Diameter-controlled synthesis of polycrystalline nickel nanowires and their size dependent magnetic properties

Abstract

Polycrystalline nickel nanowires (NWs) with a length reaching tens of micrometers were prepared in large quantities by a facile and rapid microwave-assisted polyol method. Characterization by X-ray diffraction and electron microscopy reveal that the NWs are uniform with a narrow size distribution. Their diameters (D) are controllable in a wide range via the concentration of precursor solutions. The diameters of the NWs as well as those of the contained grains increase surprisingly linearly with the concentration. The NWs have rough surfaces at low concentrations but become smoother at high ones. The growth mechanisms leading to the grain and wire diameters and the surface morphology are discussed. The coercivity (H_C) decreases with increasing diameter whereas the saturation magnetization increases to approach the bulk value, indicating a strong size dependence of the magnetic properties. Owing to the anisotropic shape and collective grain interactions, these Ni NWs exhibit enhanced H_C. At 70 nm, H_C reaches as high as 382 Oe at 5 K, which is more than two orders of magnitude higher than that of bulk Ni (0.7 Oe) and exceeds even that of NWs synthesized with strong magnetic fields. Temperature dependent magnetization shows a wide separation between the field-cooled and zero-field-cooled M(T) curves due to the strong nanowire shape anisotropy.