The structural and magnetic properties of CoAl _x Fe_2−x O₄/SiO₂ nanocomposite films by sol–gel spin-coating method

CoAl _x Fe_2−x O₄/SiO₂ (0 ≤ x ≤ 1.0) nanocomposite films were synthesized on Si (100) substrates by sol–gel spin-coating method. The effects of Al³⁺ content and annealing temperature on the structure and magnetic properties of the samples were investigated by X-ray diffraction, atomic force microscopy, field emission scanning electron microscopy, and vibrating sample magnetometer. The results showed CoAl _x Fe_2−x O₄ in the films could be crystallized with a single phase spinel structure after being annealed. When the Al³⁺ contents x increased from 0 to 1.0, the lattice constants of CoAl _x Fe_2−x O₄ nanoparticles in the films decreased from 0.8370 to 0.8251 nm, the average crystallite sizes decreased from 37 to 25 nm, and the in-plane and out-of-plane coercivity of the films decreased from 258 and 315 kA/m to 46 and 63 kA/m, respectively. As the annealing temperatures rose from 800 to 1150 °C, the lattice constants of CoAl_0.2Fe_1.8O₄ nanoparticles in the films decreased from 0.8369 to 0.8352 nm, while the average crystallite sizes increased from 7 to 63 nm. And the CoAl_0.2Fe_1.8O₄ nanoparticles in the films had obvious (111) preferred orientation at annealing temperature of 1150 °C. The in-plane and out-of-plane coercivity of CoAl_0.2Fe_1.8O₄/SiO₂ nanocomposite film annealed at 1100 °C showed the maximum values of 198 and 259 kA/m, respectively. The critical sizes of single-domain and superparamagnetism for CoAl_0.2Fe_1.8O₄ nanoparticles in the films at room temperature were also estimated.