Europium Underneath Graphene on Ir(111): Intercalation Mechanism, Magnetism, and Band Structure

TL;DR

This study investigates how europium intercalates beneath graphene on Ir(111), revealing different magnetic behaviors and electronic structure modifications depending on the Eu superstructure formed.

Contribution

It provides a comprehensive analysis combining experiments and theory on Eu intercalation, elucidating the intercalation mechanisms, electronic structure shifts, and magnetic properties of two distinct superstructures.

Findings

Eu forms (2x2) and (√3×√3)R30° superstructures with graphene.

Electronic structures are similar, with n-doping and reduced hybridization.

Magnetic behavior differs: paramagnetic for (2x2), ferromagnetic tendencies for (√3×√3).

Abstract

The intercalation of Eu underneath Gr on Ir(111) is comprehensively investigated by microscopic, magnetic, and spectroscopic measurements, as well as by density functional theory. Depending on the coverage, the intercalated Eu atoms form either a $(2 \times 2)$ or a $(\sqrt{3} \times \sqrt{3})$R$30^{\circ}$ superstructure with respect to Gr. We investigate the mechanisms of Eu penetration through a nominally closed Gr sheet and measure the electronic structures and magnetic properties of the two intercalation systems. Their electronic structures are rather similar. Compared to Gr on Ir(111), the Gr bands in both systems are essentially rigidly shifted to larger binding energies resulting in n-doping. The hybridization of the Ir surface state $S_1$ with Gr states is lifted, and the moire superperiodic potential is strongly reduced. In contrast, the magnetic behavior of the two intercalation systems differs substantially as found by X-ray magnetic circular dichroism. The $(2 \times 2)$ Eu structure displays plain paramagnetic behavior, whereas for the $(\sqrt{3} \times \sqrt{3})$R$30^{\circ}$ structure the large zero-field susceptibility indicates ferromagnetic coupling, despite the absence of hysteresis at 10 K. For the latter structure, a considerable easy-plane magnetic anisotropy is observed and interpreted as shape anisotropy.