  In this paper, we present an experimental study of $\text{L1}_0$-FePt
granular films with crystalline/amorphous boron nitride (BN) grain boundary
materials for heat assisted magnetic recording (HAMR). It is found that an
adequate RF substrate bias yields the formation of hexagonal boron nitride
(h-BN) nanosheets in grain boundaries, facilitating the columnar growth of FePt
grains during sputtering at high temperatures. The h-BN monolayers conform to
the side surfaces of columnar FePt grains, completely encircling individual
FePt grains. The resulting core-shell FePt/h-BN nanostructures appear to be
highly promising for HAMR application. The high thermal stability of h-BN grain
boundaries allows the deposition temperature to be as high as 800oC such that
high order parameters of FePt $\text{L1}_0$ phase have been obtained. For the
fabricated FePt/h-BN thin film, excellent granular microstructure with FePt
grains of 6.5nm in diameter and 11.5nm in height has been achieved along with
good magnetic hysteresis properties.
