Ni²⁺ induced high ferromagnetic ordering and dielectric relaxation phenomena in KBiFe₂O₅ brownmillerite

This work primarily focuses on the high ferromagnetic ordering and dielectric relaxation mechanism in KBiFe₂O₅ brownmillerite owing to the substitutions of Ni, which is otherwise a canted antiferromagnetic system. A nominal composition of KBiFe_2-<i>x</i>Ni_<i>x</i>O₅ (<i>x</i> = 0, 0.025, 0.05, and 0.1) was synthesized <i>via</i> a solid-state method, and a single phase with monoclinic structure (space group-<i>P</i>2/<i>c</i>) of the series was probed through X-ray diffraction. X-ray photoelectron and Mössbauer spectroscopies predicted a mixed valence state of Fe (Fe²⁺ and Fe³⁺). Thermomagnetic studies revealed an antiferromagnetic transition in the parent system, while Ni substitution resulted in a ferromagnetic transition at around 815 K. The thermomagnetic irreversibility predicted a cluster glass-like state well below the room temperature, which could be associated with the phase competition among the two phases of contrasting order. Isothermal magnetization revealed a ferromagnetic ordering in Ni-substituted KBiFe₂O₅, with a fifteen-fold increase in the room-temperature magnetic moment compared with that of the parent compound. This drastic change in the magnetization is analysed in the framework of lattice distortion and change in the tilting angle of Fe³⁺-O^2--Fe³⁺ chain due Ni²⁺ along with the exchange interactions among Ni²⁺, Fe³⁺ and Fe²⁺ ions in the system. Furthermore, a dielectric transition was observed at around 753 K, which hardly changed with Ni substitution. The tangent loss and electric modulus studies demonstrated a relaxation behavior, and the relaxation peaks were shifted towards lower frequencies with Ni substitution with an increase in activation energy. The improved magnetic and electrical characteristics make these materials potential candidates for multifunctional applications.