Electrical- and magneto-resistance control for magnetite nanoparticle sinter by regulation of heat treatment temperature

Abstract

We investigated electrical- and magneto-resistance control in magnetite (Fe3O4) nanoparticle sinter (MNPS) by the regulation of heat treatment (HT) temperature. MNPS was produced from hematite (α-Fe2O3) nanoparticles (HNP’s) using a deoxidization reaction. The average size of HNP was 30nm, and HT was carried out between 400 and 800°C. X-ray diffraction, magnetization, electrical resistivity (ER), and magneto-resistivity (MR) measurements were performed at temperatures ranging from 5 to 300K. The ER and MR behaviors were considerably different at HT temperatures above and below ∼600°C. After HT below ∼600°C, ER followed the Mott-type variable-range-hopping conduction, and MR showed large values over a wide temperature range. After HT above ∼600°C, ER indicated a Verwey transition near 110K and MR showed small values, except in the vicinity of the Verwey transition temperature. Changing the HT temperature altered the coupling between adjacent magnetite nanoparticles (MNPs) and affected the crystallinity of MNPS. Below ∼600°C, ER and MR were dominated by grain-boundary conduction, while above ∼600°C they were determined by inter-grain conduction. The application of a magnetic field to the grain-boundary region, which had random localized spins, caused a large enhancement in MR.