Study On The Structure And Magnetic Properties Of Spinel Ferrite Nanocrystal Doped With Rare-earth Ions

In the paper, the preparation conditions, their structure and magnetic behaviorswere studied. Now many methods are used to synthesize the ferrites, samples areprepared by emulsion method in the paper. Heat treatment temperature and timedecide the crystallite sizes. The samples are investigated by X-ray diffraction (XRD),Brunauer–Emmett–Teller (BET), thermal gravity analysis (TGA) and differentialthermal analysis (DTA), transmission electron microscope (TEM), FT-IR,M?ssbauer spectroscopy and a vibrating sample magnetometer (VSM).Ferrite with spinel structure shows ferrimagnetism, which has high saturationmagnetization and low coercivity, except CoFe2O4 ferrite. Among spinel ferrites,cobalt ferrite, CoFe2O4 is especially interesting because of its high cubicmagnetocrystalline anisotropy, high coercivity and moderate saturationmagnetization. First, the influence of radius and effective magnetic moment of rare-earth ions onstructure and magnetic properties of samples are investigated in detail. Theroom-temperature tested result shows the crystallite sizes and saturationmagnetization of samples doped with Nd3+ and Gd3+ ions are decreased relative tothe un-doped sample. While the sample doped with La3+ ions has larger saturationmagnetization than the un-doped sample with similar crystallite size. Theassumption that only some rare-earth ions can enter into spinel lattice can be provedby the results of lattice parameter. Ni-Mn ferrite doped with rare-earth ions withcrystallite size about 10nm shows superparamagnetism. SPM photos show rare-earthions can improve the the densification of samples.The critical diameter ofsingle-domain is about 35 nm for Ni0.7Mn0.3RE0.1Fe1.9O4 ferrite nanocrystal. Superconducting quantum interference device (SQUID) magnetometer with afield 5T was used to investigate the influence of rare-earth ions on Ni-Mn ferrite atlow temperature. Rare-earth atoms play an important role in deterring themagnetocrystalline anisotropy in 4f–3d intermetallic compounds. Hence, the valuesof saturation magnetization and coercivity of the samples increase with decreasingtemperature, which are rightly explained by low temperature spin-wave theory. Theun-doped sample has the maximum value of saturation magnetization, while theLa3+-doped sample has the maximum value of coercivity at 2K. This reveals a fineforeground for ferrite doped rare-earth ions at low temperature.Second, the maximum specific surface of CoLa0.08Fe1.92O4 ferrite nanocrystalsynthesized by emulsion method is 93.85m2/g. When the specific surface is 18.51m2/g, the coecivity and saturation magnetization values of CoLa0.08Fe1.92O4 ferritenanocrystal are 1506.9Oe and 61.0emu/g, respectively. Furthermore, on the surfaceof the nanocrystalline ferrite, the non-magnetic layer thickness of theCoLa0.08Fe1.92O4 powders is equal to 1.1nm.In order to make ferrite nanocrystal have more extensive application, CoFe2O4ferrite nanocrystal was prepared by sol-gel method and coated by amorphous silica.The coated thickness of silica is about 2nm, which is proved by TEM image.The magnetic properties can be influenced by particle shape, and particle shapecan be controlled by preparation method. So the magnetic field was applied tosynthesized process. The particle shape can be controlled by the magnetic fieldintensity and acting time. The hexagonal CoLa0.1Fe1.9O4 ferrite nanocrystal wassuccessfully prepared. At the same time, both the saturation magnetization andcoercivity are decreased with increase of magnetic field intensity and acting time.Finally, composites of Fe/Fe3O4 and Fe/Co-Fe3O4 have been prepared bydisproportion method. The metal Fe has b.c.c. structure and the metal Co has f.c.c.structure. The combination of metal and ferrite produces more excellent magneticproperties. Fe/Fe3O4 has high saturation magnetization and low coercivity, whichmake it have more extensive application. The maximum saturation magnetization ofsample is 108.4emu/g, and coercivity is 61.2Oe. Fe/Co-Fe3O4 has moderatesaturation and high coercivity. The maximum coercivity of sample reaches2504.4Oe, and saturation magnetization is 73.0emu/g. Such excellent performancecan satisfy the need of permanent information storage.We also synthesized Fe/Fe3O4 composite by the method of disproportion reactionin an external magnetic field. The Fe/Fe3O4 rod having length about 2-4μm andwidth about 0.5μm was prepared.