Effect In La0 Research Articles

Room temperature spontaneous exchange bias in (La,Sr)MnO3/PbZr0.8Ti0.2O3/(La,Sr)MnO3 sandwich structure

We report on a room temperature spontaneous exchange bias (SEB) effect in La0.67Sr0.33MnO3/PbZr0.8Ti0.2O3/La0.67Sr0.33MnO3 (LSMO/PZT/LSMO) sandwich structure, where PZT is a discontinuous film producing somewhat the direct touch of two LSMO layers. The exchange coupling between the top and bottom LSMO, which, respectively, behaves as the antiferromagnetic and ferromagnetic layer, is demonstrated to be responsible for the SEB effect. The formation of antiferromagnetic LSMO is ascribed to the large strain imposed through the PZT seed layer, whose thickness profoundly affects the SEB behavior. Thus, our finding provides an appealing alternative way to generate exchange bias by strain engineering.

Simulation of Magnetocaloric Effect in La0.7Ca0.3MnO3 Ceramics Fabricated by Fast Sintering Process

The enhanced low-field magnetocaloric effect (MCE) is simulated for La0.7Ca0.3MnO3 (LCMO) ceramics that were fabricated by fast sintering process with different Al2O3 contents. It is shown that LCMO exhibits magnetic entropy change (ΔSM) much more uniform than that of gadolinium. The results show that the peak in the MCE at the ferromagnetic to paramagnetic phase transition is improved as the sintering temperature decreases. Furthermore, the samples open up a new way in which to tune the intrinsic properties of mixed-valence manganites. Through these results, LCMO has some potential applications for magnetic refrigerants in an extended high-temperature range. It is suggested that the fast sintering process with different Al2O3 contents for LCMO is an efficient way to obtain a working material of an apparatus based on the active magnetic regenerator cycle that cools hydrogen gas.

Giant magnetoimpedance and grain boundary effect in La0.75Ba0.25MnO3 sol–gel manganite

The La0.75Ba0.25MnO3 sol–gel manganite shows a typical dc low field magnetoresistance (LFMR) effect at low temperature. Besides the sharp resistance peak representing the metal–insulator transition around Curie temperature, another broad peak associated with the effect of grain boundaries could be observed around 260K. At low frequencies, such double peaks in ac resistance also occur. With increasing ac frequency, skin effect gradually enhances, the peak associated with the grain boundaries becomes less pronounced. At 0.5MHz, beside the peak of ac magnetoresistance ΔR/R0 at Curie temperature, the ac ΔR/R0 increases with decreasing temperature, since the ac resistance is strongly dependant on dc resistivity at low frequency. With increasing ac frequency, the peak of ac ΔR/R0 at Curie temperature becomes weak and almost disappears, as the ac ΔR/R0 strongly depends upon variation of magnetic-permeabilities at high frequencies. The magnetoreactance originates mainly from the change of permeability induced by the field. At 300K, the value of magnetoimpedance ΔZ/Z0 for La0.75Ba0.25MnO3 sol–gel polycrystalline sample can reach −49% at f=4MHz under a very small field of 500Oe.

Proximity effect in La0.7Sr0.3MnO3/Bi2Sr2CaCu2O8 hybrid bi-layers

The electrical resistivity and magnetization of half-metallic/superconducting La0.7Sr0.3MnO3/Bi2Sr2CaCu2O8 (LSMO/Bi-2212) heterostructures are investigated with the thickness of Bi-2212 top layer fixed at 50 nm and that of the LSMO underlayer (t) being 40, 80 and 160 nm. Both the normal state resistivity ρ(290 K) and the superconducting transition width (δTC) increase with increasing t. On the other hand, the superconducting transition onset temperature (Tcon) of Bi-2212 monotonically decreases with increasing t, which is attributed to pair breaking via the polarized spins from the ferromagnetic LSMO due to strengthening of the magnetization. However, the rate of reduction of Tcon in Bi-2212 with respect to the thickness of LSMO is found to be much less in LSMO(t)/Bi-2212 compared with that of YBCO in the analogous system PCMO(t)/YBCO. From the qualitative analysis of the electrical and magnetic data we conclude that growing domains enhancing the magnetization is the reason for the thickness-dependent suppression of superconductivity in LSMO(t)/Bi-2212 bi-layers.

Strain dependent magnetocaloric effect in La0.67Sr0.33MnO3 thin-films

The strain dependent magnetocaloric properties of La0.67Sr0.33MnO3 thin films deposited on three different substrates (001) LaAlO3 (LAO), (001) SrTiO3 (STO), and (001) La0.3Sr0.7Al0.65Ta0.35O9 (LSAT) have been investigated under low magnetic fields and around magnetic phase transition temperatures. Compared to bulk samples, we observe a remarkable decrease in the ferromagnetic transition temperature that is close to room temperature, closely matched isothermal magnetic entropy change and relative cooling power values in tensile strained La0.67Sr0.33MnO3 films. The epitaxial strain plays a significant role in tuning the peak position of isothermal magnetic entropy change towards room temperature with improved cooling capacity.

Phase separation and direct magnetocaloric effect in La0.5Ca0.5MnO3 manganite

A series of phase separated La0.5Ca0.5MnO3 manganite samples with different grain sizes were studied by ac susceptibility, direct magnetocaloric effect (ΔT), and heat capacity measurements. The ac susceptibility shows that fractions of ferromagnetic and antiferromagnetic phases and consequently the phase separated state can be controlled by means of sintering temperature. Lower sintering temperature leads to a ferromagnetic state, while higher sintering temperature increases antiferromagnetic phase fraction, resulting in a phase separated state. In the phase separated samples, ΔT shows a conventional positive peak near TC and an anomalous positive peak at lower temperature near TN. The anomalous positive peak appears at higher magnetic field and is accompanied with thermal hysteresis. It is suggested that the anomalous magnetocaloric behaviors result from phase separation and first order magnetostructural phase transition. This study shows that direct magnetocaloric effect is a useful technique for the study of manganites.

A hybrid-exchange density functional study of Ca-doped LaMnO3

In this work, hybrid-exchange density functional theory calculations are carried out to determine the effects of A-site doping on the electronic and magnetic properties of the manganite series La1−xCaxMnO3. This study focuses on the nature of the ground state for an ordered Ca distribution. We show that the hybrid exchange functional, B3LYP, provides an accurate and consistent description of the electronic structure for LaMnO3 and La0.75Ca0.25MnO3. The magnetic ground states for the compositions studied are predicted correctly and comparisons have been made to available experimental data. This provides a basis for a first principles description of the magnetocaloric effect in La1−xCaxMnO3.

Thermal properties, magneto- and baro-caloric effects in La0.7Pb0.3MnO3 single crystal

The results of heat capacity, thermal dilatation and T-p phase diagram studies on the La0.7Pb0.3MnO3 single crystal are reported. Direct measurements of intensive magnetocaloric effect are performed by means of adiabatic calorimeter. Barocaloric effect is determined using data of heat capacity and susceptibility to hydrostatic pressure. Caloric efficiency of manganite in the vicinity of ferromagnetic phase transition is discussed and compared with that of other magnetic materials.

X-ray photoelectron diffraction study of dopant effects in La0.7X0.3MnO3 (X = La, Sr, Ca, Ce) thin films

We present and discuss element-specific x-ray photoelectron diffraction (XPD) patterns of La, Mn, O and the dopant ions Ca, Sr and Ce of various La0.7X0.3MnO (LXMO) films grown epitaxially on SrTiO3(001) substrates. The recorded XPD polar scans are explained in the framework of multiple-scattering cluster calculations, where in general a very good agreement between experimental and theoretical data has been found. Results for all thin films are compatible with a tetragonally distorted cubic perovskite structure with similar MnO6 network. Strong evidence for Mn-site termination was found in all thin films. Dopant locations on A-type sites were clearly confirmed for Sr in LSMO and Ca in LCMO films by means of XPD polar scans. The absence of surface-sensitive Ce3d diffraction features for Ce in LCeMO points to non-equivalent Ce sites and related near-surface disorder.

Resistivity, specific heat, and magnetocaloric effect of La0.8Ag0.1MnO3: Effect of isotopic substitution of 16O → 18O

Results of the experimental studies of resistivity, specific heat, and magnetocaloric effect in lanthanum deficient manganite of La0.8Ag0.1MnO3 are presented. The influence of isotopic substitution of oxygen 16О → 18О on the resistivity, specific heat, and magnetocaloric effect in La0.8Ag0.1MnO3 is estimated. The magnetocaloric effect has been studied by two different methods, which agree well enough. The entropy change as a result of the magnetocaloric effect varies between 2 and 4.1 J/kg K in the magnetic field of 26 kOe in the study samples.

Anisotropy of the upper critical fields and the paramagnetic Meissner effect in La1.85Sr0.15CuO4 single crystals

Optimally doped La1.85Sr0.15CuO4 single crystals have been investigated by dc and ac magnetic measurements. These crystals have rectangular needle-like shapes with the long needle axis parallel to the crystallographic c axis (c-crystal) or parallel to the basal planes (a-crystal). In both crystals, the temperature dependence of the upper critical fields (HC2) and the surface critical field (HC3) were measured. The H–T phase diagram is presented. Close to TC = 35 K, for the c-crystal, , whereas for the a-crystal the obtained is much higher than 1.69, predicted by the ideal mathematical model. At low applied dc fields, positive field-cooled branches known as the ‘paramagnetic Meissner effect’ (PME) are observed; their magnitude is inversely proportional to H. The anisotropic PME is observed in both a- and c-crystals, only when the applied field is along the basal planes. It is speculated that the high γa and the PME are connected to each other.

Magnetoelectric effect in La0.7Sr0.3MnO3–BaTiO3 core–shell nanocomposite

We report the magnetoelectric properties of La0.7Sr0.3MnO3–BaTiO3 core–shell nanostructure synthesized by sol–gel method. X-ray diffraction analysis suggests that the BaTiO3 undergoes large strain in the core–shell structure which can be attributed to lattice induced strain. Interestingly, the core–shell nanoparticle exhibits enhanced magnetization (143emu/g) compared to the ferromagnetic La0.7Sr0.3MnO3 bare core (20emu/g). The core–shell structure also exhibits inherent magnetoelectric coupling which is reflected as an anomaly at the ferroelectric transition in the magnetization measurement. The direct magnetoelectric measurement on La0.7Sr0.3MnO3–BaTiO3 nanocomposite revealed the magnetoelectric voltage coefficient value as 54.5mV/cmOe at 1kOe, which is a promising result compared to the other perovskite based nanocomposites.

Theoretical work on magnetocaloric effect in La0.75Ca0.25MnO3

In this work, a phenomenological model is applied to describe the magnetocaloric effect for the La0.75Ca0.25MnO3 system near a second-order phase transition from a ferromagnetic to a paramagnetic state. Based on this model, it can predict the values of the magnetocaloric properties from calculation of magnetization as a function of temperature under different external magnetic fields. The magnetic entropy change reaches a peak of about 5.39 J/(kg·K) at 257 K upon 4 T applied field variation. The ΔSM distribution is much more uniform than that of gadolinium, which is desirable for an Ericson-cycle magnetic refrigerator.

Tuning of the magnetocaloric effect in La manganites

The crystallographic and the magnetocaloric effects of La0.7Ca0.3Mn0.9957Fe0.01O3 (LCMO) powders were studied by using X-ray diffraction, vibrating sample magnetometer, and Mossbauer spectroscopy. To investigate the effect of sintering conditions on LCMO, the samples were sintered in air and in an evacuated sealed quartz tube at different temperatures. The calculated magnetic entropy changes in a magnetic field of 1.5 T for the samples sintered in air and in an evacuated sealed quartz tube were 1.6 J/kg K and 3.2 J/kg K, respectively. The magnetic entropy change increased considerably with increasing annealing temperature, but the Curie temperature Tc slightly decreased. However, the vacuum-annealed sample showed increases in both the Tc and the magnetic entropy change. The magnetocaloric effect and the working temperature of lanthanum manganites are tunable by using annealing conditions.

Magnetic and magnetocaloric properties of perovskite La0.7Sr0.3Mn1−xCoxO3

In this paper, the magnetic properties and magnetocaloric effect in La0.7Sr0.3Mn1−xCoxO3 prepared by conventional solid state reaction have been studied. The samples have rhombohedra crystal structure as was determined by room temperature X-ray diffraction. From the measurements, it was found that the Curie temperature (TC) and magnetic entropy change (ΔSM) are strongly dependent on the Co content. The Curie temperature was found to be 338, 300 and 260K for x=0, 0.05 and 0.1, respectively. Although the variation of maximum entropy change shows a similar tendency with TC, the phase transition temperatures were widened and the |ΔSM|max values, under a change of the magnetic field of 15kOe, amount to −1.36, −1.17 and −0.92J/kgK for x=0, 0.05 and 0.1, respectively. The results suggest that Mn-site substitution should be used to tailor Curie temperature and achieve large relative cooling power (RCP).

Giant and reversible extrinsic magnetocaloric effects in La0.7Ca0.3MnO3 films due to strain

Large thermal changes driven by a magnetic field have been proposed for environmentally friendly energy-efficient refrigeration, but only a few materials that suffer hysteresis show these giant magnetocaloric effects. Here we create giant and reversible extrinsic magnetocaloric effects in epitaxial films of the ferromagnetic manganite La(0.7)Ca(0.3)MnO(3) using strain-mediated feedback from BaTiO(3) substrates near a first-order structural phase transition. Our findings should inspire the discovery of giant magnetocaloric effects in a wide range of magnetic materials, and the parallel development of nanostructured bulk samples for practical applications.

Visible-light sensitive La1−xBaxCoO3 photocatalyst for malachite green degradation

Abstract In this study, a series of photocatalyst La1−xBaxCoO3 (x=0, 0.1, 0.3, 0.5, 0.7) powders were successfully prepared using the sol–gel method. The crystal structure and band gap of La1−xBaxCoO3 were characterized by X-ray diffraction and Ultraviolet–visible diffuse reflection spectroscopy for photocatalysis investigation. The results showed that the Ba-doped LaCoO3 exhibited obvious absorption characteristics in visible light regions. In addition, the effect of La1−xBaxCoO3 on the photocatalytic degradation of malachite green under different light sources was also investigated. Ba-doped LaCoO3 showed excellent visible-light sensitive photocatalytic activities and a maximum photocatalytic degradation of malachite green could be achieved with a degradation degree of 97% by La0.7Ba0.3CoO3 in visible light for 60 min.

Magnetic properties and magnetocaloric effect of Ti-doped La0.7Sr0.3MnO3

We have studied the influence of Ti doping on the magnetic properties and the magnetocaloric effect of La0.7Sr0.3Mn0.92Ti0.08O3 prepared by using solid-state reaction. The temperature dependence of magnetization reveals that the presence of the Ti dopant strongly reduces the Curie temperature (TC) to a value of ∼235 K. The magnetic-entropy change (ΔSm) has been assessed by means of isothermal-magnetization data versus the magnetic field, M-H curves, recorded at different temperatures. Around TC, −ΔSm reaches maximum values of 1.1, 2.0, and 2.7 J/(kg·K) for applied fields of 10, 20, and 30 kOe, respectively. A relative cooling power (RCP) of 135 J/kg is, thus, achieved for an applied field of 30 kOe. Detailed analyses of the M-H curves around TC indicate that the sample exhibits a second-order magnetic phase transition, where the variations of the saturation magnetization (Ms) and the inverse initial susceptibility (χ0−1) versus temperature obey asymptotic relations. Using the Arrott-Noakes method, we have obtained the critical exponents β = 0.425 ± 0.016, γ = 1.017 ± 0.055, and δ = 3.393 ± 0.039. These exponents are in good agreement with the magnetic equation of state. The β value determined in our case is between those expected for the mean-field (β = 0.5) and the 3D Heisenberge (β = 0.365) models, demonstrating the existence of ferromagnetic clusters.

Investigation of magnetocaloric effect in La0.45Pr0.25Ca0.3MnO3 by magnetic, differential scanning calorimetry and thermal analysis

We investigated magnetocaloric effect in La0.45Pr0.25Ca0.3MnO3 by direct methods (changes in temperature and latent heat) and indirect method (magnetization isotherms). This compound undergoes a first-order paramagnetic to ferromagnetic transition with TC=200K upon cooling. The paramagnetic phase becomes unstable and it transforms into a ferromagnetic phase under the application of magnetic field, which results in a field-induced metamagnetic transition (FIMMT). The FIMMT is accompanied by release of latent heat and temperature of the sample as evidenced from differential scanning calorimetry and thermal analysis experiments. A large magnetic entropy change of ΔSm=−7.2Jkg−1K−1 at T=212.5K and refrigeration capacity of 228Jkg−1 are found for a field change of ΔH=5T. It is suggested that destruction of magnetic polarons and growth of ferromagnetic phase accompanied by a lattice volume change with increasing magnetic field is responsible for the large magnetocaloric effect in this compound.

Above room temperature magnetic transition and magnetocaloric effect in La0.66Sr0.34MnO3

In this study, the magnetic and the magnetocaloric properties of the La0.66Sr0.34MnO3 (LSMO) compound were investigated. The X-ray diffraction result indicates that the LSMO sample has a single phase of rhombohedral symmetry without any impurity phase. The magnetic study reveals that the specimen La0.66Sr0.34MnO3 exhibits a ferromagnetic-paramagnetic transition at TC ∼ 376 K. Using Arrott plots, the phase transition from ferromagnetic to paramagnetic is found to be of second order. A maximum magnetic entropy change of 1.25 J/kgK has been observed for a low applied magnetic field of 1T. The relative cooling power values exhibit a nearly linear dependence on the applied magnetic field. Moreover, the analysis of the magnetocaloric effect (MCE) using the Landau theory o f phase transitions shows good agreement with the experimental results, confirming the importance of magnetoelastic coupling and electron interactions in the magnetocaloric properties of perovskite manganites. This investigation suggests that La0.66Sr0.34MnO3 can be used as a potential magnetic refrigeration material.