Influence of zirconium incorporation on the structural, linear, and nonlinear optical characteristics, and electron spin resonance behavior of CoCr2O4 nanoparticles

This study investigates how substituting Cr 3+ with Zr 4+ affects the structural, morphological, and optical properties of CoCr 2−x Zr x O 4 nanoparticles, with their spectroscopic characteristics examined through electron spin resonance (ESR) spectroscopy. As Zr 4+ substitution increased, the crystallite size decreased significantly (25.43 to 12.11 nm), while the lattice parameter showed a slight increase (8.307 to 8.319 Å). SEM and TEM confirmed a quasi-spherical, agglomerated morphology, and EDX verified successful zirconium incorporation without impurities. Optical characterization using UV–Vis and PL spectroscopy revealed a quantum confinement effect, with band gap widening (3.02–3.61 eV) as crystallite size decreased. The linear refractive index and its dispersion were analyzed using the Wemple–DiDomenico single-oscillator model, and nonlinear optical properties, specifically third-order susceptibility χ ⁽³⁾ χ ( 3 ) and nonlinear refractive index n₂ n 2 , were evaluated as functions of Zr content. As the zirconium (Zr) content increases, the nanoparticles exhibited a marked reduction in their nonlinear optical response, dropping from 3.61 × 10 –11 to 0.097 × 10 –11 esu. This decline was accompanied by a decrease in crystallite size and a corresponding increase in microstrain. Electron spin resonance (ESR) measurements were performed to examine the effect of Zr 4+ substitution on key spectroscopic parameters, including the g g -factor, ( Hᵣ H r ), peak-to-peak linewidth ( Δ Hₚₚ Δ H pp ), and spin relaxation times. Overall, CoCr 2−x Zr x O 4 nanoparticles exhibited a tunable combination of linear optical, nonlinear optical, and spin relaxation properties, allowing their behavior to be tailored for applications in optoelectronic devices, magnetic resonance imaging, and spin-based memory.