  One of the fundamental applications for monolayer-thick 2D materials is their
use as protective layers of metal surfaces and in-situ intercalated reactive
materials in ambient conditions. Here we investigate the structural,
electronic, and magnetic properties, as well as the chemical stability in air
of a very reactive metal, Europium, after intercalation between a hexagonal
boron nitride (hBN) layer and a Pt substrate. We demonstrate that Eu
intercalation leads to a hBN-covered ferromagnetic EuPt$_2$ surface alloy with
divalent Eu$^{2+}$ atoms at the interface. We expose the system to ambient
conditions and find a partial conservation of the di-valent signal and hence
the Eu-Pt interface. The use of a curved Pt substrate allows us to explore the
changes in the Eu valence state and the ambient pressure protection at
different substrate planes. The interfacial EuPt$_2$ surface alloy formation
remains the same, but the resistance of the protecting hBN layer to ambient
conditions is reduced, likely due to a rougher surface and a more discontinuous
hBN coating.
