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.
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