Spin blockade in a charge-switchable molecular magnet

We consider the effect of adding electrons to a single molecule on its magnetic properties and the resulting transport fingerprints. We analyze a generic model for a metal-organic complex consisting of orbitals with different Coulomb repulsions. We find that by modulating the charge of the molecule by a single electron the total spin can be switched from zero to the maximal value supported by the added electrons, $S=3/2$. The Nagaoka mechanism is responsible for this charge-sensitivity of the molecular spin. It is shown that fingerprints of these maximal spin states, either as groundstates or low-lying excitations, can be experimentally observed in current-spectroscopy. as either spin blockade at low bias voltage or negative differential conductance and complete current suppression at finite bias.