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We have investigated the hydrogen-annealing influence on the crystalline and electronic structures, and magnetic properties of La2/3Ca1/3MnO3 (LCMO). The results have indicated that the annealing at 700 and 900°C (labeled as LCMO-700 and LCMO-900, respectively) readily reduced single-phase LCMO, resulting in a complex phase composition of several isostructural oxygen-deficient perovskite-type phases coexisting with the Ruddlesden-Popper (La/Ca)2MnO4-type phase. While a Mn3+/Mn4+ mixed valence is present in LCMO, the hydrogenannealed samples mainly have Mn2+ and Mn3+ ions. Under such circumstance, large changes in magnetic parameters have been recorded, such as remarkable decreases in values of the magnetization, the Curie temperature (from 251 K for the as-prepared LCMO through 240 K for LCMO700 to ³221 K for LCMO-900), and the magnetic-entropy change. Particularly, the crystal and electronic-structure changes also enhance the magnetic inhomogeneity, resulting in a strong development of the Griffiths phase, and cause the first-to-second-order phase transformation. These results reflect the instability of the LCMO perovskite-type manganite versus hydrogenation. [ABSTRACT FROM AUTHOR]

We have systematically investigated the crystalline and electronic structures, magnetic and magnetocaloric properties of polycrystalline SrRu1−xFexO3 (x = 0 and 0.1) samples fabricated by the solid-state reaction method. The X-ray diffraction analyses of the samples indicated single phase orthorhombic perovskite structure. A thorough analysis of X-ray-absorption-based electronic structure revealed that both Fe2+ and Fe3+ ions are present in x = 0.1, in which concentration of Fe3+ ions is higher than that of Fe2+ ions. More careful analyses on the isothermal magnetization data derived from the Banerjee’s criterion demonstrated that the ferromagnetic-paramagnetic phase transition in all samples belongs to the second-order phase transition type. These results were also confirmed by a recently proposed quantitative criterion, which considered an exponent n from the magnetic field and temperature dependences of the magnetic entropy change (ΔSm). Especially, around TC, we have found that the ΔSm reaches the maximum values of 1.65 and 1.32 J.kg−1.K−1 for x = 0 and 0.1, respectively, for a field change of ΔH = 50 kOe. Magnetic-field dependences of the maximum magnetic-entropy change (ΔSmax) obey a power law of ΔSmax(H) ∝ Hn, where the values of n = 0.92–0.94 are far from the mean-field-theory value (2/3), indicating short-range magnetic order existing in the samples. [ABSTRACT FROM AUTHOR]

This paper presents the magnetocaloric properties of polycrystalline La0.8–xAgxCa0.2MnO3 manganites ( x = 0.0 , 0.05, and 0.1) prepared by solid-state reaction. The results reveal that an Ag-concentration increase leads to a slight increase of the Curie temperature ( T_{C}) , and broadens the ferromagnetic–paramagnetic phase-transition region. The temperature dependences of the magnetic entropy change, \Delta S_{m}(T) , for the samples have also been calculated using Maxwell’s relation. As a function of temperature, -\Delta Sm(T) curve shows the maximum value (denoted as \vert \Delta S\mathrm {max}\vert ) around TC . The field dependences of \vert \Delta S\mathrm {max}\vert and refrigerant capacity (RC) can be expressed by the power laws of \vert \Delta S\mathrm {max}\vert = a\times \Delta H^{n} and \mathrm RC =b\times \Delta H^N , respectively. Based on M(H) isotherms and the universal entropy method for the \Delta S_{m}(T) data measured at different values of \Delta H$ , we point out that the samples exhibit the crossover of the first-to-second-order phase transformation. A partial replacement of La by Ag in La0.8–xAgxCa0.2MnO3 leads to modify the first-order to second-order phase transition in the samples. [ABSTRACT FROM PUBLISHER]

We have investigated the magnetic properties and magnetocaloric effect in \La0.7{\Sr}0.3{\Mn}1\-x{\Co}_x{\O}3 prepared by a conventional solid-state reaction method. Magnetic measurements versus temperature revealed that the Curie temperature (TC) decreased gradually with increasing Co content (x); TC values are about 360, 310, 296 and 280 K for x = 0.0, 0.06, 0.08 and 0.1, respectively. Magnetic entropy change (\Delta Sm) of the samples under a magnetic field change of 10 kOe was calculated by using isothermal magnetization data. We have found its maximum (\vert\Delta S\max\vert) achieved around TC, which is not changed much (\sim 1.5~\J\cdot\kg^\-1\cdot\K^\-1) with increasing Co-doping concentration. If combining the samples with x = 0.06, 0.08 and 0.10 for magnetic refrigeration, particularly, the temperature range can be used in between 266 K and 322 K, where \vert\Delta S\max\vert values are stable at about 0.98~\J\cdot\kg^\-1\cdot\K^\-1. The relative cooling power is accordingly about 55~\J\cdot\kg^\-1 and comparable to that of other magnetocaloric alloys. These results suggest \La0.7{\Sr}0.3{\Mn}1\-x{\Co}_x{\O}3 compounds to be a potential candidate for magnetic refrigerators at room temperature. [ABSTRACT FROM PUBLISHER]

We have studied magnetic and magnetotransport properties of a polycrystalline ceramics of La0.7Sr0.2Ca0.1MnO3 (LSCM) prepared from nanoparticles. The Curie temperature (TC) of the sample is about 340 K. Basing on the magnetic-field dependences of magnetization measured around TC, maximum magnetic-entropy changes under the applied fields of 10, 20 and 30 kOe are about 1.73, 2.74, 3.52 J/kg.K, respectively, which are higher than those reported on a parent compound of La0.7Sr0.3MnO3. To understand magnetic-interaction mechanisms taking place in the system, the critical behavior has been taken into account. By using the modified Arrott plot method, \beta and \gamma values associated with the spontaneous magnetization (Ms) and inverse initial susceptibility (\chi0^{-1}) have been obtained to be 0.472 \pm 0.016 and 1.061 \pm 0.024, respectively. These critical parameters are close to those expected for the mean-field theory (with \beta=0.5, and \gamma=1.0). A small deviation in value between our results and those expected for the mean-field theory could be due to an inhomogeneity in magnetism and/or interaction competitions between antiferromagnetic pairs (Mn^3+-Mn^3+ and Mn^4+-Mn^4+) and a ferromagnetic pair of Mn^3+-Mn^4+ in LSCM. Interestingly, in the temperature range of 180–300 K, magnetoresistance values keep quite stable at about 16 and 27% for the magnetic fields of 1 and 7 kOe, respectively. This makes LSCM more suitable to applications of magnetic sensitive sensors and reading heads working in a large temperature range. [ABSTRACT FROM PUBLISHER]

We have studied the influence of the Cu-doping on the critical properties of a polycrystalline sample of La0.7Ca0.3Mn0.95Cu0.05O3 prepared by solid-state reaction. Analyses of static magnetization data in the vicinity of the ferromagnetic-paramagnetic phase transition based on the modified Arrott plot reveal critical parameters of TC\approx 197.3\ \rm K, \beta=0.49\pm 0.03 and \gamma=1.04\pm 0.04. These parameters are in good agreement with a magnetic equation of state M(H,\varepsilon)=\varepsilon^\betaf\pm(H/\varepsilon^{\beta+\gamma}), where \varepsilon=(T-TC)/TC (the reduced temperature), f+ for T>TC and f- for T