We synthesized nanoparticles of Ni0.5Co0.5(Fe2−xLax)O4 (x = 0.0–0.30) spinel ferrites through sol–gel process and investigated how lanthanum substitution affects their structural, magnetic, and optical characteristics. Our Rietveld-refined XRD analysis confirmed single-phase cubic ( \(Fd\stackrel{-}{3}m\) space group) spinel formation throughout the composition range. As we increased La3+ content, we observed three key structural changes: unit cell contraction, microstrain enhancement, and reduction in both crystallite and particle sizes. Magnetization versus temperature study reveals the ferrimagnetic (FIM) transition above 300 K with a large bifurcation at low temperature in all samples. We found that increasing lanthanum concentration produced three correlated effects: lower Curie temperatures, decreased saturation magnetization, and stronger magnetic irreversibility. We attribute these trends to decrease in magnetocrystalline anisotropy and heightened lattice strain effects. Our optical studies demonstrated systematic band gap widening with lanthanum addition, which we interpret as electronic structure modification. The most intriguing finding emerged at x = 0.15, where we detected concurrent anomalies in lattice distortion, magnetic properties, and optical band gap. We propose this composition represents an optimal balance point where cation distribution, superexchange interactions, and electronic band structure achieve synergistic alignment. Our results reveal a coupled spin–lattice–charge response mechanism operating in these La-doped Ni–Co ferrites.
