Effect of Localized Spin Concentration on Giant Magnetoresistance in Molecular Conductor TPP[FexCo1−x(Pc)(CN)2]2

We studied the effect of the localized spin concentration on the giant magnetoresistance (GMR) and the charge order (CO) in the phthalocyanine (Pc) molecular conductors TPP[FexCo1-x(Pc)(CN)(2)](2) (TPP = tetraphenylphosphonium), in which the localized spin concentration can be controlled by adjusting the Fe concentration x. Resistivity studies indicated that the CO state was stabilized by localized spins. The negative magnetoresistance was observed even in the region of a localized spin concentration of as low as about 5%. In the high-concentration region, the negative magnetoresistance changed into a positive one at low temperatures. This result shows that a magnetic field stabilizes the CO state, unless the magnetic field is sufficiently strong to break the antiferromagnetic order of the localized spins.