  The state of the art ab initio calculations reveal the effect of a scanning
tunnelling microscopy tip on magnetic properties and conductance of a
benzene-adatom sandwich on Cu(001). We concentrate on a benzene-Co system
interacting with a Cr tip. Our studies give a clear evidence that magnetism and
conductance in molecule-adatom junctions can be tailored by the STM tip.
Varying the tip-substrate distance the magnetic moment of the Co adatom can be
switched on/off. The interplay between spin-polarized electron transport
through the junction and its magnetic properties is demonstrated. A spin-filter
effect in the junction is predicted.
