Single electron charging effects were observed for gold nanoclusters grown on octanedithiol self-assembled monolayers by scanning tunneling microscopy and X-ray photoelectron spectroscopy (XPS). Strong interaction of gold with the terminal sulfur atoms of dithiol molecules on Au(111) suppresses effectively the penetration of deposited gold atoms through the dithiol layer and results in the formation of uniform metal nanoclusters. Decoupling of the clusters from Au(111) by the octanedithiol layer and the small self-capacitance of the nanoclusters realize the observation of the Coulomb blockade in scanning tunneling spectroscopy and the Au 4f core level shifts in XPS at room temperature. Both phenomena originate from a common physics, the Coulomb energy of charged particles.

PACS 71.55. Jv, 72.25.Mk, 75.10.Nr, 75.60.EjThe magneto-resistance and magnetization have been studied for magnetite (Fe 3 O 4 ) sinter made from low size-dispersion hematite (α-Fe 2 O 3 ) nanoparticles (LSDHN's) with high temperature calcination. Two kinds of LSDHN's were grown by the hydrothermal synthesis. The average particle sizes of them are 30 nm and 60 nm. The magnetite sinter was produced by calcining the LSDHN's at 800 ºC for 5 hours in the atmosphere of Ar(90%)/H 2 (10%) mixed gases. We have observed an abrupt change of the electrical resistivity by one order of magnitude in the vicinity of the Verwey temperature (123 K) of a bulk single crystal. From the X-ray diffraction experiment, we have found that the magnetite sinter includes crystalline region. The magnetite sinter is considered to be composed of relatively narrow grain-boundary regions of amorphous-like magnetite and large grain regions of crystalline magnetite. It is regarded that the grainboundary-conduction is dominant below the Verwey temperature and the inter-grain-conduction is dominant above the Verwey temperature. We have observed the positive differential magnetoresistance (PD-MR) in low temperature regions and the negative differential magnetoresistance (ND-MR) in high temperature regions. The ND-MR is an ordinary phenomenon for magnetite, but the PD-MR is a peculiar one. In addition, an unusual localized-spin-freezing phenomenon has been also observed. Below the Verwey temperature, the magnetization difference between zero-field-cooling and field-cooling has been observed. The magnetization difference shows a sudden change in the vicinity of the Verwey temperature. 1 Introduction Magnetite (Fe 2 O 3 ) recently attracts considerable attention as a candidate of the material for spintronics devices (spin-valve, spin filter, spin transistor etc.) due to the half-metallicity, despite the oldest known magnetic material. Half-metallic materials are characterized by the presence of an energy gap in the majority spin band and no gap in minority spin band at the Fermi level. Therefore it is considered that they have complete spin-polarized conduction electrons. If we employ the assembly of magnetite nano-particles, we expect large electronic spin-dependent transport (SDT), because this system has many domains with random spin-orientation and the spin-orientation can be ordered by the applied magnetic field. In this case, the large negative differential magneto-resistance (ND-MR) is expected to be observed. The large ND-MR leads to the possibility of the application to a high sensitive magnetosensor like a magnetic head of the hard disk drive. Decreasing the diameter of a magnetite particle, it has been reported that a single magnetic domain is formed in the particle below ~100nm, and it shows the super-paramagnetic behavior below ~20 nm. The ND-MR for the assembly of magnetite particles has been reported for powder compacts [1] and poly-crystalline thin films [2].

PACS 71.55. Jv, 72.25.Mk, 75.10.Nr, 75.60.EjThe spin-dependent-transport (SDT) has been studied for magnetite (Fe 3 O 4 ) nano-particle sinter (MNPS) made from low size-dispersion hematite (α-Fe 2 O 3 ) nanoparticles (LSDHN's) with low temperature calcination. Two kinds of LSDHN's are grown by the hydrothermal synthesis. The average sizes of them are 30 nm and 60 nm. The MNPS is produced by calcining the LSDHN's at 500 ºC for 5 hours in the atmosphere of Ar(90%)/H 2 (10%) mixed gases. As compared with a bulk single crystal, the considerable intensification of negative-differential-magnetoresistance (ND-MR) has been observed for the MNPS. We have not observed abrupt change of the electrical resistivity in the vicinity of the temperature of the Verwey transition (which is the metal-insulator transition) appeared for a bulk single crystal. The ND-MR for 30 nm shows larger values than that of 60 nm on the temperature dependence. From the X-ray diffraction experiment, the MNPS is found to include crystalline magnetite regions. We consider that the MNPS is composed of large amorphous-like grain-boundaries and small crystalline grains. The electrical current is inferred to flow in grain-boundary regions. In grain-boundary regions, since the localized spins are relatively random distributed, the spin-polarized conduction electrons show the SDT. Below the Verwey temperature, we have observed the magnetization difference between zero-field cooling (ZFC) and fieldcooling (FC). This phenomenon indicates that the localized spins in the amorphous-like grain-boundaries are frozen in some degree. Above the Verwey temperature, the magnetoresistance is well fitted by the square of the Langevin function. We consider that the localized spins in the amorphous-like grainboundaries do not form perfectly random configuration and are somewhat ordered in a short range region.

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