Effect of A-site ionic disorder on the structure, magnetic, and magnetocaloric properties of La_(0.7-x)Nd_xCa_(0.3-y)(Ba,Sr)_yMnO_3

摘要

The effects of A-site cation size mismatch on La_(0.7-x)Nd_xCa_(0.3-y)(Ba,Sr)_yMnO_3 perovskite materials with a constant A-site average ionic radius (r_A) = 1.2052 A have been investigated. Polycrystalline samples have been prepared through the standard solid-state reaction method at high temperatures, and their structures, magnetic properties, and magnetocaloric effect (MCE) have been investigated in detail. X-ray diffraction patterns show that all of the samples crystallized in an orthorhombic structure with the space group Pnma. XPS analysis confirmed the Mn~(3+)/Mn~(4+) ratio remained constant as σ~2 was varied. Meanwhile, upon increasing σ~2 from 0.0002 to 0.007, the Curie temperature (T_C) decreased linearly from 254 K to 108 K. Based on M-H isotherms, Arrott plots and the maximum magnetic entropy prove a crossover of first-to-second-order transition with increasing σ~2. Samples with σ~2 = 0.0002, 0.001, and 0.003 exhibited a first-order transition and had larger MCEs than those showing a second-order transition.