  Spectroscopic measurements with low-temperature scanning tunneling
microscopes have been used very successfully for studying not only individual
atomic or molecular spins on surfaces but also complexly designed coupled
systems. The symmetry breaking of the supporting surface induces magnetic
anisotropy which lead to characteristic fingerprints in the spectrum of the
differential conductance and can be well understood with simple model
Hamiltonians. Furthermore, correlated many-particle states can emerge due to
the interaction with itinerant electrons of the electrodes, making these
systems ideal prototypical quantum systems. In this manuscript more complex
bipartite and spin-chains will be discussed additionally. Their spectra enable
to determine precisely the nature of the interactions between the spins which
can lead to the formation of new quantum states which emerge by interatomic
entanglement.
