Cotunneling current affected by spin-polarized wire molecules in networked gold nanoparticles

Abstract

As a bottom-up approach toward spintronics, a network structure of gold nanoparticles connected with spin-polarized wire molecules has been studied. A spinless network is prepared as a reference system. The network of gold nanoparticles with an average diameter of 4 nm form granules (average diameter of 100 nm), which in turn, connect themselves with each other to bridge $2\text{ }\ensuremath{\mu}\text{m}$-gap gold electrodes. Since the charging energy of a 4-nm gold nanoparticle amounts to 160 meV, it works as a Coulomb island and the conduction through the network is dominated by Coulomb blockade effect at room temperature. Thermal-activation-type conduction is found in a temperature range of 300 K--30 K, below which cotunneling is suggested to dominate. Important findings reported here are as follows: (1) The cotunneling occurs at elevated temperatures as high as $T=30\text{ }\text{K}$ due to the small size of gold nanoparticles: Nonlinear characteristics featured by $I\text{\ensuremath{-}}{V}^{3}$ are found, suggesting that the number of tunnel junctions relevant to the cotunneling is two. (2) The cotunneling current is substantially smaller in spin-polarized network than in spinless network, suggesting that spin-flip scattering caused by localized spins on wire molecules suppresses cotunneling process: The interpretation is supported by negative magnetoresistance observed in spin-polarized networks.