Controlled synthesis and enhanced tunnelling magnetoresistance in oriented Fe3O4 nanorod assemblies

Abstract

This paper focuses on the effect of shape anisotropy on the magnetic and tunnelling magnetoresistance (TMR) properties of oleylamine-coated magnetite (Fe3O4) nanorod assemblies. Fe3O4 nanorods with a wide variation in length (35–180 nm) and diameter (5.5–24 nm) are obtained by a facile two-step process where β-FeOOH nanorods are first prepared hydrothermally, followed by a transformation to the Fe3O4 phase in an oleylamine medium via a solvo-thermal reaction. An observation of the Verwey transition at 120 K and analyses of Mössbauer spectra indicate that the as-synthesized Fe3O4 nanorods are highly stoichiometric. In an assembly of nanorods, a surface-functionalizing oleylamine layer acts as the insulating dielectric layer to form multiple tunnel junctions between the semi-metallic nanorods, and intergrain tunnelling takes place. A 14% TMR is recorded in the nanorod assemblies at room temperature, which interestingly increases by a factor of 1.4 when the nanorods are pre-aligned under an external magnetic field. For aligned nanorods, the magnetic moments stay in parallel to each other and result in higher spin-polarized current. The observed TMR value increases with a decrease in temperature, and attains a maximum value of 31% at the Verwey transition temperature. Spin polarization of the nanorod assemblies is estimated to be 46% at room temperature.