On-line phase transition in La1− x Sr x MnO3 (0.28 ≤ x ≤ 0.36) perovskites through ultrasonic studies

La_{1? x} Sr _x MnO₃ perovskite manganite materials with different compositions (x?=?0.28, 0.31 and 0.36) have been prepared employing solid-state reaction technique. On-line evaluation of ultrasonic velocities and longitudinal attenuation of the above samples has been done over a wide range of temperatures using the transmission method. The observed anomalies in velocities, attenuation and elastic moduli reveal the occurrence of lattice softening and hardening near Curie temperature. The observed dramatic hardening in sound velocities and softening in attenuation are correlated with the phase transition, i.e. ferromagnetic to paramagnetic. The increase in magnitude of maximum velocity with change in Sr content at T _c indicates the existence of linear magnetostriction effect. The elastic moduli study elucidates the observations made from the above-mentioned studies. The variation in the ultrasonic velocities, longitudinal attenuation and its derived parameters help us to understand the competitions between ferromagnetism and paramagnetism.     

Phase diagram of perovskite manganites Sm1 − x Sr x MnO3

Physics of the Solid State - The magnetic and structural phase diagram of perovskite manganites Sm1 ? x Sr x MnO3 (0.16 ≤ x ≤ 0.67) are constructed based on systematic studies by...     

Percolation conductivity of single-crystal manganite Pr1 − x Sr x MnO3 (x = 0.22; 0.24) in the phase-separation state

This paper presents an experimental study of the temperature dependences of the electrical resistivity, thermopower, and magnetoresistance of single-crystal manganite Pr_{1 ? x} Sr _x MnO₃ (x = 0.22, 0.24). The results obtained have been analyzed in terms of the theory of hopping electrical conductivity of manganites in the phase-separation state and the main postulates of percolation theory. In the 2–400-K interval, hopping conductivity with temperature-dependent activation energy has been observed. Near the Curie temperature, the monotonic exponential growth of electrical resistivity with decreasing temperature becomes distorted by formation of finite metallic clusters. For T ≤ 36 K, the conductivity of Pr_0.8Sr_0.22MnO₃ follows the Mott law.     

On the ferromagnetic perovskite with potential GMR properties: The manganite La1−x K x MnO3

We investigate structural features (cell distortions), magnetic and electric properties versus temperature for La_1-x K _x MnO₃ (0.05 < x < 0.2) perovskites. All the phases crystallize in a rhomboedral symmetry (R3c) and are ferromagnetic. Curie temperatures are strongly dependent of x, with a maximum around 308 K for La_0.8K_0.2MnO₃. A sharp decrease of the resistivity is observed just below the Curie temperature, accompanied by a very steep increase of the spontaneous magnetization, characteristic of a conventional Brillouin type curve. The average magnetic moment of Mn is about 88 e.m.u/g at 5 K for the composition La_0.8K_0.2MnO₃. The transition from ferromagnetic-metallic to paramagnetic-semiconducting states is explained from the suggested distribution of the cations (La³⁺ _1-x K⁺ _x )_A(Mn⁴⁺ _1-2x Mn⁴⁺ _2x )_BO₃ by the double exchange of Mn³⁺-Mn⁴⁺ pairs at the B-sublattice. Preliminary magnetoresistance measurements on bulk ceramic samples display an MR effect of the same amplitude as in the case of the alkaline earth substituted La manganites.     

Modelling of structural domains and elastic strain calculation in rhombohedral La1−x Sr x MnO3 films on (110) SrTiO3

As reported in the literature, structural domains in (110) oriented rhombohedral La_{1? x} Sr _x MnO₃ (LSMO) films on (110) SrTiO₃ show a pattern with alternating domain widths and inclined domain walls. An appropriate one-dimensional periodic domain model was developed and the non-uniform strain field in a coherently grown film was calculated by applying the coherency-defect approach. The strain is sharply peaked at the junction of the domain walls and the film/substrate interface. The dependence of the equilibrium domain-width ratio and domain-pattern period on film thickness was determined by minimizing the sum of elastic and domain-wall energies. The proposed domain structure can be formed only in a certain range of the film's crystal-structure data compared to the substrate. Corresponding composition-dependent structure data of LSMO are compiled.     

Heat capacity and the magnetocaloric effect in Pr0.6Sr0.4Mn1–x Fe x O3 manganite

The heat capacity and the magnetocaloric effect of Pr_0.6Sr_0.4Mn_1–xFe_xO₃(x = 0 and 0.1) manganite have been studied in the temperature range 80–350 K and magnetic fields to 18 kOe. The magnetocaloric effect is estimated using two independent methods: the method of magnetic field modulation (direct method) and from the data on the heat capacity in magnetic field and without magnetic field (indirect method). The substitution of Fe atoms for Mn atoms (x = 0.1) shifts T _C by 167 K to lower temperatures; in this case, the magnetocaloric effect (MCE) is changed insignificantly in magnetic field 18 kOe with ΔS _M = 2.05 and 2.31 J/kg K for x = 0 and 0.10, respectively.

     

Polaronic and charge ordering effects in Pr 0.5 Sr 0.5 Mn 1− x Fe x O 3

In this work we report the results of X-ray diffraction and Mössbauer spectroscopy for the systems Pr_0.5Sr_0.5Mn_1???x Fe _x O₃ (with x?=?0.05, 0.10, 0.15, 0.20, 0.25, 0.30). XRD patterns indicated that all samples were single phase with slightly distorted orthorhombic symmetry. Room temperature Mössbauer spectra are all quadrupole split, indicating paramagnetic relaxation of the Fe moment for all values of Fe concentrations. The spectra are fitted with two doublet components associated with Fe^3?+? ions in octahedral sites with different distortions. Mössbauer spectra recorded at liquid nitrogen temperature for this system also indicate paramagnetic relaxation of the Fe moments down to liquid nitrogen temperature (LNT). In these spectra a third quadrupole component with quadrupole splitting close to zero develops. This component is associated with the delocalization of the charge carriers and the consequent disappearance of lattice distortions produced by the polaronic effect at room temperature. The component with the high quadrupole splitting (0.81 to 1.07 mm/s) results from Jahn–Teller distortion as a consequence of charge ordering transition at low temperature.     

Electron paramagnetic resonance studies on manganite Pr0.5Sr0.5Mn1−x Ga x O3 (x=0 and 0.05)

In this paper, we present the investigations of electron paramagnetic resonance on perovskite manganite Pr_0.5Sr_0.5MnO₃ and Ga-doped Pr_0.5Sr_0.5Mn_0.95Ga_0.05O₃. The temperature dependent paramagnetic resonance spectra parameters (effective g-factor, peak-to-peak linewidth ΔH _pp and double integrated intensities) have been used to study the paramagnetic spin correlations and spin dynamics. The gradual increase of effective g-factor is attributed to the presence of orbital ordering above T _C. The model fittings of temperature dependent double integrated intensities reveal Arrhenius law is appropriate for describing Pr_0.5Sr_0.5Mn_0.95Ga_0.05O₃ instead of Pr_0.5Sr_0.5MnO₃ system. As for Pr_0.5Sr_0.5MnO₃, the broadening of linewidth with the temperature increase origins from the contribution of small polaron hopping in the PM regime. However, as for Pr_0.5Sr_0.5Mn_0.95Ga_0.05O₃, the broadening of EPR linewidth can be understood with the spin-lattice relaxation mechanism.     

Thermal-expansion and magnetostriction anomalies in phase transitions in La1−x SrxMnO3

A study is reported on phase transitions in the La_1?x Sr_xMnO₃ system, both spontaneous and induced by a pulsed magnetic field of up to 250 kOe, accompanied by anomalies in magnetoelastic properties. The temperatures of the polaron (charge) and magnetic ordering, as well as those of structural transitions, are observed to be in good agreement with the results obtained by other methods. Jumps in the field dependence of longitudinal and transverse magnetostriction associated with field-induced orbital ordering have been found. A strong temperature dependence of the corresponding threshold fields is observed.     

Lattice,electrical, and magnetic effects in lanthanum manganites La1−x Sr x MnO3 (x = 0.125, 0.150, 0.175)

The temperature dependences of the velocities of transverse and longitudinal hypersound in lanthanum strontium manganites of the composition La_1−x Sr _x MnO₃ (x = 0.125, 0.150, 0.175) have been measured at frequencies of 0.5–0.7 GHz. The structural phase transitions have been revealed, and their positions have been confirmed by data on the electrical resistance and magnetic measurements performed using the same samples. The results obtained have been analyzed in the framework of the model of competing Jahn-Teller distortions and magnetic ordering. The anomalies observed in the behavior of the velocities of longitudinal hypersound have been attributed to the local Jahn-Teller distortions, and their suppression due to the magnetic ordering has been considered a possible factor responsible for the colossal magnetoresistance.     

Enhanced polarization and magnetoelectric response in Tb1−x Ho x MnO3

A series of manganites Tb_1−x Ho _x MnO₃ (0≤x≤0.6) with orthorhombic structure are synthesized and detailed investigations on their multiferroicity are performed. Successive magnetic transitions upon temperature variation are evidenced for all the samples, and both the Mn³⁺spiral spin ordering and rare-earth spin ordering are suppressed with increasing x. A significant enhancement of both the polarization and magnetoelectric response within 0.2<x<0.4 is observed, which may probably result from the shortening of the spiral-spin-ordering period, due to the competition between the spiral spin order and E-type antiferromagnetic order. This argument is supported by further theoretical calculations based on the two e _g -orbital double-exchange model.     

The influence of disorder on magnetocaloric effect and the order of transition in ferromagnetic manganite (La1−x Nd x )2/3(Ca1−y Sr y )1/3MnO3

Magnetocaloric effect and order of transition in (La_1?x Nd _x )_2/3(Ca_1?y Sr _y )_1/3MnO₃, prepared by conventional solid-state reaction, have been investigated. Using Banerjee criterion, we demonstrate first-order transition for (J1) and (J2 ) as well as second-order transition for (J3 ), (J4 ), and (J5 ) samples. The ΔS _M ^max is ranging between 9.18 Jkg^?1 K^?1 and 4.87 when Nd and Sr content changes leading to relative cooling power (RCP) varying between 330 and 229.35 J/kg. Both ΔS _M ^max and the RCP are found sensitive to the disorder σ ². The universal behavior obtained from ΔS variation curves confirmed the first-order transition for (J1) and (J2 ) samples and second-order transition for (J3), (J4), and (J5 ) samples obtained by Banerjee criterion. All samples with second-order phase transition exhibit inhomogeneous character estimated from local exponent n.     

Structural and magnetic properties with reversible magnetocaloric effect in PrSr1–xPbxMn2O6 (0.1 ≤ x ≤ 0.3) double perovskite manganite structures

In this study, the effect of Pb substitution with Sr on structural, magnetic and magnetocaloric properties in Pr(Sr_1–xPb_x)Mn₂O₆ (0.1?≤?x?≤?0.3) double perovskite manganite compounds prepared by the sol–gel method has been reported. Crystallinity has been worked by X-ray diffraction measurement and the Rietveld refinement method, which shows all samples are in monoclinic structure with P21/n space group without impurity phase. Scanning electron microscope analysis shows that all samples constituted from square-shaped grains with distinct grain boundaries, and energy-dispersive X-ray spectroscopy analysis shows that all expected elements are detected and there is no impurity element in the sample. All samples exhibit a magnetic phase transition from ferromagnetic to paramagnetic phase at T_C determined as 274, 282 and 286?K for x?=?0.1 (PSPM-1), 0.2 (PSPM-2) and 0.3 (PSPM-3) in the PrSr_1–xPb_xMn₂O₆ structure, respectively. Isothermal magnetisation curves near T_C are used to determine the nature of the magnetic phase transition and the magnetic entropy change values (?ΔS_M), which are found as 2.78, 3.11 and 3.52?J?kg^?1?K^?1 for PSPM-1, PSPM-2 and PSPM-3 samples, respectively. Relative cooling power (RCP) values of the samples are also determined as 241, 311 and 246?J?kg^?1 for PSPM-1, PSPM-2 and PSPM-3 samples, respectively. ΔS_M and RCP values are also calculated by using the Landau theory and the results are in accordance with the experimental results, which indicate the role of electron condensation energy and magneto-elastic coupling in magnetic entropy change value of the samples.     

In-plane and out-of-plane anisotropic magnetoresistances in La1 − x Pb x MnO3 thin films

The temperature and field dependences of in-plane (IP) and out-of-plane (OP) anisotropic magnetoresistances (AMRs) have been measured in La_{1 ? x} Pb _x MnO₃ (LPMO) thin films having different microstructures, namely the single-crystal (SC), nanocrystalline (NC) and polycrystalline (PC) microstructures. The OP AMR, irrespective of the microstructure, is found to be considerably larger than the IP AMR. The magnetization data show that the larger OP AMR arises because magnetization is favoured in the film plane owing to strain anisotropy. In addition, the temperature and field dependences of both IP and OP AMRs are governed by the crystallinity of the films, indicating that the AMRs are strongly influenced by the magnetization process.     

Structural and dielectric anomaly in Na1−XKxNbO3, at x = 0.315

Structural and dielectric properties of sodium-potassium niobate (Na_1?x K_xNbO₃) (0.250?≤?x?≤?0.350) ceramics, prepared by the solid-state reaction method were investigated. Observed dielectric properties show a strong compositional dependence, at all the measured temperatures (from room temperature to 150°C) and frequencies (from 1 to 1000?kHz). In the prepared compositions, the lowest values of dielectric permittivity, loss tangent and electrical conductivity were found, and an anomaly in the shifting pattern of X-ray diffraction (XRD) peaks and change in the slope of a/c vs. x plot were observed at x?=?0.315. The observed structural and dielectric anomaly gives a morphotropic phase boundary-like indication, in Na_1?xK_xNbO₃ system, at x?=?0.315.     

Effect of Sr content on structure and electrical properties of La1−x Sr x MnO3 from ITSOFC cathode view point

The samples belonging to La_1−x Sr _x MnO₃ series, prepared by combustion route without using fuel, exhibit crystal structural phase transition with the change in Sr content. Less than 40 mol% Sr is partially substituted in the crystal structure of LaMnO₃. The structural phase transition from rhombohedral to cubic, cubic to tetragonal, and tetragonal to orthorhombic takes place on 40, 60, and 80 mol% addition of Sr. The highest electrical conductivity in La_0.4Sr_0.6MnO₃ is understood to be due to the maximum concentration of polaron. The polarons are formed due to the conversion of Mn³⁺ to Mn⁴⁺ so as to achieve electroneutrality after substitution of Sr²⁺ for La³⁺.     

Transport effects in La0.67−x R x Sr0.33MnO3 (R=Eu, Gd)

An experimental study is reported on the temperature dependences of the magnetic susceptibility, electrical resistivity, magnetoresistance, thermo-and magnetothermopower, and on the Hall effect of the polycrystalline lanthanum manganites La_0.67?x R _x Sr_0.33MnO₃ (x=0 and 0.07, R=Eu, Gd) within the 77-to 430-K temperature range. Replacement of a small amount of lanthanum by europium or gadolinium was found to result in a considerable decrease in the resistivity and in a change in the pattern of its temperature dependence. The temperature dependence of both the ordinary and anomalous Hall coefficients near the Curie temperature is shown to be determined by the change in the number of carriers in delocalized states. A method is proposed for separation of the hole and electronic contributions to thermopower in the ferromagnetic region. The conduction mechanisms are discussed in terms of the concept of mobility edge motion.