Magnetic and Pseudocapacitive Enhancement in Gd-Substituted NiFe2O4

Abstract

Gadolinium-doped nickel ferrite nanoparticles Gd_xNiFe_2-xO₄ (x = 0, 0.025, 0.075, 0.125 and 0.175) were synthesized via a sol-gel auto-combustion method to investigate how Gd substitution influences their structural, magnetic, and electrochemical properties. XRD confirmed a predominantly cubic spinel phase across the series, with secondary peaks emerging at higher substitution levels. Increasing Gd content reduced crystallite size through lattice strain and grain-boundary pinning. Magnetic measurements showed a pronounced increase in saturation magnetization at 2.5 mol% Gd, followed by a gradual decrease at higher substitution levels owing to weakened magnetic interactions. Electrochemical testing in 1 M KOH indicated pseudocapacitive behavior governed by reversible Ni²⁺/Ni³⁺ and Fe²⁺/Fe³⁺ transitions, with the 2.5 mol% composition also delivering the highest specific capacitance and lowest charge-transfer resistance. This improvement is attributed to Gd-induced oxygen vacancies that enhance ion diffusion and contribute to the simultaneous optimization of magnetic and electrochemical performance.