Synthesis and Structural, Dielectric and Magnetic Properties of Al-Substituted Co-Li FNPs Synthesized by the Sol–Gel Method

In this work, we used the sol–gel method with synergistic auto-combustion to fabricate Co_0.8Li_0.4Fe_2−xAl_xO₄ ferrite nanoparticles (FNPs) with Al³⁺ substitution, and investigated their structural, dielectric, and magnetic properties. Citric acid (C₆H₈O₇.H₂O) was chosen as a chelating agent with a metal nitrate-citrate ratio of 1:3. The x-ray diffraction (XRD) pattern confirmed the crystalline development in the Co_0.8Li_0.4Fe_2−xAl_xO₄ FNPs to a single phase with cubic geometry and spinel structure belonging to the space group Fd-3m o^h₇. The lattice constant ( \(a\) $a$ ) of Co-Li FNPs (x = 0.0) was recorded as 8.348 ± 0.002 Å, which was found to be decreased after Al-substitution up to 8.315 ± 0.002 Å for x = 0.6. The crystallite size (D) employing the Debye-Scherrer formula was found in the range of ~ 11–22 nm. High-resolution transmission electron microscopy (HRTEM) images showed single-crystal Co_0.8Li_0.4Fe_2−xAl_xO₄ FNPs with a nearly spherical shape. The average particle size ( \({D}_{{\text{avg}}}\) $D_{\text{avg}}$ ) determined through skewed histogram charts suggested a range up to ~ 21–28 ± 10 nm. The optical properties were studied by UV-visible spectroscopy. Adsorption-desorption curves were used to obtain pore size and volume. The dielectric constant ( \(\varepsilon^{\prime }\) ${\epsilon }^{\prime }$ \()\) $)$ , dielectric loss ( \(\varepsilon^{\prime \prime }\) ${\epsilon }^{″}$ ) and dielectric loss tangent (tan δ) all decreased exponentially with an increase in frequency. The substitution of Al³⁺ ion led to an increase in all dielectric parameters. The plot of magnetization versus applied magnetic field (M-H) showed that saturation magnetization decreased with Al³⁺substitution. The decrease in magnetization was attributed to the decrease in A–B interaction and may be due to the substitution of non-magnetic Al³⁺ ions.