Physical Properties of Ca-Doped Double Perovskite La₂NiMnO₆

Abstract

Samples of double perovskite manganese La_2−xCa_xNiMnO₆ (x = 0, 0.05, 0.15, 0.2) are prepared by a traditional solid-state reaction. X-ray diffraction patterns show that four samples all exhibit a good uniform single phase and have perovskite crystal structures. From the temperature dependence of the magnetization in a 5 mT magnetic field (M–T) and the field dependence of the magnetization near the Curie temperature (M–H) for La_2−xCa_xNiMnO₆ (x = 0, 0.05, 0.15, 0.2), the ferromagnetism of the samples is weakened, while the antiferromagnetism is enhanced with Ca doping. Hysteresis loops at 2 K, enlarged views of the low-field hysteresis loops at 2 K under a field cooling process with a cooling field of 1T and Raman spectra at room temperature for La_2−xCa_xNiMnO₆ (x = 0, 0.05, 0.15, 0.2) show that when the Ca doping amount is x = 0.05, compared with La₂NiMnO₆, the antisite disorder degree of the system is almost the same, but the number of antiphase boundaries increases. With further Ca doping, the antisite disorders and numbers of antisite defects and antiferromagnetic antiphase boundaries increase, simultaneously strengthening the enhancement in the antiferromagnetic coupling. In addition, as the Ca doping increases, the upward deviation of the temperature-dependent χ⁻¹ ~ T for bulk La_2−xCa_xNiMnO₆ (LCNMO) from the Curie–Weiss law becomes more obvious, which shows that Ca doping enhances the long-range ferromagnetic order, antiferromagnetic clusters increase, and long-range ferromagnetic order dominates the magnetic behaviour near the ferromagnetic transition point. Finally, we discuss the magnetic entropy and electrical changes of four samples and confirm that Ca doping causes the systems to undergo a weak first-order phase transition.