Research On The Structure And Magnetic Properties Of Cobalt Ferrite Nanocrystal

In the paper, the preparation conditions, their structure and magneticbehaviors of spine ferrite were studied. Now many methods are used tosynthesize the nano cobalt ferrites, my samples are prepared by soft chemistrymethod in the paper,such as Tsol-gel and TGlycine-nitrate process. Heat treatmenttemperature and time decide the crystallite sizes. The structure of the samplesare investigated by X-ray diffraction (XRD), Differential Scanning Calorimetry(DSC) and Scanning Electron Microscope(SEM), the magnetic properties arestudied by Vibrating Sample Magnetometry (VSM).Ferrite with spinel shows ferrimagnetism, and cobalt ferrite is especiallyinteresting because of modrate saturation magnetization , high coercivity, highchemical stability and larger magneto-optical deflection angle in visible region.So cobalt ferrite has been considered as an attractive candidate formagneto-optical recording materials. Increasing the coercivity and rectangular isan effective way to improve the performance of magnetic recording media. Inthis paper we use different preparation processes, different annealingtemperature and doping rare earth elements to improve the magnetic propertiesof cobalt ferrite.Firstly, Cobalt ferrites nanocrystalline powders were prepared by sol-gel andglycine-nitrate process(GNP). Then the magnetic properties of the twonano-powder were compared. We found that the particles of powder prepared bythe sol-gel are smaller and its coercivity is larger. But the saturation and theremanence magnetization of the powder samples prepared by GNP method are larger. And also the GNP method is simpler than sol-gel method, so in moststuation we use GNP method to synthesize the samples.Secondly, the influence on the sample structure and magnetic properties bycobalt ion content and annealing temperature is studied systematically.We foundthat monophase with a spinel structure was formed only at high cobalt content(x=1.0). And at this ratio of cobalt ion, although the coercivity is smaller, thesaturation and remanence magnetization is larger, and the rectangular is better.So we get better integrated magnetic properties at the standard ratio of cobaltferrite. We annealed the samples in 600℃, 700℃, 800℃, 900℃, 1100℃and 1200℃. As the annealing temperature increasing, the phases becomingbetter, and the particles growing larger.The saturation magnetization increasedslowly, but the coercivity decrease quickly. Considering the saturationmagnetization, coercivity and remanence ratio,the samples which sintered at 700℃got the best performance. The coercivity Hc is 1549 Oe, the saturationmagnetization is 63.05 emu /g.Thirdly, the influence of rare-earth-doped cobalt ferrite was studied. Thestructure and magnetic properties can be changed by Sm/La cation dopedrespectively. And the range of substituted content is from 0.05 to 0.1. The spinelstructure is not changed by rare-earth cation doping, but with the increase ofdoping content, some other phases come out, and the lattice size will beincreased. The doping of rare-earth ions will decrease the magnetic moments ofthe B crystal lattice, and make the saturation magnetization of the samplesdecreased .Because of larger magnetic anisotropy of the rare earth ions, withless-doped, the coercivity of samples increased, but with the increase of doping,the number of miscellaneous makes coercivity declined. As the magnetic anisotropy of samarium ion is greater than lanthanum ions'.With micro-doped,the coercivity of Sm-doped is larger than La-doped. When the doping content is0.06, we got the greatest coercivity. The value is 1613Oe for La-doped and1654Oe for Sm-doped, which is very favourable for cobalt ferrite in use formagneto-optical recording media materials.Finally, when we investigated the effect of annealing temperature on themagnetic properties of cobalt ferrite. We found that some initial magnetizationcurve lies outside of the hysteresis loop when annealing temperature above 800℃. With the increase of annealing temperature, the phenomenon is moreobvious. Being based on the magnetic anisotropy theory, the influence ofannealing temperature is little on the magnetic anisotropy of the samples. Withlarge magnetic anisotropy field, cobalt ferrite magnetize more difficult at initialphase. But as annealing temperature increasing, the coercivity decreases rapidly,which caused some initial magnetization curve lies outside the main hysteresisloop. And then through the principle of magnetic domain, a single domaincritical dimensions of cobalt ferrite is 74 nm. The samples which sintered at 700℃were in a single domain particle state. Only effected by the rotating magneticviscosity, the coercivity is larger, and the magnetic hysteresis loop is normal.When the annealing temperature at 800℃or above, the samples getmulti-domain structure. The existence of domain wall between the magneticdomain caused irreversible movement. The irreversible movement of the domainwall which caused the hysteresis loop anomaly. As the annealing temperaturerising ,the domain wall increasing, the irreversible movement of magneticdomain strengthening,and the unusual behavior of the magnetic hysteresisbecame more and more obvious.