Giant magnetoresistance in phthalocyanine. Computer illustration showing giant magnetoresistance (GMR) through a single phthalocyanine (H2Pc) non-magnetic molecule (ball and stick), with an atomic cluster (spheres) representing the bottle neck of the H2Pc molecular junction with a cobalt ferromagnetic tip and substrate. Magnetoresistance is the property of a material to change the value of its electrical resistance when an external magnetic field is applied to it. GMR is a quantum mechanical magnetoresistance effect observed in thin-film structures, such as this, composed of alternating ferromagnetic and non-magnetic conductive layers.