  Heat transfer in nanocomposite materials has attracted great interest for
various applications. Multilayer structures provide an important platform to
study interfacial thermal transport and to engineer materials with ultralow
thermal conductivity. Here we report on the fabrication and thermal
characterization of [h-BN/$L1_0$-FePt]xN multilayers, where hexagonal boron
nitride (h-BN) nanosheets (2.5 nm thick) and $L1_0$-FePt layers (6.5 nm thick)
alternate periodically. Differential three-omega($3\omega$) measurements reveal
an ultralow effective thermal conductivity of $ 0.60 \pm 0.05 W \cdot
m^{-1}K^{-1}$ across the multilayer films, and a low thermal boundary
conductance (TBC) of $ 67.9 \pm 6.6 MW \cdot m^{-2}K^{-1}$ for the
[FePt/h-BN(2.5nm)/FePt] interface at room temperature. We attribute the
ultralow thermal conductivity to the weak van der Waals bonding at h-BN/FePt
interfaces, which dominates the thermal resistance of the multilayer structure.
These findings provide insights into the thermal transport in 2D-material/metal
multilayer nanostructures and suggest the [h-BN/FePt] superlattice as a
promising material for nanoscale thermal barrier coating. Furthermore, the
obtained TBC lays the foundation for analyzing heat transfer in FePt-(h-BN)
nanogranular films, a promising magnetic recording media which can potentially
provide high thermal gradient for heat-assisted magnetic recording (HAMR). This
work advances the understanding of thermal transport in 2D-material/metal
nanocomposites and demonstrates interface engineering as an effective approach
to achieve materials with ultralow thermal conductivity.
