The Research On Preparation And Surface Modification Of FePt Magnetic Materials

Because of its high coercive force, ultra-high single axis magnetic crystalanisotropy, very low super paramagnetic critical size and good chemical and thermalstability, FePt magnetic materials has great potential application for future ultra-highdensity perpendicular magnetic recording media, biomedical domain and so on. In thisthesis the FePt films, Fe/Pt multilayers and FePt nanoparticles（NPs） were depositedby pulse electroplating method （PEM）. The FePt NPs also were synthesized by wetchemical method, and the surfaces were modified by SiO₂. The relationships betweenthe preparation conditions of samples and the composition, morphology, crystalstructure, magnetic performance were investigated by XRD, FESEM, TEM, EDS,FT-IR and VSM.The results were as follows: All FePt co-deposition films prepared by PEMcontained a lot of oxygen content. Pulse potential had great effects on thecompositions of films. The thickness of thin film was proportional to deposition time.FePt co-deposition films with different compositions and thicknesses could beobtained through changing pulse potential and deposition time. The compositions ofthin films were more sensitive to pulse potential and with higher oxygen containingwhen the ratio of Fe/Pt was closer to1. All FePt co-deposition films as-depositionwere face-centred cubic （fcc） structure. A small amount of fcc structure FePt changedinto face-centred square （fct） structure after the films were heated treatment inhydrogen atmosphere at550℃for30min, and the films became some hard magnetic.Multilayer technology could effectively reduce the oxygen content of filmsas-deposition and the FePt phase transition temperature from fcc structure to fctstructure. The Fe/Pt multilayers with compositions of both Fe₃₃Pt₃₇O₃₀andFe₃₇Pt₃₅O₂₈were deposited through adjusting electrodeposition parameters. Theoxygen content of Fe/Pt multilayers with compositions of Fe₃₇Pt₃₅O₂₈reduced by71%after600℃treatment in hydrogen for30min, and the crystal structure of FePtchanged from promiscuous fcc structure into ordered fct structure in which the （110）texture appeared; The soft magnetic of the multilayers changed into hard magnetic.The coercive force of the multilayers increased obviously to about1420Oe.The FePt nanoparticles with the composition of Fe₅₂Pt₄₈were synthetized by wet chemistry method. The as-synthetized FePt NPs were spherical with promiscuous fccstructure. They showed superparamagetism at room temperature and the coerciveforce of them was approximately zero. After heat treatment in argon at700℃for30min, the FePt NPs changed into ordered L10-FePt and the degree of order was0.8around. Spherical FePt NPs were prepared by PEM. They are rich-Pt and the averagediameter was about2.5nm. The as-prepared NPs were also superparamagetism atroom temperature with promiscuous fcc structure. After heat treatment in argon at700℃for30min, the rich-Pt NPs changed into ordered fcc structure L1_2-FePt₃phasewith a small quantity of L10-FePt phase and showed some hard magnetic.In the process of FePt@SiO₂composite NPs prepared by reverse microemulsionmethod，the primary particles of SiO₂were heterogeneous nucleated, aggregated andgrown on the surface of FePt NPs. The thickness of SiO₂shell layers could range from2nm to20nm through adjusting the addition amount of tetraethoxysilane. Thedispersion of composite NPs could be improved by SiO₂modification. Theas-synthetized FePt@SiO₂composite NPs showed a good superparamagnetic withlittle change in the saturation magnetization. The growth and aggregation ofFePt@SiO₂composite NPs and the phase transition of FePt from fcc structure to fctstructure in the heat treatment process were effectively restrained by SiO₂modification. After heat treatment, compared with FePt, the saturation magnetizationand remanent magnetization of FePt@SiO₂composite NPs reduced slightly but thecoercive force kept unchanged.