  A systematic investigation has been performed to optimize the microstructure
of $\mathrm{L1_0-FePt-SiO_x}$ granular thin film as recording media for
heat-assisted magnetic recording. The FePt-BN nucleation layer, which is stable
even at $700^\circ \text{C}$, is used to control the grain sizes and
microstructure during high-temperature processing. The study finds that films
of high-aspect-ratio FePt grains with well-formed silicon oxide (SiOx) grain
boundaries require the grading of the deposition temperature during film growth
as well as the grading of the silicon oxide concentration. Well-isolated
columnar grains of $\mathrm{L1_0-FePt}$ with an average height greater than 11
nm and diameters less than 7 nm have been achieved. Transmission electron
microscopy (TEM) analysis of the microstructures of samples produced under a
variety of non-optimal conditions is presented to show how the microstructure
of the films depends on each of the sputtering parameters.
