Reversible Eu2+↔Eu3+ transitions at Eu-Si interfaces

Valence switching at Eu‐Si interfaces is demonstrated by resonant photoemission during repeated oxidation‐reduction cycles performed by room‐temperature O₂ exposure and mild heating. The ${\mathrm{Eu}}^{\text{2+}}\leftrightarrow {\mathrm{Eu}}^{\text{3+}}$ transitions are accompanied by Fermi level switching associated with changes in the stoichiometry of the surface heterostructure. The ability to cycle between two well‐defined magnetic states at a surface may be attractive in technological applications.