EDS results confirmed the doping of Gd and Sm into LaB₃O₆ lattice. The IR and Raman spectra of these solid solutions gave distinctive bands corresponding to planar BO₃ and tetrahedral BO₄ groups. The photoluminescence (PL) spectra of La_1?xGd_xB₃O₆ gave a strong emission band, ⁶P_J?→?⁸S_7/2, at 310 nm. The PL spectra of La_1?xSm_xB₃O₆ phosphor showed orange-red emission at 598 nm when excited using light of wavelength of 402 nm. The results were obtained by the transition ⁴G_5/2?→?⁶H_7/2 of Sm³⁺ ions. The influence of dopant concentration on the emission profiles was studied. The ESR spectra of La_1?xGd_xB₃O₆ (x?=?0.02) gave a typical U-spectrum and spin-Hamiltonian parameters are deduced.

     

First-principles investigations of structural, elastic, electronic and magnetic properties of Ga1−x Mn x P and In1−x Mn x P

We employ the full-potential linearized augmented plane wave plus local orbital (FP L/APW + lo) method based on the density functional theory (DFT) in order to investigate the structural, elastic, electronic, and magnetic properties of ordered dilute ferromagnetic semiconductors Ga_1?x Mn _x P and In_1?x Mn _x P at (x = 0.25) in the zinc blende phase, using generalized gradient approximation, GGA (PBE). To our knowledge the elastic constants of these compounds have not yet been measured or calculated, hence our results serve as a first quantitative theoretical prediction for future study. Results of calculated electronic structures and magnetic properties reveal that both Ga_0.75Mn_0.25P and In_0.75Mn_0.25P have stable ferromagnetic ground state, and they are ideal half-metallic (HM) ferromagnetic at their equilibrium lattice constants. Also we show the nature of the bonding from the charge spin-densities calculations. The calculated total magnetic moments are 4.0 μ_B per unit cell for both Ga_0.75Mn_0.25P and In_0.75Mn_0.25P, which agree with the Slater–Pauling rule quite well, and we observe that p–d hybridization reduces the local magnetic moment of Mn from its free space charge value and produces smaller local magnetic moments on the nonmagnetic Ga, In and P sites. The values of N ₀α and N ₀β exchange constants confirm the magnetic nature of these compounds. From the robust half-metallicity of Ga_0.75Mn_0.25P and In_0.75Mn_0.25P as a function of lattice constant is also investigated.     

Structural,optical, magnetic,and photoluminescence properties of Sn0.7−xMo0.3 NdxO2+δ (0.0 ≤ x ≤ 0.3)

Journal of Materials Science: Materials in Electronics - In this study, the properties of a series of (Sn0.7?xMo0.3 NdxO2+δ) (0.0?≤?x?≤?0.3) Nd3+ thin...