Formation of Europium-transition metal surface compounds and protection of Eu below hexagonal boron nitride (h-BN)

In this work, the formation and protection of two-dimensional (2D) ferromagnetic surface alloys are achieved by intercalating europium (Eu) beneath a protective monolayer of hexagonal boron nitride (h-BN) on various transition metal (TM) substrates. Low energy electron diffraction (LEED), scanning tunnelling microscopy (STM), x-ray photoelectron spectroscopy (XPS), angle-resolved photoemission spectroscopy (ARPES), and x-ray magnetic circular dichroism (XMCD) were employed to study structural, electronic, and magnetic properties across both flat and curved (vicinal) surfaces including c-Pt(111),  Rh(111)    and      c-Ni(111).

The structural characterisation of h-BN grown on flat and curved TM substrates revealed that substrate crystallographic orientation and lattice mismatch critically influence h-BN growth with insights supported by DFT calculations. h-BN growth was found to induce faceting in all studied systems. For strongly interacting systems, only a few stable facets were observed, such as in the h-BN/c-Ni(111) system, while weakly interacting systems exhibited more stable facets, exemplified by the h-BN/c-Pt(331) system. The latter was further explored through DFT to understand the mechanism of stable facet formation.

Furthermore, Eu intercalation below h-BN on Pt(111) and on Rh(111), led to the formation of ordered EuPt2 and EuRh2 surface alloys, respectively, which are protected by h-BN. Their formation confirmed by (√3 × √3)R30◦ superstructure observed in the LEED, predominantly divalent Eu signal in Eu 3d core level XPS spectrum, and ferromagnetic ordering with Curie temperature of 14.1 K for EuRh2 as demonstrated by XMCD. h-BN provided partial oxidation protection in all systems, especially in c-Pt(111) at (111) and, surprisingly, in c-Ni(111) away from (111).