Evolution of FePt Nanoparticle Shape and L10 Order During In-Situ Annealing

In the L1 0 (CuAuI) ordered state, equiatomic FePt exhibits exceptional magnetic properties with uniaxial-magnetocrystalline anisotropy (K u ) greater than 10 7 ergs/cm 3 . However, monodispersed FePt nanoparticles produced by chemical synthesis methods [1] are face-centered cubic (FCC) and require annealing to chemically order into the tetragonal L1 0 structure. L1 0 -ordered FePt nanoparticles display outstanding magnetic properties with coercivity that can exceed 22 kOe [2]. Therefore, understanding the L1 0 -ordering phase transformation is critical for incorporating these FePt nanoparticles into magnetic nanostructures [3]. The current study employs in-situ heating using Protochips Aduro TM heater chips and high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM), also known as atomic-number contrast or Z-STEM, to investigate the development of L1 0 order and particle-shape changes during annealing of FePt nanoparticles. The FePt nanoparticles were synthesized using a standard method that is described elsewhere [1]. A surfactant of oleic acid and oleyl amine stabilizes the FePt particle-size distribution with a standard deviation of less than 5%. One drop of the FePt nanoparticles suspended in hexane was deposited onto a Protochips device (a prototype of the Aduro TM heating holder [4]). Heating the sample to 500 o C for 60 sec in the vacuum of the microscope removed the surfactant layer and allowed for contamination-free imaging.