  We explore theoretically the spin transport in nanostructures consisting of a
gold quantum dot bridging nonmagnetic electrodes and two Mn12-Ph single
molecule magnets (SMMs) that are thiol-bonded to the dot but are not in direct
contact with the electrodes. We find that reversal of the magnetic moment of
either SMM by the application of a magnetic field leads to a large change in
the resistance of the dot, i.e., a strong spin valve effect. We show that this
phenomenon arises from a novel physical principle: The spin-dependent molecular
orbitals that extend over the dot and both SMMs change drastically when the
magnetic moment of either SMM is reversed, resulting in a large change in the
conduction via those orbitals. The same physics may also be responsible for the
spin valve phenomena discovered recently in carbon nanotube quantum dots with
rare earth SMMs by Urdampilleta, Klyatskaya, Cleuziou, Ruben and Wernsdorfer
[Nature Mater. 10, 502506 (2011)]
