Intensified spin‐dependent‐transport and localized‐spin freezing in magnetite sinter made from low size‐dispersion hematite nanoparticles with low temperature calcination

Abstract

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.