Evolution of Griffiths phase and critical behaviour of La1-xPbxMnO3 +/- y solid solutions

Polycrystalline La1-xPbxMnO3 +/- y (x = 0.3, 0.35, 0.4) solid solutions were prepared by solid state reaction method and their magnetic properties have been investigated. Rietveld refinement of x-ray powder diffraction patterns showed that all samples are single phase and crystallized with the rhombohedral structure in the R-3c space group. A second order paramagnetic to ferromagnetic (FM) phase transition was observed for all materials. The Griffiths phase (GP), identified from the temperature dependence of the inverse susceptibility, was suppressed by increasing magnetic field and showed a significant dependence on A-site chemical substitution. The critical behaviour of the compounds was investigated near to their Curie temperatures, using intrinsic magnetic field data. The critical exponents (beta, gamma and delta) are close to the mean-field approximation values for all three compounds. The observed mean-field like behaviour is a consequence of the GP and the formation of FM clusters. Long-range FM order is established as the result of long-range interactions between FM clusters. The magnetocaloric effect was studied in terms of the isothermal entropy change. Our study shows that the material with the lowest chemical substitution (x = 0.3) has the highest potential (among the three compounds) as magnetic refrigerant, owing to its higher relative cooling power (258 J kg(-1) at 5 T field) and a magnetic phase transition near room temperature

Impact of Annealing Temperature on the Physical Properties of the Lanthanum Deficiency Manganites

International audienceThe lanthanum deficiency manganites La0.8-x□xCa0.2MnO3 (x = 0, 0.1 and 0.2), where □ is a lanthanum vacancy, were prepared using the classic ceramic methods with different thermal treatments (1373 K and 973 K). The structural, magnetic, and magnetocaloric properties of these compounds were studied as a function of annealing temperature. It was noted that the annealing temperature did not affect the crystal structure of our samples (orthorhombic structure with Pnma space group). Nevertheless, a change in the variation of the unit cell volume V, the average bond length dMn–O, and the average bond angles θMn–O–Mn were observed. Magnetization versus temperature study has shown that all samples exhibited a magnetic transition from ferromagnetic (FM) to paramagnetic (PM) phase with increasing temperature. However, it can be clearly seen that the annealing at 973 K induced an increase of the magnetization. In addition, the magnetocaloric effect (MCE) as well as the relative cooling power (RCP) were estimated. As an important result, the values of MCE and RCP in our Lanthanum-deficiency manganites are reported to be near to those found in gadolinium, considered as magnetocaloric reference material

Transport and magneto-transport properties in La0.8K0.2-xxMnO3-delta (x =0 and 0.1) manganites

We discuss the effect of the nanometric grain size on the behavior of the electrical and magnetoresistive response of La0.8K0.2-xxMnO3-delta (x = 0 and 0.1) nanocrystalline samples that were prepared by a sol-gel method. The results from transport and magneto-transport measurements evidence a robust dependence on the nanometric grain size. The temperature dependence of the resistivity was evaluated using different transport models. The results reveal a field-dependent minimum of the resistivity in the low-temperature region, which can be described in terms of intergranular spin-polarized tunneling. Remarkably, a considerable increase of the magnetoresistance (MR) with the decrease of nanoparticle size was found, which might open a new way for the search for potential candidates for magnetoresistive devices. Besides, the magnetic field dependence of the MR was also analyzed, and a distinct drop of MR at low fields was noticed. This behavior was primarily explained by the spin-polarized tunneling transport of conduction electrons across grain boundaries

Effect of small cation occupancy and anomalous Griffiths phase disorder in nonstoichiometric magnetic perovskites

The structural, magnetic, magnetocaloric and Griffiths phase (GP) disorder of non-stoichiometric perovskite manganites La0.8-xSr0.2-yMn1+x+yO3 are reported here. Determination of valence states and structural phases evidenced that the smaller cations Mn2+ and Mn3+ will not occupy the A-site of a perovskite under atmospheric synthesis conditions. The same analysis also supports that the vacancy in the A-site of a perovskite induces a similar vacancy in the B-site. The La3+ and Sr2+ cation substitutions in the A-site with vacancy influences the magnetic phase transition temperature (TC) inversely, which is explained in terms of the electronic bandwidth change. An anomalous non-linear change of the GP has been observed in the Sr substituted compounds. The agglomeration of Mn3+-Mn4+ pairs (denoted as dimerons), into small ferromagnetic clusters, has been identified as the reason for the occurrence of the GP. A threshold limit of the dimeron formation explains the observed non-linear behaviour of the GP formation. The Sr-substituted compounds show a relatively large value of isothermal entropy change (maximum 3.27 J/kgK at mu H-0 = 2T) owing to its sharp magnetic transition, while the broad change of magnetization in the La-substituted compound enhances the relative cooling power (maximum 98 J/kg at mu H-0 = 2T)

Field induced crossover in critical behaviour and direct measurement of the magnetocaloric properties of La0.4Pr0.3Ca0.1Sr0.2MnO3

La0.4Pr0.3Ca0.1Sr0.2MnO3 has been investigated as a potential candidate for room temperature magnetic refrigeration. Results from X-ray powder diffraction reveal an orthorhombic structure with Pnma space group. The electronic and chemical properties have been confirmed by X-ray photoelectron spectroscopy and ion-beam analysis. A second-order paramagnetic to ferromagnetic transition was observed near room temperature (289 K), with a mean-field like critical behaviour at low field and a tricritical mean-field like behaviour at high field. The field induced crossover in critical behaviour is a consequence of the system being close to a first-order magnetic transition in combination with a magnetic field induced suppression of local lattice distortions. The lattice distortions consist of interconnected and weakly distorted pairs of Mn-ions, where each pair shares an electron and a hole, dispersed by large Jahn–Teller distortions at Mn3+ lattice sites. A comparatively high value of the isothermal entropy-change (3.08 J/kg-K at 2 T) is observed and the direct measurements of the adiabatic temperature change reveal a temperature change of 1.5 K for a magnetic field change of 1.9 T

Effect of reduced local lattice disorder on the magnetic properties of B-site substituted La0.8Sr0.2MnO3

Disorder induced by chemical inhomogeneity and Jahn-Teller (JT) distortions is often observed in mixed valence perovskite manganites. The main reasons for the evolution of this disorder are connected with the cationic size differences and the ratio between JT active and non-JT active ions. The quenched disorder leads to a spin-cluster state above the magnetic transition temperature. The effect of Cu, a B-site substitution in the La0.8Sr0.2MnO3 compound, on the disordered phase has been addressed here. X-ray powder diffraction reveals rhombohedral (R-3c) structures for both the parent and B-site substituted compound with negligible change of lattice volume. The chemical compositions of the two compounds were verified by ion beam analysis technique. With the change of electronic bandwidth, the magnetic phase transition temperature has been tuned towards room temperature (318 K), an important requirement for room temperature magnetic refrigeration. However, a small decrease of the isothermal entropy change was observed with Cu-substitution, related to the decrease of the saturation magnetization

Structural and Magnetocaloric Properties of ErFe 2 . 4 Al 0 . 6 Compound

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Structural and Magnetocaloric Properties of ErFe 2 . 4 Al 0 . 6 Compound

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