M. Bejar

Structural and magnetic characterisation of the perovskite oxides La0.7Ca0.3−xNaxMnO3

X-ray powder diffraction (XRD) and magnetic measurements were performed in order to investigate the effect of Na+ ion substitution for Ca2+ ions on the crystallographic structure, the character of magnetic ordering, and the effect of transition temperature in La0.7Ca0.3−xNaxMnO3 manganites series (0 ⩽ × ⩽ 0.2). All samples crystallise in an orthorhombic structure with the Pnma space group. We have found a strong dependence of structural and magnetic properties on the cation-size disorder parameter σ2. The temperature dependence of magnetization of all samples obeys the Bloch T3/2 law. The values of the spin wave constant at low temperature B increase with the increase of x and the Curie temperature decreases. It is concluded that the substitution of Ca by Na+ ions causes a decrease in total exchange integral Aof the samples.

Magnetocaloric study, critical behavior and spontaneous magnetization estimation in La0.6Ca0.3Sr0.1MnO3 perovskite

A detailed study of structural, magnetic and magnetocaloric properties of the polycrystalline manganite La0.6Ca0.3Sr0.1MnO3 is presented. The Rietveld refinement of X-ray diffraction pattern reveals that our sample is indexed in the orthorhombic structure with Pbnm space group. Magnetic measurements display a second order paramagnetic (PM)/ferromagnetic (FM) phase transition at Curie temperature Tc = 304 K. The magnetic entropy change (ΔSM) is calculated using two different methods: Maxwell relations and Landau theory. An acceptable agreement between both data is noted, indicating the importance of magnetoelastic coupling and electron interaction in magnetocaloric effect (MCE) properties of La0.6Ca0.3Sr0.1MnO3. The maximum magnetic entropy change (−ΔSmaxM) and the relative cooling power (RCP) are found to be respectively 5.26 J kg−1 K−1 and 262.53 J kg−1 for μ0H = 5 T, making of this material a promising candidate for magnetic refrigeration application. The magnetic entropy curves are found to follow the universal law, confirming the existence of a second order PM/FM phase transition at Tc which is in excellent agreement with that already deduced from Banerjee criterion. The critical exponents are extracted from the field dependence of the magnetic entropy change. Their values are close to the 3D-Ising class. Scaling laws are obeyed, implying their reliability. The spontaneous magnetization values determined using the magnetic entropy change (ΔSM vs. M2) are in good agreement with those obtained from the classical extrapolation of Arrott curves (μ0H/M vs. M2). The magnetic entropy change can be effectively used in studying the critical behavior and the spontaneous magnetization in manganites system.

Erratum to: Study of the Magneto-Resistivity and Dependence of Percolation in La0.75Ca0.1Sr0.15Mn1−xGaxO3 Compounds

We present a study of the effect of Ga-doping on the physical properties of La0.75Ca0.1Sr0.15Mn1−xGaxO3 (x=0.025, 0.075 and 0.1) compounds prepared by the sol-gel method. The variation of the magnetization (M) vs. temperature (T), under an applied magnetic field of 0.05 T, reveals a ferromagnetic–paramagnetic transition for all samples. The magnetization behavior and the Curie temperature TC have shown a large dependence on the fractional composition x. In fact, the M(T) curves have revealed the presence of a long-range ferromagnetic state below TC.The magnetotransport properties have been investigated based on the temperature dependence of the resistivity ρ(T) measurements. We have noted that these samples present an electrical transition from a paramagnetic-semiconductor state to a ferromagnetic-metallic one, when decreasing temperature. We have used the phenomenological equation for conductivity under a percolation approach, which is based upon an approach that the system consists of the phase separated ferromagnetic metallic and paramagnetic insulating regions. We have then fitted the resistivity data measured in the range of 15–300 K and found that the activation barrier decreases with the raising Ga3+ ion concentration.

Magnetic Refrigeration: Application to the Electron Doped Manganites

Magnetic refrigeration is a revolutionary, efficient, environmentally friend technology, which is on the threshold of commercialization. The magnetic rare earth materials are utilized as the magnetic refrigerants in most cooling devices. Recently, some data on the magnetocaloric effect in hole-doped manganites are reviewed. It is shown that the variation of interaction exchange energy, taking place under the effect of a magnetic field in the vicinity of the phase transformation, provides a significant contribution to the change of magnetic entropy. In this paper we report different results found for electron-doped manganites La1-xCexMnO3 (x = 0.3, 0.4 and 0.5). These manganites present a large magnetic entropy change induced by low magnetic change, which is beneficial for the household application of active magnetic refrigerant materials.

Effect of the substitution of calcium by potassium on the dielectric properties in La 0.7 Ca 0.3−x K x MnO 3 compounds

Impedance spectroscopy measurements were performed to examine the effect of substitution of calcium by potassium on the dielectric properties in La_0.7Ca_0.3-xK_xMnO₃ compounds. The impedance plane plots show semicircle arcs at different temperature and an electrical equivalent circuit has been proposed to explain the impedance results. A high frequency dielectric relaxation was also observed.