Synthesis and magnetic properties of single-crystalline magnetite nanowires

By carefully controlling the reaction conditions, nanowires of Fe3O4 are directly acquired from nanowires of alpha-Fe2O3 in a reduced atmosphere at 410-430 degrees C. X-ray diffraction, Raman spectrum, and transmission electron microscopic analyses demonstrate that the product is single-crystalline Fe3O4. The nanowires have diameters of 40-90 nm and lengths of 10-20 mu m, which are close to those of the pristine alpha-Fe2O3 nanowires. By studying different growth conditions, we find that hydrogen can push the conversion of the crystal structures, while temperature determines the chemical composition of the final products. The magnetic properties of as-prepared Fe3O4 nanowires are measured using a quantum design magnetic property measurement system. The nanowires show a ferrimagnetic behavior at room temperature and their magnetic properties are strongly influenced by surface and interface effects. The Verwey transition temperature (T-V = 116 K) is found to be a little lower than that of bulk materials, which can be attributed to the small deviation from stoichiometry caused by the oxygen vacancies near the surfaces. Below 12 K, the nanowires show a spin-glass-like behavior owing to the disordered frozen magnetic state at the surfaces. (c) 2007 Elsevier B.V. All rights reserved.