Magnetic properties and exchange coupling in Al3+-substituted multi-element spinel ferrite Co0.25Ni0.25Zn0.25Cu0.25Fe2O4 nanoparticles

The substitution effect of Al3+ ions on the magnetic features of Co0.25Ni0.25Zn0.25Cu0.25Fe2O4 (x = 0.00–0.10) spinel ferrite nanoparticles (Al3+→CoNiZnCu (x ≤ 0.1) SFNPs), prepared via the sol-gel auto-combustion method, have been investigated. The morphology, elemental composition, high phase purity, and cubic spinel structure were confirmed by SEM, TEM, SAED (Selected Area Electron Diffraction) pattern, EDX, and XRD analyses. The magnetic hysteresis loops (M-H curves) confirmed a strong dependence of the magnetic properties on both temperatures (T = 300 (RT, Room Temperature) and 10 K) and the Al3+ dopant concentration. A transition from a soft ferrimagnetic character at RT (low Hc values ranging between 73 and 204 Oe) to a hard ferrimagnetic state at 10 K (Hc up to approximately 2151 Oe) was observed, which is quantitatively justified by a dramatic increase in the effective anisotropy constant (Keff) from ~ 2.89 to ~ 162 kJ/m3 as thermal fluctuations are suppressed. The saturation magnetization (Ms) and number of Bohr magnetons ( $$\:{n}_{B}$$ ) increase with increesing Al3+ content, reaching peak values for x = 0.06, and then decrease for x > 0.06. This non-monotonic behavior is attributed to the prefferential occupancy of non-magnetic Al3+ ions in the A sites (replacing highly magnetic Fe3+ ions), which enhances the net mangetic moment difference (|MB - MA|) and optimizes A–B super-exchange interactions before magnetic dilution and spin-canting dominates at higher doping levels. Notably, the squareness ratio (SQR) values at 10 K were significantly higher than the theoretical Stoner-Wohlfarth limit of 0.5 for pristine and all doped samples (reaching up to 0.59), providing quantitative evidence of strong inter-particle exchange coupling that stabilize the remanent state. Compared to traditional single-cation or ternary ferrites, this multi-element matrix exhibit a wider tunable magnetic range, making these SFNPs suitable for both high-frequency AC applications and high-density magnetic recording.