The Synthesis And Magnetic Properties Of L1₀-FePt Nanoparticles

The L10-Fe Pt nanoparticles(NPs)with a fce-centered-tetragonal structure,also known as fct-Fe Pt,have attracted intensive attention in the field of magnetic recording,permanent magnets and catalysis,due to their properties of large magnetocrystalline anisotropy constant,high coercivity,extremely small superparamagnetic limit size,high curie temperature,excellent corrosion resistance and chemical stability.Solution phase synthesis was develped to prepare monodisperse Fe Pt NPs with a narrow size distribution,uniform size,compositional and well-defined shapes.The face-centered cubic(fcc)structure was adopted in the synthesized Fe Pt NPs.The fcc-Fe Pt NPs with the superparamagnetic properties at room temperature,is unsuitable for magnetic recording and permanent magnet material application.The phase transformation of Fe Pt NPs from fcc to fct usually requires high temperature annealing,which will lead to particles agglomeration and growth up.In this study,the aim is to propose a simple one-step solution phase synthesis for hard magnetic fct-Fe Pt NPs,explore the intermidiate reaction process of fct-Fe Pt formation,reveal the halide ions effectively promoting the ordering phase transformation process of Fe Pt NPs,and further illuminate Cu doping lowering the ordering phase transformation temperature of fct-Fe Pt NPs.The main research contents are as follows:In the atmosphere of N2,the hard magnetic cauliflower-like fct-Fe Pt NPs were prepared by one-step high-temperature liquid phase reduction using Fe(acac)3 and K2 Pt Cl6 as precursors,OAm as solvent,surfactant and reductant.The results show that the Fe/Pt mole ratio,sovlent volume,reaction temperature and time has a significant influence on the morphology,structure and magnetic properties of fct-Fe Pt NPs.The fct-Fe Pt NPs exhibit the strongest magnetic properties when prepared at 350 ? for 8 hours with a Fe/Pt mole ratio of 1:1 and 10 m L OAm.The temperature dependent experiments show that the Fe(acac)3 is reduced to Fe3O4 intermediate at 220 ?.Then,Fe3O4 intermediate and K2 Pt Cl6 are reduced to form fcc-Fe Pt with elevating the reaction temperature.When heating to 330 ?,fct-Fe Pt NPs are obtained by phase tansformation process.In the end,the OAc and TOPO were utilized as surfactants and the fct-Fe Pt NPs with a size range of 66-10 nm were separated.The synthesis of Fe Pt NPs were studied,with Fe3O4 intermediates used as Fe precursor.By simply heating the solution of Fe3O4 NPs intermediate and Pt(acac)2 with OAm to 350 ?,the formation process of Fe Pt NPs were investigated.Pt(acac)2 were reduced to branch-like at 170 ?,then the branch-like structure collaped to small Pt NPs when heated to 200 ?.Fe3O4 NPs were reduced to Fe atoms and alloy with Pt to form Fe Pt NPs at 350 ?.The phase structure of Fe Pt NPs were dependent on the size of Fe3O4 NPs.When 5 nm Fe3O4 NPs were used as precursors,fct-Fe Pt NPs were separated.The saturated magnetization and coercivity of fctFe Pt NPs were 39 emu/g and 2.4 k Oe,respectively.When the size of Fe3O4 NPs were set to be 8 nm or 20 nm,fcc-Fe Pt NPs would be obtained.This method could also be used to prepare other transition metals and Pt-based binary alloys NPs,e.g.,Co Pt,Ni Pt,Cu Pt and Zn Pt.Halide ions promoted the ordering phase transformation of hard magnetic fct-Fe Pt NPs.Comparing the phase structure and magnetic properties of Fe Pt NPs prepared with different precursors,the fct-Fe Pt NPs were obtained only precursors contains Cl-.The phase sturcutre and magnetic property of the Fe Pt NPs were tuned by adding different additives.The results demonstrated that the halide ions played a key part in the formation of fct-Fe Pt NPs.The amount of Cl-additive affected the ordering of fct-Fe Pt NPs,with the mole ratio of Cl-to Pt atoms increased from 1:1 to 6:1,the order parameter first increased and then decreased.The ordering parameter reached its maximum value when the mole ratio of Cl-to Pt was set to be 3:1.The strong binding force between the halide ions and the Fe/Pt ions decreased the reduction rate of Fe and Pt precursors,leading to a lower nucleation rate of Fe Pt NPs and large hard magnetic fct-Fe Pt NPs.The ordering phase transforamtion temperature of fct-Fe Pt was further decreased with Cu doping.The effects of Cu doping content,reaction temperature and time on the ordering of fctFe Pt Cu NPs were studied in this work.The results showed that the ordering temperature of fctFe Pt Cu NPs was decreased sharply by the doping Cu.At 270 ?,Pt and Cu precursors were reduced to form Pt and Cu atoms rich Fe Pt Cu ternary alloy,where Cu atoms took the Fe sites in the Fe Pt lattice.As the reaction temperature increased,more Fe(acac)3 precursor was decomposed to Fe atoms and entered the Pt and Cu atoms rich Fe Pt Cu lattices to form fccFe Pt Cu NPs.The c-axis was contracted due to smaller Cu atoms radius and lower melting point.As a result,phase transformation happened from fcc-Fe Pt Cu phase to fct-Fe Pt Cu phase by atoms diffusion and rearrangement at 310 ?.Compared with fct-Fe Pt without Cu doping,the ordering temperature of fct-Fe Pt Cu was reduced by 40 ?.