The Manipulation Of Magnetic Properties For Nanosized Cobalt Ferrite And The Related Composite Materials

The manipulation of magnetic properites for the nano-sized magnetic materials is the advanced topics in the fields of condensed matter physics and materials physics. The related studies have both theory significance and wide application foreground. From the aspect of theory, the magnetic structure and magnetic interaction of the nano-sized magnetic materials are quite different from that of the bulk material, such as the formation of the single domain and the destruction of the exchange interaction for the surface atoms, which results in some special magnetic properties for the nano-sized magnetic materials, such as superparamagnetism, the reduced saturation magnetization, the lower Curie temperature, the stronger magnetic anisotropy and magneto-optical effect, as well as some phenomena at low temperature, such as the blocking behaviour, the freezing of the spin-glass-like state and the macroscopic quantum tunneling. In addition, the nano-sized magnetic materials have wide application foreground, such as electronic devices, information storage, environmental protection, biomedicine, aerospace, etc. Therefore, studying the scientific problems in the field of the nano-sized magnetic materials favors the deep understanding on the magnetism of the materials in the nano-size scale, as well as promoting the wide application for the nano-sized magnetic materials. This paper are mainly focused on the nano-sized cobalt ferrite (CoFe2O4), which is a kind of ferrimagnetic compound with ferrite structure. Following are the works concerned in this paper:1. Several synthesis methods, including combustion method, coprecipitation method, hydrothermal method and thermal decomposition method, are applied to synthesize CoFe2O4 nano-particles and the related composite materials, these materials have different particle size and morphologies.2. We have studied the impact of the raw materials and reaction conditions in wet-chemistry method (coprecipitation and hydrothermal synthesis) on the composition, structure, morphologies and magnetic properties for the final products. For coprecipitation method, it is shown that adding reactant into the raw materials can inhibit the increase of the particle size, oxidize the Co2+ ions, and cause the formation of y-Fe2O3. As a result, the products have small coercivity (Hc) and magnetic anisotropy. On the contrary, adding the raw materials into the reactant can clearly enlarge the particle size, and a 10 nm layer for some materials with poor crystallization is observed on the surfaces of the nanoparticles, which may cause the drastic enhancement of Hc and inhibit the increase of the saturation magnetization (Ms). This surface layer can be destroyed by annealing treatment at high temperature. For hydrothermal synthesis, the addition of the surfactant sodium bis(2-ethylhexyl) sulphosuccinate (AOT) or the coordination compound Na3CA-2H2O can lead to the laatice expansion, which may be related with the increase of Ms. While the AOT can clear away the very small particles and enhance the cubic anisotropy and Hc, while adding a large mass of Na3CA-2H2O can modify the shape of the particles and reduce the particle size, which also impacts Ms and He. In addition, by changing reaction temperature and time, the particle size of CoFe2O4 nano-particles by the hydrothermal method can be controlled between 7.6 and 34 nm, in this size range, Ms and Hc increase with the increase of size.3. For the samples prepared the combustion method, the impact of particle size on the magnetic properties is studied. The results show that the heat treatment for the product of the combustion reaction can cause the increase of size from 15 nm to be larger than 1μm. Ms also increases with the increase of size, while the He reaches the maximum as the size of CoFe2O4 nano-particles is about 30nm, and in comparison with annealing, the samples by quenching have obviously smaller Hc.4. CoFe2O4 nano-particles with the average size of 7.6nm are synthesized by the hydrothermal method, and the temperature dependence magnetic properties has also been studied. It is shown that with the increase of temperature, the magnetic properties of the sample experience four stages:the freezing of spin-glass-like state is observed below 20 K, the blocking behavior due to magneto-crystalline anisotropy energy plays a major role between 20 K and 220 K, a small Hc is shown between 220 K and 380 K due to the inter-particle interactions, and the superparamagnetism appear as temperature is higher than 400 K.5. CoFe2O4/CoO (n(Co):n(Fe)=2:1 and 5:1)composite materials and Co3-χFexO4 (x=0, 0.09,0.14,0.27) powders are prepared by combustion and thermal decomposition method, respectively, and the exchange bias effect in these materials is also studied. For the composite material with n(Co): n(Fe)=5:1, the exchange bias occurs below 250 K, and at 10 K, by tuning the cooling field, this composite material has the maximum exchange bias field (He) of 106 Oe. For Co3-xFexO4, the increase of x makes the transition of the composition from single phase compound to the co-existence of two phases (CoFe2O4 and Co3-xFexO4), the magnetic properties enhance as well. In addition, the exchange bias effect also has clear changes:at 10 K, the He of the sample withχ=0.14 is the maximum, reaches 140 Oe, and for the samples with x=0.09 and 0.14, the change trend of He with temperature is also clearly different from that of the sample withχ=0.27, indicating that the He for the former two samples is due to the ferrimagnetic/antiferromagnetic interactions, while for the latter sample, the small He may be related with the freezing spin-glass-like state on the surfaces of the nanoparticles.