Controlled synthesis and enhanced tunnelling magnetoresistance in oriented Fe3O4 nanorod assemblies

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 beta-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 Mossbauer 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.