Formation and protection of an Eu-Ir surface compound below hexagonal boron nitride

Europium (Eu) intercalation below hexagonal boron nitride (hBN) on an Ir(111) substrate at various Eu coverages is investigated. The structural and electronic properties were examined using low energy electron diffraction (LEED), scanning tunnelling microscopy (STM), x-ray photoelectron spectroscopy (XPS) and angle-resolved photoemission spectroscopy (ARPES). Depending on the deposition temperature, different superstructures, (5 $\times$ $M$), (5 $\times$ 2), and ($ \sqrt{3}$ $\times$ $\sqrt{3})R30^{\circ}$ with respect to the Ir substrate were identified by LEED. The (5 $\times$ $M$) superstructure ($M$ $>$ 2), at 0.1 monolayer (ML), preserved the hBN/Ir Moir{é} pattern and exhibited a unidirectional ordering of Eu atoms. At higher coverage of 0.26 ML, a (5 $\times$ 2) superstructure emerged, where excess Eu atoms diffused into the bulk and were analyzed as Eu in a tri-valent state. At the highest preparation temperature with a one-third ML Eu, the formation of a ($\sqrt{3}$ $\times$ $\sqrt{3})R30^{\circ}$ superstructure indicates the presence of a EuIr$_{2}$ surface alloy beneath the hBN layer, with di-valent Eu atoms suggesting potential ferromagnetic properties. Air exposure was used to evaluate the protection of the hBN layer, and the results indicate that the EuIr$_{2}$ surface alloy was partially protected. However, the hBN layer remained intact by intercalation and air exposure, as confirmed by ARPES analysis.