<I>In-situ</I> Electron Holography Observation of FePt Nanoparticles at Elevated Temperatures

Abstract

We have developed a method to observe the magnetic field around L10 FePt nanoparticles by in-situ electron holography at elevated temperatures. FePt Nanoparticles with sharp size distribution and chemical homogeneity were synthesized by the reverse micelle method. The as-prepared FePt nanoparticles, which had a disordered fcc structure (A1) with the diameter centered at 6 nm, were coated with a surfactant, dispersed onto a glass plate, and heated in order to undergo a transformation from A1 to an ordered fct structure (L10). The particles were kept separated by the surfactant with their original diameter during annealing. A submicron-size island comprising isolated particles was removed and dispersed on an electron transparent carbon film and then magnetized along one direction. We observed a magnetic field distribution of the submicron-size island of nanoparticles by means of electron holography during heating. Although magnetization decreased between 212C and 412C to 25% of the initial strength at 25C, it increased again during cooling and recovered 67% of its initial strength. However, when an island was heated to 512C, the magnetization diminished and did not recover again during cooling. The Curie temperature (Tc) of the FePt nanoparticles was determined to be 350C and was in good agreement with the Tc determined by bulk measurements using a VSM, which was approximately 100C lower than the reported Tc for bulk Fe55Pt45. [doi:10.2320/matertrans.MD200703]