Interrelationship between cation inversion and structural, morphological, optical, and magnetic properties in Ni-substituted MgFe2O4 spinels.

Cation inversion critically governs magnetic exchange in spinel ferrites, yet it is typically treated as a fixed structural form rather than a tunable variable. In this work, we show that Ni2+ substitution in magnesium ferrite, forming the isovalent solid-solution series Mg1-x Ni x Fe2O4 (x = 0.0, 0.1, 0.3, 0.5), drives a thermodynamically controlled redistribution of cations between the tetrahedral and octahedral sublattices, which in turn governs the evolution of the saturation magnetisation and coercivity across the series. Structural characterisation via X-ray diffraction, Fourier-transform infrared spectroscopy and Raman spectroscopy confirms a single-phase cubic spinel, while UV-visible diffuse reflectance spectroscopy, vibrating sample magnetometry, and scanning electron microscopy provide complementary insight into optical, magnetic, and morphological properties, respectively. We introduce a thermodynamic framework based on a free-energy functional that incorporates exchange interactions, crystal-field effects, and configurational entropy. Within this model, the inversion parameter arises as an equilibrium order parameter determined by free-energy minimisation. This work reframes cation inversion as a controllable thermodynamic degree of freedom, offering new pathways for the rational design of functional spinel ferrites.