Fabrication and enhanced magnetoresistance of SiO_2-coated Fe_3O_4 nanosphere compact

摘要

Magnetoresistance (MR) of a nanostructured material, monodisperse Fe_3O_4 nanospheres of about 200 nm coated with thin SiO_2 and compactly cold pressed and sintered, has been investigated. A high MR, up to 10.8% at 1 T and 17% at 8 T, has been observed at 100 K. This enhanced MR is likely arising from the spin-polarized tunneling of conducting electrons through the insulating SiO_2 boundaries. The decrease of the MR with the temperature increasing was attributed to the existence of the local spins in the grain boundaries. In addition, the MR ratio decreases with the SiO_2 thickness increasing and sintering temperature decreasing. It suggests that the magnetic property of the insulating barrier is crucial for enhancing the low-field MR at a high temperature.