Magnetic coupling of single Co adatoms to a Co underlayer through a Pd spacer of variable thickness

We have performed a combined experimental and theoretical investigation of Co atoms on top of a Pd spacer on Co/Ir(111). Using spin-polarized scanning tunneling microscopy and spectroscopy we first studied the morphology and the spin-resolved electronic properties of mono-and double-layer Pd on Co/Ir(111). We find pseudomorphic growth with two stackings of the Pd monolayer, which can easily be distinguished electronically by measuring the differential tunneling conductance. Spin contrast is achieved on both the Pd mono-and double-layer and on Co adatoms on top, which means that the electron density above the surfaces and adatoms is spin-polarized. Indeed, based on our ab initio calculations, we find that the surface Pd atoms of mono-and double-layer carry an induced magnetic moment of about 0.3 and 0.2 mu(B), respectively; Co layers adsorbed on top of a Pd mono-and double-layer possess a magnetic moment of about 2.0 mu(B)/atom. In accordance with our experimental observations of Co atoms on the Pd spacer, the calculations also show that Co adlayers are ferromagnetically coupled to the Co/Ir(111) underneath. The size of the magnetic exchange coupling is reduced by a factor of three from a mono-to double-layer Pd spacer between the Co layers.