Single-electron transport and magnetic properties of Fe-SiO2 nanocomposites prepared by ion implantation

The electric transport, magnetic, and magnetotransport properties of Fe-SiO2 nanocomposites prepared by Fe-ion implantation into silica were investigated. The structural studies revealed bcc Fe nanoparticles of an average size of 3 nm dispersed in a 100-nm-thick nanocomposite layer formed within the silica substrate. Using special thin-film electrodes that were only 100 nm apart, in-plane electrical measurements were performed in a temperature range of 4-300 K. Though no external gate electrode was used, single-electron transport phenomena (Coulomb blockade and Coulomb staircase) were observed at 4 K. The presence of Coulomb steps in I-V curves implies that the electric transport was realized by the tunneling of electrons via a random quasi-one-dimensional chain of a few isolated iron nanoparticles. The magnetic properties of the nanoparticles were determined by surface effects and by the superparamagnetic behavior. The nanoparticles exhibited enhanced anisotropy and were dipolarly interacting. However, the tunneling current was found to be independent of external magnetic field; i.e., no tunneling magnetoresistivity (TMR) was measured at 77 K. At this temperature the nanoparticles were superparamagnetic. Presumably, a low volumetric concentration of Fe nanoparticles (< 14%) and a spin-flip process due to residual single Fe atoms present in the silica barriers were responsible for the absence of the TMR effect.