E. Dhahri

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.

Magnetic and specific heat studies of the frustrated Er2Mn2O7 compound

Abstract A new Er2Mn2O7 compound was synthesized by the ceramic method and its crystal structure was characterized using powder X-ray diffraction (XRD) and observed by scanning electron microscopy (SEM). The magnetic properties were investigated using a BS2 magnetometer and the heat capacity was studied using a quantum design (PPMS). The structural study revealed that this compound was monophasic and crystallized in the monoclinic system with the P2/M space group. Magnetization measurements were carried out in the temperature range of 1.8-200 K under an applied magnetic field of 0.05 T. A crossover from a room temperature paramagnetic phase to an antiferromagnetic one at low temperature was detected from the magnetic study. The magnetic susceptibility, in the paramagnetic region above 40 K, was found to present a simple Curie-Weiss type behavior. From the specific heat (CP) measurements in magnetic fields up to 5 T, we noted the presence of a wide peak characteristic of a second order magneto-structural transition.

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.