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Abstract

Thermal annealing is essential for achieving ultrasmall size ferromagnetic properties in next-generation high performance nanocomposite magnetic materials. However, during the annealing process, growth and agglomeration of nanoparticles normally occurs, which destroys the narrow size distributions. Thus, the materials become less suitable for application in high-density magnetic recording. The mechanism of nanoparticle growth and sintering has been difficult to determine because of the lack of suitable in situ tools to probe subnanometer changes at the local level. Here we report a study using high-resolution scanning transmission electron microscopy (STEM) coupled with an in situ thermal annealing stage of surfactant-free, monodispersed superparamagnetic PtFe (cubic) alloy nanoparticles (≈2 nm in diameter) stabilized in or on a KCl matrix. Ex situ experiments confirmed that annealing produces PtFe (tetragonal) ordered intermetallic nanoparticles with a mean diameter of 5 nm, and the in situ study revealed that the mechanism of nanoparticle growth is dominated by particle–particle coalescence, although Ostwald ripening is also implicated in a few regions. In addition, to determine the time dependent evolution of the size distribution of an ensemble of over 400 nanoparticles, analysis of the in situ data also allows tracking of individual nanoparticles, distinguishing coalescence from Ostwald ripening, nanoparticle by nanoparticle. This approach has provided valuable insights into changes in crystal structure and sintering that occur during the thermal annealing of Pt–Fe nanoparticles.

Impact Statement

The Fusion system was used in an FEI F20 in STEM mode to analyze the phase change and coarsening of FePt nanoparticles < 5nm in size. The ferromagnetic nanoparticles are candidates for ultra-high density media, however require an annealing step after nanoparticle synthesis to 600 C. The as synthesized nanoparticles are disordered FCC, and the anneal step initiates a phase change to a tetragonal structure which makes the particles magnetic. A KCl matrix was used as a support to help inhibit coarsening. In situ heating was used to determine the coarsening mechanism, and it was found the particles primarily coarsened via coalescence. In some incidences Ostwald ripening occurred, and direct observations of each process was reported.