In-situ 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 212°C and 412°C to 25% of the initial strength at 25°C, it increased again during cooling and recovered 67% of its initial strength. However, when an island was heated to 512°C, the magnetization diminished and did not recover again during cooling. The Curie temperature (Tc) of the FePt nanoparticles was determined to be 350°C and was in good agreement with the Tc determined by bulk measurements using a VSM, which was approximately 100°C lower than the reported Tc for bulk Fe₅₅Pt₄₅.