Large magneto-conductivity effect in Fe-Phthalocyanine conductor at low temperatures

Magnetoresistance and magnetic torque measurements of one-dimensional π-d organic conductor TPP[Fe(Pc)(CN)₂]₂ are performed in the temperature range of 30-1.5 K. The resistance shows semiconducting behavior in the whole temperature range, suggesting a charge ordered state. Quite large negative magnetoresistance is observed at low temperatures, which is associated with the change of the magnetic torque. These results show that the charge gap steeply decreases at about 14.5 T, where some magnetic transition of Fe 3d spins occurs. This is clear evidence that the π electron transport is strongly correlated with the local spins via the strong π-d interaction. Possible theoretical models, double-exchange interaction and π-d spin-singlet models are discussed.