Ferromagnetic hcp chromium in Cr/Ru(0001) superlattices

We report the first observation of a weak ferromagnetic state: of Cr in Cr/Ru(0001) superlattices, based on magnetic hysteresis and corroborated by x-ray magnetic circular dichroism at the Cr L-2,L-3 edges. In situ reflection high-energy electron diffraction, x-ray diffraction, and Cr K-edge polarized x-ray absorption investigations have shown that the Cr layers thinner than 8 Angstrom adopt a slightly distorted hcp structure, accompanied by a large atomic volume expansion of up to 14% compared to the bce packing volume. The expanded hcp structure clearly induces the observed ferromagnetism, in agreement with theory.