Investigation on magnetic and magnetocaloric properties in the Pb-doped manganites La0.78Ca0.22−xPbxMnO3(x = 0, 0.05 and 0.1)

Abstract

We present an investigation on structural, magnetic and magnetocaloric properties of La0.78Ca0.22−xPbxMnO3 (x = 0, 0.05 and 0.1) polycrystalline compounds prepared by conventional solid-state reaction method. Rietveld refinement of X-ray powder diffraction (XRD) patterns shows that all samples crystallize in the orthorhombic structure, with the Pnma space group. Magnetization as a function of temperature shows that all samples exhibit a paramagnetic (PM) – ferromagnetic (FM) phase transition at the Curie temperature TC which increases from 202 K to 242 K for x = 0–0.1, respectively. The magnetic-entropy changes,ΔSM, were calculated from the field dependence of magnetization data using the Maxwell's thermodynamic relation. The magnetic entropy change |ΔSMmax| decreases from 6.9 J kg−1 K−1 for x = 0 to 5.53 J kg−1 K−1 for x = 0.1, while the relative cooling power (RCP) also decreases from 274 to 241 J kg−1, respectively, under an applied magnetic field of 5 T. All of the investigated manganites exhibit a second-order magnetic phase transition at TC, which is also confirmed by Arrot plot and Landau's theory. Also, by normalizing the ΔSM(T,H) curves to their respective ΔS'(=ΔSM(T)/ΔSMmax) value we indicate that all these curves of the samples are collapsed onto a universal master curve. In addition, field dependence of the magnetic entropy change showing the power law dependenceΔSMmax∝(μ0H)n, at the TC is also analyzed. The obtained ‘n’ values are 0.64, 0.75 and 0.97 for x = 0, 0.05 and 0.1 respectively, which confirms that our samples does not follow the mean field model. The deviation from the mean field behavior for all the samples demonstrates an existence of short-range ferromagnetic order in the samples.