关键词: magnetism, magnetic entropy change, relative cooling power

Abstract: Structural, magnetic properties, critical behaviors and magnetic entropy changes of La$_{0.7-x}$Gd$_{x}$Ca$_{0.3}$MnO$_{3}$ ($x=0, 0.05, 0.1$) polycrystalline manganites have been investigated. The x-ray diffraction characterization shows that all the samples can be well indexed on an orthorhombic structure with $Pnma$ space group. Magnetic measurements show that polycrystalline samples sequentially display the characteristics of cluster spin glass states, ferromagnetic states, ferromagnetic paramagnetic coexistence states and pure paramagnetic states with increasing temperature. The Curie temperature ($T_{\rm c}$) increases with increasing the doping concentration $x$, and the ferromagnetic to paramagnetic phase transition is a second-order phase transition near $T_{\rm c}$. Critical behaviors have been studied through the modified Arrott plots and the Kouvel-Fisher method. The critical exponents of polycrystalline samples are determined to be close to the critical exponents of the tricritical mean field model ($x=0$), 3D Ising model ($x=0.05$) and 3D Heisenberg model ($x=0.1$), indicating that their ferromagnetic coupling may be the result of the short-range interactions between spins in this system. The maximum magnetic entropy changes reach values of 3.99~J/(kg$\cdot $K), 2.81~J/(kg$\cdot $K), and 4.20~J/(kg$\cdot $K) at the magnetic field of 7~T, and the relative cooling power (RCP) values of La$_{0.7-x}$Gd$_{x}$Ca$_{0.3}$MnO$_{3}$ ($x=0, 0.05, 0.1$) are 478.8~J/kg, 431.1~J/kg, and 536.98~J/kg respectively.

Key words: magnetism, magnetic entropy change, relative cooling power