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 Greens 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.
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