Revival of Ferromagnetic-Metallic Phase and Emergence of Griffiths Phase via Ni and Cr Substitution in Charged-Ordered Pr0.75Na0.25MnO3 Manganite

Abstract

Polycrystalline samples of Pr_0.75Na_0.25Mn_1-xA_xO₃ (A = Ni, Cr; x = 0, 0.03) were prepared using conventional solid-state method and their structural and magnetic properties were investigated. Monovalent-doped Pr_0.75Na_0.25MnO₃ exhibits insulating behavior which is related to the strong localization of charge carriers due to the presence of charge ordering (CO). The substitution of different of magnetic ions, A, which are Ni²⁺ and Cr³⁺, at a concentration of x = 0.03 at the Mn site in the CO Pr_0.75Na_0.25Mn_1-xA_x O₃ manganite, has been found to dramatically modify its structural and magnetic properties. X-ray diffraction patterns showed that all samples were present in single phase and crystallized in orthorhombic structure with Pnma space group. The Rietveld refinement analysis showed that the unit cell volume increased due to Ni²⁺ substitution, while it decreased due to Cr³⁺ substitution, which may be attributed to the different ionic radii of both ions. The suppression of the CO state and the induction of the ferromagnetic-metallic state in the Ni-substituted and Cr-substituted samples are suggested to be due to the induction of double-exchange interaction involving Ni²⁺–O–Mn⁴⁺ and Mn³⁺–O–Cr³⁺, respectively. The variation in magnetic properties between the Ni-substituted and Cr-substituted samples is suggested to be due to the existence of different strengths of ferromagnetic interaction between Ni²⁺–O–Mn⁴⁺ and Mn³⁺–O–Cr³⁺. In addition, the deviation of the temperature dependence of the inverse magnetic susceptibility curves from the Curie-Weiss law suggests the existence of Griffith’s phase-like behaviour for both substitution samples. The Ni-substituted sample induced a greater GP effect than the Cr-substituted sample due to the larger difference of TC and TG value (TC-TG).