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FePt nanoparticles have the potential to extend the data density of magnetic recording beyond 1 Tbit/in². The phase transformation from low anisotropy, face-centered cubic to face-centered tetragonal with high anisotropy is essential and complex. The samples measured in this paper were annealed in a mixture with fine ground (<20 μm) NaCI. The switching field distributions and activation volumes of two different sizes of particles are presented, 4 and 15 nm. The samples exhibit complex structural behaviour, demonstrating how difficult it is to obtain uniform nanoparticles. We find that the 4-nm particles reverse coherently whereas the 15 nm do not due to inhomogeneous crystallinity. [ABSTRACT FROM AUTHOR]