The M(?)ssbauer Study on Spinel Co-Ni Ferrite Nano-particles
|Course||Condensed Matter Physics|
|Keywords||Nickel ferrite Ferrite materials Cobalt ferrite Anisotropy constant Mixed Spinel Nanoscience Magnetite Nanomaterials Magnetic anisotropy|
The notion of magnetism dates back to the Ancient World, where magnetite, the first known ferrite, was called lodestone. After 1930s, the study on ferrite materials began to develop at a very high speed due to the technology of high frequency radio which desires materials of ferromagnet with high electrical resistivity. Some ferrite-related theories such as the theory of antiferromagnetism, the theory of superexchange interaction and the Neel theory of molecular field were established during that period of time. The development of ferrite has a far-reaching impact on science and technology. In 1933, the permanent-magnet material containing cobalt ferrite was created by Japanese. Kinds of zinc-contained ferrite with high quality were developed and relevant preparation techniques were established by Snock from Philips laboratory of Holland, which caused the industrialization of soft magnetic ferrite in 1946.Many efforts were performed on the study of mixed ferrite which usually presents a wide choice in its properties and could meet different kinds of need. Cobalt ferrite is a kind of hard ferrite with a high and positive magnetocrystalline anisotropy constant K1 （+106erg/cm3） and Nickel ferrite a kind of soft magnetic ferrite with a relatively low and negative magnetocrystalline anisotropy constant K1 （ -104erg/cm3）. So the magnetocrystalline anisotropy constant K1 of the mixed spinel ferrite Co0.027Ni0.973Fe2O4 should be zero theoretically by doing a simple math, and consequently the ferromagnetic resonance line width of the mixed spinel ferrite Co0.027Ni0.973Fe2O4 should reach to its minimum, which had been demonstrated experimentally by M. H. Sirvetz and J. H. Sounders. Most of the studies on the mixed Co-Ni ferrite CoxNi1-xFe2O4 are limited to the case that the cobalt concentration is lowNano science have been developed dramatically in recent years, in which mixed Ni-Zn ferrite have stimulated many interests due to its potential applications in microwave techniques. But the studies on the nano-sized Co-Ni ferrite CoxNi1-xFe2O4（0<x<l） have not been systematically reported so far. Nano particle system is still a interesting research field because many behaviors of nano materials have not been fully understood. Nano-sized Co-Ni ferrite CoxNii.xFe2O4 （0<x<l） particles have been synthesized by using the PVA Sol-Gel method, and their properties have been analyzed by many measuring techniques.Fe3C>4 is a kind of half-metallic ferromagnet which presents a perspective for exhibiting a large MR at room temperature. Antiferromagnetic components play a important role in half-metallic-ferromagnet /Antiferromagnet composites. So it is important to synthesize half-metallic-ferromagnet /Antiferromagnet composites with a method which could be easily controlled. Fe3C>4 fihns and Ve^O^a -Fe2C?3 composite films have been synthesized from a sintered Fe2C>3 target by rf magnetron sputtering and been examined by different measuring equipments.The main contents of this thesis are as follows:1. Nano-sized particles of mixed Co-Ni ferrite CoxNii.xFe2O4 （0<x<l） have been prepared by PVA Sol-Gel method and their properties have been examined by X-ray diffraction technique, vibrating sample magnetometer and room temperature Mossbauer spectrometer for the first time. X-ray diffraction shows an approximately linear increase of lattice constant with an increasing cobalt concentration x, which might result from the fact that the ionic radius of Co2+ is larger than that of Ni2+. The larger value of the lattice constant for the nanosizedCoxNixxFe2O4 than for the bulk material with x ranging from 0 to 1.0 may be due to the lattice expansion induced by the reduced particle size and increased surface-body ratio in CoxNilxFe2O4 ferrite nanoparticles. The increase ofcobalt concentration yields the monotonic increase of Ms, which may be caused by the substitution of Ni2+ ions by Co2+ ions on octahedral sites considering thatthe magnetic moment pi per ion for Co2+ ions （3 piB） is larger than that for Ni2+ions （2 pig ）. With the increasing cobalt concentration, the coercivities （Hc） increase in the range of low cobalt concentration and then decrease in the rangeof high concentration, which have been explained by the induced anisotropy from Co2+ ion. Room temperature Mossbauer spectroscopy have been recorded and the differences and variations of the hyperfine parameters for the two interstitial sites have been explained.2. Nano-sized ferrite CoxNii.xFe2O4 （x=0.1, 0.3, 0.5） have been measured by high temperature Mossbauer technique for the first time. According to the fitted hyperfine parameters, the area ratio （A/B） increases with increasing temperature at low temperature and then decreases at high temperature. Both Einstein and Debye model have been used to fit the resonance absorption areas and the center shifts of A and B sites respectively. The characteristic temperatures of A site is larger than that of B site for both Einstein and Debye model, which explains the increase of the area ratio （A/B） with increasing temperature at low temperature and may show that the Fe3+ ions are trapped more tightly in A site than in B site. The decrease of the area ratio （A/B） at high temperature may be due to the migration of Fe3+ ions from A site to B site. Moreover, the characteristic temperatures from the center shifts fitting are obviously higher than that from the absorption areas fitting, which might derive from the different weighting factorsin the general expressions of the mean square displacement （jc2） and the meansquare velocity （v2）.3. The splitting of Neel temperature in mixed nano ferrite CoxNii.xFe2O4 have been detected by the magnetic thermogravimetric analysis for the first time. The splitting values are 0 K, 21.6 K and 32.2 K when x equals to 0.1, 0.3 and 0.5 respectively. The dependence of the reduced hyperfine field a=Hhf（T）/Hhf（0） on the reduced temperature x=T/TN for the A and B sites of nano-sized CoxNii.xFe2O4 （x=0.1, 0.3, 0.5） have been fitted by the Brillouin function, from which Neel temperatures for both A and B sites of each sample were obtained. Neel temperature （Tn） values of A and B site from fitting procedure show a very similar splitting to that from the magnetic thermogravimetric analysis. The splitting values between TN（A） and TN（B） from fitting procedure are 0±4 K, 13±3K and 30±6 K when x equals to 0.1, 0.3 and 0.5 respectively. Superexchange interaction model has been used to explain above splitting phenomena. 4. Fe3O4 films and Fe3(V a -Fe2O3 （half-metallic-ferromagnet/antiferromagnet） composite films have been sputtered from a sintered Fe2C>3 target by the method of altering the sputtering pressure for the first time, their properties have been examined by X-ray diffraction technique, vibrating sample magnetometer and conversion electron Mossbauer spectrometer （CEMS）. Antiferromagnetic a -Fe2C>3 components of the samples have been confirmed by XRD and CEMS, and the experimental results have been explained.