We have investigated a Coulomb-blockade effect and a higher order cotunneling effect in rubrene-Co nanocomposite spin devices, where the Co nanoparticles were uniformly embedded into the rubrene matrix and a large magnetoresistance ͑MR͒ effect appeared. A clear Coulomb gap was observed between Ϯ1.1 V at low temperature. Within the gap, the enhancement of the MR ratio was observed up to ϳ50%, which has not been explained by previous theoretical studies. The enhancement is induced by higher-order cotunneling. Power-law dependence of an electric current for a bias voltage ͑I ϳ V 2N−1 ; N is an order of cotunneling͒ was observed in the Coulomb gap, which corroborates that at maximum fifth-order cotunneling is attributed to the enhancement of the MR ratio.

We have fabricated a series of copper-phthalocyanine–cobalt (CuPc–Co) nanocomposites (compositional ratio of CuPc:Co=4:1), wherein Co nanoparticles were embedded within a CuPc matrix, in order to study the spin transport properties and to investigate spin–spin interactions between conducting spins and localized spins in the CuPc. Nonlinear characteristics of electric current, I–V (2N-1), (V: bias voltage, N: order of co-tunneling) were observed, where the fifth order co-tunneling was clarified. The order of the co-tunneling effect in CuPc–Co was almost equal to that in a rubrene–Co nanocomposite with almost the same diameter of the Co nanoparticles. We have not found clear evidence of spin-flip processes, as observed in other molecular systems, which may be attributed to the spin-coupling between localized spins in the CuPc and spins in Co.