Characterization of the magnetic properties of multilayer magnetostrictive iron-gallium nanowires

We investigate the magnetic properties of magnetostrictive iron-gallium (Galfenol, Fe(100-x)Ga(x), 10 <= x <= 25 at. %) nanowires with magnetic force microscopy (MFM) and using micromagnetic modeling software (magpar). Wires with diameters of 150 nm were fabricated in alternating multilayered structures with Fe-Ga and Cu having aspect ratios of similar to 2 and similar to 0.5, respectively, with the goal of minimizing the relative contribution of shape anisotropy to magnetic domain alignment. Micromagnetic simulations of isolated Fe-Ga segments with these dimensions predict that (1) at remanence, two opposing vortex structures will form at the ends of a Fe-Ga segment, with a single domain wall in the middle of the segment and (2) traditional magnetic dipoles will form at the ends of the segment to align with a large (saturation) external magnetic field. MFM results are presented that support these models. At remanence, no contrast is observed in the MFM phase images. Magnetic poles become evident at the ends of the Fe-Ga segments when a magnetic field exceeding similar to 300 Oe is applied along the length of the nanowire. The direction of the pole alignment is readily flipped by changing the direction of applied field by 180 degrees. Additionally, MFM images show rotation of the magnetic poles in each Fe-Ga segment as they align with fields of similar to 550 Oe applied at angles of similar to 55 degrees and similar to 105 degrees relative to the length of the nanowire. The MFM results support the simulation results and demonstrate that an aspect ratio of similar to 2 will reduce shape anisotropy effects sufficiently in Fe-Ga nanowire that magnetization can lie off of the nanowire axis. (C) 2010 American Institute of Physics. [doi:10.1063/1.3359852]