  The electronic origin of a large resistance change in nanoscale junctions
incorporating spin crossover molecules is demonstrated theoretically by using a
combination of density functional theory and the non-equilibrium Green's
functions method for quantum transport. At the spin crossover phase transition
there is a drastic change in the electronic gap between the frontier molecular
orbitals. As a consequence, when the molecule is incorporated in a two terminal
device, the current increases by up to four orders of magnitude in response to
the spin change. This is equivalent to a magnetoresistance effect in excess of
3,000 %. Since the typical phase transition critical temperature for spin
crossover compounds can be extended to well above room temperature, spin
crossover molecules appear as the ideal candidate for implementing spin devices
at the molecular level.
