Periodic Chiral Magnetic Domains In Single-Crystal Nickel Nanowires

We report on experimental and computational investigations of the domain structure of single-crystal Ni nanowires (NWs). The similar to 200x200x8000 nm(3) Ni NWs were grown by a thermal chemical vapor deposition technique that results in single-crystal structures. Magneto resistance measurements of individual NWs suggest the average magnetization points largely off the NW long axis at zero field. X-ray photo emission electronmicroscopy images obtained at room temperature show a well-defined periodic magnetization pattern along the surface of the nanowires with a period of lambda(avg) = 239 +/- 37 nm. Finite element micromagnetic simulations reveal that when the material parameters of the modeled system match those of nickel crystal at T = 10 K, an oscillatory magnetization configuration with a period closely matching experimental observation (lambda = 222 nm) is obtainable at remanence. This magnetization configuration involves a periodic array of alternating chirality vortex domains distributed along the length of the NW. Vortex formation is attributable to the relatively high cubic anisotropy of the single crystal Ni NW system at T = 10 K and its reduced structural dimensions. The periodic alternating chirality vortex state is a topologically protected metastable state, analogous to an array of 360 degrees domain walls in a thin strip. Simulations show that other remanent states are also possible, depending on the field history. At room temperature (T = 273 K), simulations show vortices are no longer stable due to the expected reduced cubic anisotropy of the system, suggesting a disparity between the fabricated and modeled nanowires. Negative uniaxial anisotropy and magnetoelastic effects in the presence of compressive biaxial strain are shown to promote and restore formation of vortices at room temperature.