Inducement of ferromagnetic–metallic phase and magnetoresistance behavior in charged ordered monovalent-doped Pr0.75Na0.25MnO3 manganite by Ni substitution

In this communication, the study on the effect of Ni²⁺ substitution on structural, magnetic and electrical transport properties were performed in Pr_0.75Na_0.25Mn_1-xNi_xO₃ (x = 0–0.10) ceramics synthesized using conventional solid-state method. X-ray diffraction patterns showed that all samples were present in single phase and crystallized in orthorhombic structure with Pnma space group. Rietveld refinement analysis revealed unit cell volume slight increase with increase Ni concentration, thereby indicating partial substitution of Ni²⁺ at Mn³⁺. The presence majority of Ni²⁺ states in the compound were confirmed by X-ray photoelectron spectrum. Tolerance factor calculation suggested that Ni substitution exerted no strong effect on structural distortion. For un-doped sample (x = 0), AC susceptibility (χ′) against temperature (T) curve showed paramagnetic (PM)–antiferromagnetic(AFM) behavior at Neel temperature (T_N) of approximately 170 K. Furthermore, resistivity (ρ) against temperature (T) curve showed an insulating behavior for the whole measured temperature range. The χ′ against T curve of x = 0 sample showed broad peak at approximately 218 K which was attributed to the onset of charge ordered (CO) state. No such broad peak was observed in Ni-substituted samples which indicated the weakening of CO state. Moreover, χ′ measurements exhibited successful inducement of PM–FM transition with Curie temperature (T_C), decreasing from 132 K (x = 0.02) to 92 K (x = 0.08). Electrical resistivity measurement on samples (x = 0.02–0.08) displayed inducement of metal–insulator transition, where transition temperature (T_MI) decreased and resistivity increased, with x before re-entrant insulating behavior at x = 0.10. Notably, upturn resistivity was observed below 40 K for x = 0.06 and 0.08 samples. The suppression of CO state and inducement of ferromagnetic-metallic (FMM) state beginning from x = 0.02 sample was attributed to the reduced degree of Jahn–Teller distortion and Coulomb interaction among Mn ions, as well as the presence of ferromagnetic superexchange (FM SE) interaction among Ni²⁺–O–Mn⁴⁺ which improved the alignment charge carrier spins and induced the double-exchange (DE) interaction among Mn³⁺–O–Mn⁴⁺. The decrease in T_C and T_MI with increased x may be due to the enhanced AFM SE interactions of Mn³⁺–O–Mn³⁺, Mn⁴⁺–O–Mn⁴⁺ and Ni²⁺–O–Ni²⁺ which decreased the FM SE interaction of Ni²⁺–O–Mn⁴⁺. Consequently, the effective DE interaction was decreased. In addition, the decreased metallic behavior and re-entrant insulating behavior for x = 0.10 sample was due to the strong AFM interaction between Ni²⁺ ions which consequently contributed to the suppression of FM SE and DE interactions. The observed upturn resistivity below 40 K for x = 0.06 and 0.08 samples was attributed to the Kondo-like effect which resulted from the interaction between itinerant conduction electron spin and localized spin impurity.