文摘
The effects of K doping in the A-site on the structural, magnetic and magnetocaloric properties in La0.65Ca0.35−xKxMnO3 (0x0.2) powder samples have been investigated. Our samples have been synthesized using the solid-state reaction method at high temperature. The parent compound La0.65Ca0.35MnO3 is an orthorhombic (Pbnm space group) ferromagnet with a Curie temperature TC 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 x0.1 and in the rhombohedral system with R3¯c space group for x=0.2 while La0.65Ca0.2K0.15MnO3 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, ΔSmmax, 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 ΔSm presents two maxima which may arise from structural inhomogeneity.
