  For many spin systems with constant isotropic antiferromagnetic
next-neighbour Heisenberg coupling the minimal energies E_{min}(S) form a
rotational band, i.e. depend approximately quadratically on the total spin
quantum number S, a property which is also known as Lande interval rule.
However, we find that for certain coupling topologies, including recently
synthesised icosidodecahedral structures this rule is violated for high total
spins. Instead the minimal energies are a linear function of total spin. This
anomaly results in a corresponding jump of the magnetisation curve which
otherwise would be a regular staircase.
