  We consider electrical transport through molecules with Heisenberg-coupled
spins arranged in a ring structure in the presence of an easy-axis anisotropy.
The molecules are coupled to two metallic leads and a gate. In the charged
state of the ring, a Zener double-exchange mechanism links transport properties
to the underlying spin structure. This leads to a remarkable contact-site
dependence of the current, which for an antiferromagnetic coupling of the spins
can lead to a total suppression of the zero-bias conductance when the molecule
is contacted at adjacent sites.
