Spin-dependent tunneling and Coulomb blockade in ferromagnetic nanoparticles

In this paper we review studies on spin-dependent transport in systems contain-ing ferromagnetic nanoparticles. In a tunnel junction with a nanometer-scale-island, the charging effect leads to an electric current blockade phenomenon inwhich a single electron charge plays a significant role in electron transport, re-sulting in single-electron tunneling (SET) properties such as Coulomb blockadeand Coulomb staircase. In a tunnel junction with a ferromagnetic nano-islandand electrode, it was expected that the interplay of spin-dependent tunneling(SDT) and single-electron tunneling (SET), i.e., spin-dependent single-electrontunneling (SD-SET), would give rise to remarkable tunnel magnetoresistance(TMR) phenomena. We investigated magnetotransport properties in both se-quential tunneling and cotunneling regimes of SET and found the enhancementand oscillation of TMR. The self-assembled ferromagnetic nanoparticles we haveemployed in this study consisted of a Co-Al-O granular film with cobalt nanopar-ticles embedded in an Al-O insulating matrix. A Co36Al22O42 film prepared bya reactive sputtering method produced a TMR ratio reaching 10 % and su-perparamagnetic behavior at room temperature. The TMR ratio exhibited ananomalous increase at low temperatures but no indication of change with biasvoltage. In Ch. 4, we show that the anomalous increase of the MR provided ev-idence for higher-order tunneling (cotunneling) between large granules throughintervening small granules. We emphasize that the existence of higher-ordertunneling is a natural consequence of the granular structure, since broad distri-bution of granule size is an intrinsic property of granular systems. In Ch. 5, weconcentrate on SD-SET properties in sequential tunneling regimes. We fabri-cated two types of device structures with Co-Al-O film using focused ion-beammilling or electron-beam lithography techniques. One had a granular nanobridgestructure: point-shaped electrodes separated by a very narrow lateral gap filledwith the Co-Al-O granular film. The other had a current-perpendicular-to-plane(CPP) geometry structure: a thin Co-Al-O granular film sandwiched by ferro-magnetic electrodes with the current flowing in the direction perpendicular to