Directly Ordering Preparation And Characterization Of Monodisperse FePt Nanoparticles

Magnetic nanoparticles represent an important class of functional nanoparticles and have been investigated extensively for their interesting nanoscale magnetism and potential applications.Among various magnetic nanomaterials,ordered L1₀-FePt have high uniaxial magnetic crystal anisotropy,strong ferromagnetism,very small superparamagnetic limit size,excellent chemical stability and unique catalytic activity,making them the most promising candidates for future ultra-high density data storage media,high-performance permanent magnetic nanocomposites and highly active catalytic materials.Numerous studies have revealed that the chemical solution-phase syntheses are capable of producing high-quality fcc-FePt nanoparticles,which exhibit superparamagnetic properties at room temperature.It has been observed that a fully ordered L1₀-structured FePt can only be formed at high annealing temperatures at which the particles often sinter.A large number of improved synthesis methods still suffer from operational complexity,unavoidable high temperature annealing,generation of impurity phases,large particle size and wide range of particle size distribution,which greatly limit their usefulness.A single small-sized,monodisperse FePt nanoparticle with ferromagnetism at room temperature is a condition that must be satisfied for future single-particle magnetic recording,and how to prepare small-sized,monodisperse,hard magnetic ordered L1₀-FePt nanoparticles in one simple step has become the focus and difficulty of current research.Based on this,a halogen-containing surfactant cetyltrimethylammonium chloride(CTAC)-assisted liquid-phase synthesis is proposed in this thesis as a strategy for the direct one-step preparation of small-sized,monodisperse,hard magnetically ordered L1₀-FePt nanoparticles without further high-temperature heat treatment or doping with a third metal element.The current CTAC-assisted synthetic strategy makes it possible to deeply understand the formation of the ordered Pt-based intermetallic NP in solution phase synthesis.And this article mainly includes the following two parts:The first is the study on direct ordered preparation and magnetic properties of FePt nanoparticles.The halogen-containing surfactant CTAC was selected as the ordered structure inducer,and the hard magnetic L1₀-FePt nanoparticles with small size and good dispersion were successfully prepared by chemical liquid-phase synthesis.Through the optimization of the synthesis process conditions,it was determined that the amount of CTAC,reaction temperature,surfactant carbon chain length and molar ratio of Fe/Pt precursors have important effects on the morphology,structure and magnetic properties of the prepared FePt nanoparticles,and the reaction mechanism of the direct ordering phase transition process of FePt nanoparticles induced by CTAC was revealed.This strategy provides a new method for avoiding high-temperature annealing and directly growing ordered Pt-based intermetallic nanoparticles.The results show that hard magnetic L1₀-FePt nanoparticles with small size,good dispersion and high coercivity can be prepared with appropriate amount of CTAC at 350°C and close molar ratio of Fe and Pt atoms.The second is the morphological improvement and characterization of L1₀-FePt nanoparticles.The L1₀-FePt nanoparticles prepared by CTAC alone assisted with irregular morphology and wide particle size distribution.To gain small-sized,monodisperse,egular morphology and hard magnetic L1₀-FePt nanoparticles,surfactants such as tri-n-octylphosphine oxide(TOPO)and oleic acid(OA)were introduced to the reaction system to cover the nanoparticle surface,which could introduce the repulsive force around the nanoparticles to stabilize the nucleated particles for stable growth and maturation to obtain monodisperse FePt particles under the conditions of preparing L1₀-FePt nanoparticles with optimal magnetic properties.The results showed that TOPO and OA successfully improved the morphology of the prepared L1₀-FePt nanoparticles at the expense of the magnetic properties of FePt nanoparticles,but the addition of 0.5 m L OA allowed the preparation of small-sized,monodisperse,hard magnetically ordered L1₀-FePt nanoparticles with an average particle size of 10.7 nm,room temperature coercivity of 3.15 k Oe,room temperature exhibit strong ferromagnetism and high Curie temperature,which have good application prospects.