Site-Selective Zn²⁺ substitution induced magnetic phase transition from ferrimagnetism to superparamagnetism in N,N,N-trimethylhexadecan-1-aminium bromide stabilized Co–Zn ferrite nanoparticles

Herein, we report a composition-driven magnetic phase transition from ferrimagnetism to superparamagnetism in N,N,N-Trimethylhexadecan-1-aminium bromide (CTAB)-stabilized Co₁₋ₓZnₓFe₂O₄ (CZFO) nanoparticles (0 ≤ x ≤ 0.9), governed by site-selective Zn²⁺ substitution and its cascading effects on structure and spin dynamics. The saturation magnetization (Mₛ) exhibits a nonmonotonic trend with Zn^{2 +} substitution, peaking at 70.56 emu/g for x = 0.1 and declining to 14.73 emu/g at x = 0.9, while coercivity (H_c) drops sharply from 479.64 Oe to 15.17 Oe. These magnetic changes are closely linked to cation redistribution, where Zn²⁺ preferentially occupies tetrahedral sites, displacing Fe³ ⁺ to octahedral positions and weakening A–B superexchange interactions (J_AB), as confirmed by Rietveld refinement. The structural evolution, confirm by X-ray diffraction (XRD), reveals phase-pure cubic spinel formation (Fd3̅m), with lattice constants expanding from 8.426 Å to 8.483 Å and crystallite sizes shrinking from 45.8 nm to 29.5 nm. This lattice expansion and size reduction correlate with increased microstrain (from 0.0013 to 0.0024), as shown by Williamson–Hall analysis, indicating enhanced surface stress and spin disorder, factors that contribute to reduced anisotropy energy (K_eff) and promote thermal activation of single-domain moments. Ferromagnetic resonance measurements further support this transition, showing a resonance field shift from 3456.2 Oe to 703.8 Oe, linewidth narrowing from 3272.6 Oe to 645.2 Oe, and g-factor reduction from 2.14 to 2.02. These trends reflect diminished exchange stiffness, increased spin canting, and orbital quenching, consistent with the relation g ≈ ge(1 + λ/Δ_CF), where Δg ≈ 0.12 suggests significant perturbation of crystal field symmetry. Spectroscopic analyses reinforce the structural-magnetic interplay: FTIR and Raman spectra show vibrational mode softening (A-site band: 534–521 cm⁻¹; A₁_g: 680–650 cm⁻¹), indicative of weakened metal–oxygen bonding due to Zn²⁺ incorporation. HRTEM and FESEM confirm spherical morphology and reduced agglomeration, while TRPL analysis reveals shorter carrier lifetimes (4.8 ns to 2.3 ns), pointing to increased surface defect density and its role in spin relaxation. Together, these results establish a coherent framework linking site-selective cation substitution, structural distortion, and spin dynamics, enabling tunable magnetic behavior in CZFO nanoparticles for GHz-range spintronic and magneto-functional applications.