Zinc ion-substituted cobalt ferrite powders Co_1−xZn_xFe₂O₄ (x = 0–0.7) were prepared by the sol-gel auto-combustion process. The structural properties and magnetic of the samples were investigated with X-ray diffraction (XRD), superconducting quantum interference device, and a Mössbauer spectrometer. The results of XRD showed that the powder of a single cubic phase of ferrites calcined when kept at 800 °C for 3 h. The lattice constant increases with increase in Zn concentration, but average crystallite size does not decrease constantly by increasing the zinc content, which is related to pH value. It was confirmed that the transition from ferrimagnetic to superparamagnetic behaviour depends on increasing zinc concentration by Mössbauer spectra at room temperature. Magnetization at room temperature increases for x ≤ 0.3, but decreases for increasing Zn²⁺ ions. The magnetization of Co_0.7Zn_0.3Fe₂O₄ reached maximum value (83.51 emu/g). The coercivity decreased with Zn²⁺ ions, which were doped on account of the decrease of the anisotropy constant.

A sol-gel autocombustion method was used to synthesize Al³⁺ ion-substituted cobalt ferrite CoAl_xFe_2−xO₄ (x = 0–1.5). According to X-ray diffraction analysis (XRD), cobalt ferrite was in a single cubic phase after being calcined at 1000 °C for 3 h. Moreover, the lattice constant decreased with increase in aluminum substituents. When the sample was analyzed by Scanning Electron Microscopy (SEM), we found that uniformly sized, well-crystallized grains were distributed in the sample. Furthermore, we confirmed that Al³⁺ ion-substituted cobalt ferrite underwent a transition from ferrimagnetic to superparamagnetic behavior; the superparamagnetic behavior was completely correlated with the increase in Al³⁺ ion concentration at room temperature. All these findings were observed in Mössbauer spectra. For the cobalt ferrite CoAl_xFe_2−xO₄, the coercivity and saturation magnetization decrease with an increase in aluminum content. When the annealing temperature of CoAl_0.1Fe_1.9O₄ was steadily increased, the coercivity and saturation magnetization initially increased and then decreased.

The present study envisages the preparation of chromium substituted Nickel ferrite NiCr_xFe_2−xO₄ (x = 0~1.0) powders by a sol-gel auto-combustion method. X-ray diffraction analysis (XRD) showed that the specimens with x > 0.2 exhibited a single-phase spinel structure, and that more content of Cr within a specimen is favorable for the synthesis of pure Ni-Cr ferrites. The lattice parameter decreased with an increase in the Cr concentration. The sample without calcining exhibited a good crystallinity. Scanning Electron Microscopy (SEM) showed the formation of ferrite powders nano-particles, and that the substitution of Cr weakened the agglomeration between the particles. Mössbauer spectra of NiCr_xFe_2−xO₄ showed two normal Zeeman-split sextets that displayed a ferrimagnetic behavior. Furthermore, the spectra indicated that iron was in the Fe³⁺ state, and the magnetic hyperfine field at the tetrahedral tended to decrease with an increase in the Cr substitution. The saturation magnetization decreased by the Cr³⁺ ions, and reached a minimum value (Ms = 4.46 emu/g). With an increase in the annealing temperature, the coercivity increased initially, which later decreased.

Magnetic Functional Nanomaterials Co_1−yZn_yRE_xFe_2−xO₄ (RE (rare-earth) = La,Sm,Gd) were prepared using the sol–gel combustion method. XRD characterization confirms that the ferrite samples we synthesized are single-phase cubic structures. The variation in the average crystalline size and lattice parameter is related to RE ion doping. The Mössbauer spectra of CoRE_xFe_2−xO₄ are two sets of magnetic six-wire peaks that indicate the ferrimagnetic behavior of the sample. The calcination temperature greatly influences the absorption area of Mössbauer for CoFe₂O₄, indicating that the calcination temperature affects the iron ion content at the octahedral B and tetrahedral A sites. Additionally, scanning electron microscopy measurements of the substituted specimens reveal that the ferrite powders are nanoparticles. With an increase in RE ions, the coercivity increases, and the saturation magnetization changes obviously. The XRD characterization of Co_0.7Zn_0.3La_xFe_2−xO₄ shows that the main crystalline phase of the sample is the cubic spinel structure phase, and there are fewer secondary crystalline phases. The lattice parameter tends to decrease with the substitution of La³⁺ ions. The average grain size decreased significantly with the increase in La content. From ferrimagnetic state transition to relaxation behavior, the hyperfine magnetic field decreases in La concentration by room temperature Mössbauer spectra. With the substitution of La³⁺ ions, both the saturation magnetization and coercivity of the samples were reduced, and the coercivity of all samples was lower.

Perovskite is the largest mineral on earth and has a variety of excellent physical and chemical properties. La_1−xR_xFeO₃ (R = Co, Al, Nd, Sm) were synthesized using the sol-gel method and analyzed by XRD, TG-DTA, and VSM. With the increase in the Co²⁺ doping content, the diffraction peak drifted in the direction of a larger angle. The grain size of La_1−xR_xFeO₃(R = Co) is mainly concentrated between 50.7 and 133.5 nm. As the concentration of Co²⁺ increased, the magnetic loop area and magnetization increased. La_1−xR_xFeO₃(R = Al) is an orthorhombic perovskite structure, the grain size decreased with the increase in Al³⁺ doping concentration, and the minimum crystallite is 17.9 nm. The magnetic loop area and magnetization increased with the increase in Al³⁺ ion concentration. The enclosed area of the M-H curve of the sample decreased, and the ferromagnetic order gradually weakened and tended to be antiferromagnetic, which may be due to the increase in sintering temperature, decrease in the iron oxide composition, and changes in the magnetic properties. Proper doping can improve the magnetization of La_1−xR_xFeO₃(R = Nd), refine the particles, and obtain better magnetic performance. As the Nd³⁺ ion concentration increased, the magnetic properties of the samples increased. Ms of La_0.85Co_0.15FeO₃ prepared by different calcination time increases with the increase in calcination time. As the Sm³⁺ ion concentration increased, the magnetic properties of the samples increased. Proper doping can improve the magnetization of La_1−xR_xFeO₃(R = Sm), refine the particles, and generate better magnetic performance.

LiFePO₄ takes advantage of structure stability, safety and environmental friendliness, and has been favored by the majority of scientific researchers. In order to further improve the properties of LiFePO₄, AO-type metal oxides (MgO and ZnO) and LiFePO₄/C composites were successfully prepared by a two-step sol-gel method. The effects of AO-type metal oxides (MgO and ZnO) on LiFePO₄/C composites were studied. TG, XRD, FTIR, SEM and VSM analysis showed that the final product of the MgO and LiFePO₄/C composite was about 70.5% of the total mass of the precursor; the complete main diffraction peak of LiFePO₄ and MgO can be found without obvious impurity at the diffraction peak; there is good micro granularity and dispersion; the particle size is mainly 300 nm; the saturation magnetization (Ms), the residual magnetization (Mr) and the area of hysteresis loop are increased with the increase in MgO content; and the maximum Ms is 11.11 emu/g. The final product of ZnO and LiFePO₄/C composites is about 69% of the total mass of precursors; the complete main diffraction peak of LiFePO₄ and ZnO can be found without obvious impurity at the diffraction peak; there is good micro granularity and dispersion; the particle size is mainly 400 nm; and the coercivity (Hc) first slightly increases and then gradually decreases with the increase of zinc oxide.