  The ability to design and control the spin and charge order of low
dimensional materials on the molecular scale offers an intriguing pathway
towards the miniaturization of spintronic technology towards the nanometer
scale. In this work, we focus on the adsorption induced modifications of the
magnetic and electronic properties of a low dimensional ferromagnetic surface
alloy after the adsorption of the prototypical organic molecule
perylene-3,4,9,10-tetracarboxylic dianhydride (PTCDA). For this metal-organic
interface, we observe the formation of a localized $\sigma$-like bond between
the functional molecular groups and the surface alloy atoms. This strong
chemical bonding coincides with a lifting of the characteristic surface alloy
band structure and a reduction of the magnitude of the local magnetic moments
of the Dy atoms by 18%. We attribute both findings to a mixing of
spin-degenerate molecular states with spin-split states of the Dy-Ag surface
alloy via the sigma-like bonds between PTCDA and the Dy surface alloy atoms.
Our findings clearly demonstrate the potential of tailored molecule-surface
sigma-bonds to control not only the electronic but also the magnetic order of
low dimensional materials.
