  In this work, we report on the growth of hexagonal boron nitride (hBN)
crystals from an iron flux at atmospheric pressure and high temperature and
demonstrate that (i) the entire sheet of hBN crystals can be detached from the
metal in a single step using hydrochloric acid and that (ii) these hBN crystals
allow the fabrication of high carrier mobility graphene devices. By combining
spatially-resolved confocal Raman spectroscopy and electrical transport
measurements, we confirm the excellent quality of these crystals for
high-performance hBN-graphene-based van der Waals heterostructures. The full
width at half maximum of the graphene Raman 2D peak is as low as 16 cm$^{-1}$,
and the room temperature charge carrier mobilitiy is around 80000 cm$^2$/(Vs)
at a carrier density 1$\times$10$^{12}$cm$^{-12}$. This is fully comparable
with devices of similar dimensions fabricated using crystalline hBN synthesized
by the high pressure and high temperature method. Finally, we show that for
exfoliated high-quality hBN flakes with a thickness between 20 nm and 40 nm the
line width of the hBN Raman peak, in contrast to the graphene 2D line width, is
not useful for benchmarking hBN in high mobility graphene devices.
