Structures And Properties Of FePt Nanoparticles

Due to the ultrahigh magnetocrystalline anisotropy constant Ku(above107erg/cm3), highCurie temperature and good chemical stability, FePt alloys with a face-centered tetragonal (FCT)structure (the so-called L10structure) have been attracting considerable attention as the ultrahighdensity magnetic recording media, high performance permanent magnet,biotechnology/biomedicine and environmental remediation.In this paper, Fe-Pt magnetic alloys have been synthesized by sol-gel method. The structure,morphology, phase transformation and magnetic properties of the samples were characterized bydifferential thermal analyses (TG–DTA), X-ray diffraction (XRD), X-ray photoelectronspectroscopy (XPS), transmission electron microscope (TEM), high-resolution transmissionelectron microscopy (HRTEM),energy dispersive X-ray spectroscopy (EDS), selected areaelectron diffraction (SAED) and vibrating sample magnetometer (VSM).The main conclusions are as following:1. In this work, FexPt100-x(x=30,50,55,60,65,70,75, and80) NPs have been prepared bythe sol–gel method. By adjusting Fe/Pt atomic ratio, we could obtain all three intermetallics,namely, FePt3, FePt and Fe3Pt of the Fe–Pt binary alloy. The structure, morphology and magneticproperties of the samples were investigated. The results indicated that the atomic ratio of Fe andPt is very important for the structural and magnetic properties in the FexPt100-xNPs.2. The Hc increased with the Fe content from30to60at.%, while the Hc decreased when the Fecontent further increases to80at.%. The Hc might be influenced by the chemical order of theL10-FePt NPs. The Ms of FexPt100-xNPs increased with the increase of the Fe content.3. The relationship between the annealing temperature (400,500,600and700°C) and thestructure of the L10-FePt NPs was discussed. And the possible growth mechanism was given in termsof atomic diffusion.4. Differential thermal analyses (DTA) showed the dehydration of various water moleculesand combustion/decomposition of organic groups at the temperature below600°C. The x-ray diffraction (XRD) patterns revealed that when the annealing temperature increased to500°C, theL10FePt NPs were formed as indicated by the appearance of superlattice peaks (001) and (110).Moreover, based on XRD results, we found that the chemical order parameter S increased withthe increase of the annealing temperature.5. The transmission electron microscope (TEM) images showed that the NPs produced at700°C had a mean diameter of14nm. The energy dispersive X-ray spectroscopy (EDS) analysisshowed that no impurity phases existed in FePt NPs. The maximum coercivity (10148Oe) wasobtained for the sample annealed at700°C.6. The addition of Au, Cu and Ag reduced the phase transition temperature fromface-centered cubic (FCC) to face-centered tetragonal (FCT) structure for the FePt NPs. Inaddition, additive Au, Cu and Ag promoted the chemical ordering of L10FePt NPs and increasedthe grain size of L10FePt NPs.7. Due to the silica coating, agglomeration was restrained even if the annealing temperatureincreases up to700°C, and the silica shells decreased magnetic dipole coupling between FePtnanoparticles. The SiO2-coated L10-FePt nanoparticles showed a high coercivity of8096Oe atroom temperature in spite of their core size of only7nm in diameter.