  The characterization and manipulation of deposited magnetic clusters or
molecules on surfaces is a prerequisite for their future utilization. In recent
years techniques like spin-flip inelastic electron tunneling spectroscopy using
a scanning tunneling microscope proved to be very precise in determining e.g.
exchange constants in deposited finite spin chains in the meV range. In this
article we tackle the problem numerically by investigating the transition from
where a pure spin Hamiltonian is sufficient to the point where the interaction
with the surface significantly alters the magnetic properties. To this end we
study the static, i.e. equilibrium impurity magnetization of antiferromagnetic
chains for varying couplings to a conduction electron band of a metal
substrate. We show under which circumstances the screening of a part of the
system enables one to deduce molecular parameters of the remainder from level
crossings in an applied field.
