Two ferromagnetic phases with different spin states of Mn and Ni in LaMn_0.5Ni_0.5O₃

Abstract

A ferromagnetic transition below 280 K is observed in the temperature variation of the magnetization, measured using high magnetic fields, for samples of LaMn_0.5Ni_0.5O₃ synthesized by the high-temperature solid-state reaction method. On the other hand, two well-defined magnetic transitions, at ~150 and ~280K, are observed in low-field zero-field-cooled (ZFC) magnetization (M_ZFC) measurements on these samples, indicating the possibility for the existence of two different ferromagnetic phases of the compound. LaMn_0.5Ni_0.5O₃, synthesized by a low-temperature method, shows a single magnetic transition at ~150K for samples annealed below 500°C and a single magnetic transition at 280 K for samples annealed above 1000°C in the low-field M_ZFC measurements. Broad or multiple magnetic transitions are observed for the low-temperature samples annealed in the temperature range 500-1000°C. High-temperature magnetic susceptibility, powder X-ray-diffraction, and core-level X-ray photoelectron spectroscopic studies on the single-phase samples (annealed at 400 and 1300°C) indicate that the nanocrystalline material (obtained at 400°C) with spin states of Mn and Ni as Mn⁴⁺ and Ni²⁺ undergoes a magnetic transition below 150 K, whereas high temperature (1300°C) annealed material with spin states like Mn³⁺ and low-spin Ni³⁺ becomes ferromagnetic below 280 K. A charge disproportionation of the type Mn⁴⁺+Ni²⁺→Mn³⁺+Ni³⁺ takes place when the low-temperature synthesized LaMn_0.5Ni_0.5O₃ is annealed above 500°C. The results give evidence for the existence of two different phases of LaMn_0.5Ni_0.5O₃ with different crystal structures and different spin states of Mn and Ni.