Structure,Magnetic Properties and Electromagnetic Wave Absorption Proper-ties of Neodymium Substituted Cobalt Ferrite
High-frequency devices with frequencies up to gigahertz have extreme advantages such as long-distance response,fewer wires,and high-speed response,realizing the interconnection of everything.The electromagnetic radiation and interference generated by these high-frequency smart devices,that is,electromagnetic pollution,has become a new type of pollution source after water,at-mosphere,and noise.Its characteristic is that it interferes with the normal communication of electromagnetic devices and causes great harm to the human body,and it is not easy to protect.In recent years,cobalt ferrite has been widely used in the field of electromagnet-ic pollution prevention because of its high coercivity and resistivity,suitable saturation magnetization,large magnetocrystalline anisot-ropy,good chemical stability,and high Curie temperature.However,cobalt ferrite cannot obtain excellent electromagnetic wave ab-sorption performance and application limitations due to poor impedance matching and high density.Rare earth elements have a large magnetic moment and magnetocrystalline anisotropy due to their special electronic structure,which makes them widely used in a vari-ety of materials.Doping a small amount of trivalent rare-earth ions into spinel ferrite is a promising method to improve its electromag-netic properties.In this paper,cetyltrimethylammonium bromide was used as a surfactant,C4H6CoO4·6H2O,FeCl3·6H2O,and NdCl36H2O were used as raw materials to synthesize spinel-type nano-cobalt ferrite with Nd substitution by the co-precipitation method.For cobalt ferrite,the substitution amount of Nd was x(x=0,0.02,0.04,0.06 and 0.08).X-ray diffractometer(XRD),transmission elec-tron microscope(TEM),inductively coupled plasma emission spectrometer(ICP-OES),vibrating sample magnetometer(VSM)and vector network analyzer(VNA)were used to character samples,the research focused on the substitution of Nd ions influence on the microstructure,magnetic properties and electromagnetic wave absorption properties of cobalt ferrite nanoparticles.The results showed that Nd ions substitution would reduce the saturation magnetization of cobalt ferrite,which was because the magnetic behavior of spi-nel ferromagnetic materials was mainly controlled by the negative Fe3+-Fe3+interaction(spin coupling of three-dimensional electrons).At the same time,there were relatively weak Nd3+-Fe3+interaction(4f-3d coupling)and Nd3+-Nd3+interaction(indirect 4f-5d-4f electronic coupling).When the large-radius Nd ions replaced Fe ions in the octahedral position,the negative exchange interaction be-tween Fe3+-Fe3+in the two sublattices was reduced,thereby reducing the saturation magnetization.When Nd ion substitution rate was high(x≥0.06),the secondary phase NdFeO3 produced was antiferromagnetic in nature,which would further reduce the saturation magnetization of the cobalt ferrite.When the substitution amount of Nd ion was low(x≤0.04),it could refine the cobalt ferrite grains and increase the lattice constant.This was because Nd-O bond energy was higher than that of Fe-O,and the introduction of Nd ions into the octahedral position required more energy.The energy required for this process was provided at the expense of crystallization,thus hindering the growth of crystallites.The reduction of grain size played a key role in increasing the coercivity and magnetocrystal-line anisotropy of cobalt ferrite,which was because the coercivity was inversely proportional to the grain size.The larger the grain size,the lower the volume fraction of grain boundaries and the less pinning effect of domain walls.However,when x≥0.06,the secondary phase NdFeO3 would be produced,and the grain size would become larger,the lattice constant would become smaller,and the mor-phology of the cobalt ferrite would change from spherical to quadrilateral.This was because there was a solubility limit for the replace-ment amount of Fe ions by Nd ions,and excess Nd ions tended to accumulate near the grain boundaries in the form of NdFeO3.Exces-sive substitution of Nd ions would reduce the coercivity and magnetocrystalline anisotropy,and this was because the magnetocrystal-line anisotropy of cobalt ferrite was caused by Co ions in the octahedral position.Due to the substitution of Nd ions with a larger radius,Co ions migrated from the octahedral position to the tetrahedral position,which showed that the substitution of Nd ions caused the tetra-hedral and octahedral cation rearrangement of the cobalt ferrite to have a greater impact on the coercivity and magnetocrystalline anisot-ropy.By calculating the area of the hysteresis loop,it could be found that Nd ions replacing cobalt ferrite increased the attenuation con-stant(α)in the form of enhanced hysteresis loss.In this way,the electromagnetic waves entering the material could be dissipated in other forms of energy,thus effectively enhancing the electromagnetic waves'absorption performance of cobalt ferrite.When x=0.04,and the thickness of the absorption layer was 2.5 mm,and the frequency was 13.75 GHz,the minimum reflection loss value was-12.94 dB,and the effective absorbing frequency band was 1.02 GHz(13.07～14.09 GHz).Compared with cobalt ferrite,the reflec-tion loss of CoFe1.96Nd0.04O4 was increased by 61.46％,and the effective absorption band was widened by 1.02 GHz.
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