Surface Magnetism and Proximity Effects in Hexaboride Thin Films*

The unexpected ferromagnetism in alkaline hexaborides has attracted extensive research efforts to understand the exceptionally high Curie temperature sustained by low magnetic moments density. Here, we report the coexistence of ferromagnetic and superparamagnetic orders with 2D-type magnetic anisotropy in CaB₆ and SrB₆ thin films. Boron deficiency and closely correlated local oxidization were found at the film surface and its interface with the oxide substrate. These results suggest that the magnetism in hexaboride films is likely generated by two thin surface/interface layers with high concentration of boron vacancies which are energetically stabled by oxygen cation. Additionally, clear suppression of the superconductivity proximity effect in SmB₆ film was observed when SmB₆ was grown on top of the magnetic CaB₆. Such results demonstrate the interesting perspective of engineering multifunctional heterostructures by coupling the surface magnetic layer with the diversity of other hexaboride materials.