Synthesis of Fe<SUB>3</SUB>O<SUB>4</SUB> Nanowire by Oxidation

Abstract

Technology of elongated magnetically hard nanoparticles is far less developed than that of spherical magnetic nanoparticles. This article reports on new fabrication method and properties of magnetically hard magnetite nanowires in anodized alumina membrane (AAM). First, Fe nanowires with 60 nm diameter and 100 nm interpore distances have been synthesized by electrodeposition using AAM template. A two-step annealing process was used to transform Fe nanowires into Fe3O4 nanowires. Fe nanowires were annealed at 500 � C for 2 hours in pure oxygen. This oxidation led to the formation of nanowires with mixed composition of Fe3O4 and Fe2O3. A subsequent 2 hour annealing in pure flowing hydrogen at the temperature of 350 � C reduced the oxygen content and transformed the nanowires into Fe3O4. TEM images show that the polycrystalline nanowires are compact and uniform. Field cooled (FC) and zero-field cooled temperature dependent magnetization curves for Fe3O4 nanowires indicate the Verwey transition at 130 K. Hysteresis loop measurements revealed strongly anisotropic magnetization characteristics. Room temperature coercivity of Fe3O4 nanowires was 732 Oe for the magnetic field parallel to nanowire axis and 310 Oe for the perpendicular field; the saturation fields were 5274 Oe and 8792 Oe, respectively.