Effect of Calcium Doping on Magnetocaloric Properties and Critical Behaviour of La0.55CaxSr0.45-xMnO3 around FM-PM Phase Transition

Abstract

Effect of calcium doping on magnetocaloric properties and critical behaviour of La0.55CaxSr0.45-xMnO3 (x = 0.00, 0.05, 0.1, 0.2 and 0.25) prepared by solid state reaction technique has been reported here. XRD measurements reveal rhombohedral structure for all the samples. Lattice parameters as well as Mn-O-Mn bond angle decreases with increasing Ca content. These structural changes control ferromagnetism in the materials via the double-exchange mechanism [1]. Temperature dependent magnetization measurement reveals a reduction of saturation magnetization and decrease in Curie temperature (TC) from 354 K to 311 K with increase in Ca content. A second order ferromagnetic (FM) to paramagnetic (PM) phase transition of all the samples is confirmed by Arrott plots. The magnetocaloric effect is calculated from the isothermal magnetization measurements using Maxwell’s thermodynamic relations. The maximum entropy change ((-ΔSm)max) and relative cooling power (RCP) are observed to decrease while TC shifted toward the room temperature with increase in Ca content. Therefore the sample La0.55CaxSr0.45-xMnO3 could be used as good solid state refrigerant for magnetic refrigerator near room temperature. Critical behaviour around PM-FM phase transition has been investigated using various methods including modified Arrott plots (MAP) [2], Kouvel–Fisher (KF) [3] method and critical isotherm analysis. The M-H data taken around TC was investigated in the framework of the mean field model, the 3D–Heisenberg model, the 3D–Ising model and tricritical mean field model [2]. The well agreement between MAP and KF methods confirms that the calculated values of the critical exponents are reliable. Calculated critical exponents reveal that the samples with x=0.00, 0.05, 0.1 possess a long-range ferromagnetic interaction belonging to the mean-field model, and x=0.2, 0.25 possess ferromagnetic short-range interactions belonging to the 3D Heisenberg model. Chemical pressure due to substitution of larger Sr ions with smaller Ca ions, cooperative Jahn–Teller distortions, and presence of ferromagnetic clusters all have significant influence on the nature of the FM-PM phase transition in the studied samples.  K-F plot  Entropy chang vs. T