  We theoretically study the transport properties of a supramolecular spin
valve, consisting of a carbon nanotube with two attached magnetic molecules,
weakly coupled to metallic contacts. The emphasis is put on analyzing the
change of the system's transport properties with the application of an external
magnetic field, which aligns the spins of the molecules. It is shown that
magnetoresistive properties of the considered molecular junction, which are
associated with changing the state of the molecules from superparamagnetic to
the ferromagnetic one, strongly depend on the applied bias voltage and the
position of the nanotube's orbital levels, which can be tuned by a gate
voltage. A strong dependence on the transport regime is also found in the case
of the spin polarization of the current flowing through the system. The
mechanisms leading to those effects are explained by invoking appropriate
molecular states responsible for transport. The analysis is done with aid of
the real-time diagrammatic technique up to the second order of expansion with
respect to tunneling processes.
