Effect of K doping on the physical properties of La 0.65Ca 0.35−xK xMnO 3 (0⩽ x⩽0.2) perovskite manganites

Abstract

The effects of K doping in the A-site on the structural, magnetic and magnetocaloric properties in La 0.65Ca 0.35− x K x MnO 3 (0⩽ x⩽0.2) powder samples have been investigated. Our samples have been synthesized using the solid-state reaction method at high temperature. The parent compound La 0.65Ca 0.35MnO 3 is an orthorhombic (Pbnm space group) ferromagnet with a Curie temperature T C of 248 K. X-ray diffraction analysis using the Rietveld refinement show that all our synthesized samples are single phase and crystallize in the orthorhombic structure with Pbnm space group for x⩽0.1 and in the rhombohedral system with R3¯c space group for x=0.2 while La 0.65Ca 0.2K 0.15MnO 3 sample exhibits both phases with different proportions. Magnetization measurements versus temperature in a magnetic applied field of 50 mT indicate that all our investigated samples display a paramagnetic–ferromagnetic transition with decreasing temperature. Potassium doping leads to an enhancement in the strength of the ferromagnetic double-exchange interaction between Mn ions, and makes the system ferromagnetic at room temperature. Arrott plots show that all our samples exhibit a second-order magnetic-phase transition. The value of the critical exponent, associated with the spontaneous magnetization, decreases from 0.37 for x=0.05 to 0.3 for x=0.2. A large magnetocaloric effect (MCE) has been observed in all samples, the value of the maximum entropy change, |Δ S m| max, increases from 1.8 J/kg K for x=0.05 to 3.18 J/kg K for x=0.2 under a magnetic field change of 2 T. For x=0.15, the temperature dependence of |Δ S m| presents two maxima which may arise from structural inhomogeneity.