Self-Organization And Magnetotransport Properties Of Periodically Structured Cobalt Filament Array

Ultrathin layer electrochemical deposition is a novel approach for fabricating metallic filament arrays with periodical microstructures.In this thesis,a variety of submicron cobalt filament arrays with two-dimensional or three-dimensional periodical structures were fabricated by tuning the thickness of the thin layer.The crystal structure of the electrochemical deposits is analyzed by transmission electron microscope(TEM).Magnetotransport properties and self-organization of magnetic domains of the deposits show anomalous behavior owing to their novel morphology.The main parts of this thesis are included as follows:(1)For the first time we demonstrate the remarkable contribution of the melt of ice in front of the growth front on the formation of a unique helical cobalt micro structure.Magnetoresistance(MR)measurement indicates an anomalous magnetotransport behavior introduced by the novel helical morphology.Large-area helical cobalt microwire arrays with periodically rotated stairs along them are electrodeposited on substrates without using templates or additives.By introducing a relatively high growth temperature,these wires are grown in a relatively thick electrolyte layer at the interface between ice and solution.When growing at a temperature between-1 to-1.7?,the cobalt wires may twist during the growth,forming helical wires with periodical micro-stairs along them.Magnetoresistance(MR)measurement indicates two symmetrical anomalous extreme values of longitudinal MR and two abrupt decreases of transverse MR during the magnetization process.It can be ascribed to the contribution of the helical symmetry of the cobalt wires.Additionally,we would like to point out that this method of fabricating helical metallic microwires has already been used in the formation of copper helix.Based on a number of experimental observations,we arise that the helical growth phenomenon can be ascribed to the synergistic effect between localized melt of ice in front of the growth front and anisotropic nucleation of metallic deposits on solid substrates.(2)Large-area zigzag cobalt wire arrays are fabricated by ultrathin layer electrochemical deposition.Spatial period of the zigzag wires can be adjusted from 500nm to 1.3?m by changing the driving voltage.TEM analyze indicates that the zigzag morphology can be ascribed to the periodical rotation of crystal orientation during the growth of cobalt wires.In addition,the novel shape anisotropy of zigzag wires affects the self-organization of magnetic domains strongly.MFM observation indicates that the magnetic domains can be arranged regularly by magnetizing the wires.Compared with the previous experimental conditions,a lower growth temperature(-2?)and lower pH value of the solution(pH=3)are introduced in the deposition of zigzag cobalt wires.These zigzag cobalt wire arrays are electrodeposited through a thinner liquid layer.The higher the driving voltage is,the smaller the spatial period will be.Selected area electron diffraction(SAED)result of the bending area of the zigzag wire is found to be two series of six symmetrical diffraction arcs,the angle between which is 30 degree.While SAED result of unbent area shows only one series of six symmetrical diffraction arcs.These results suggest the periodical bend originates from the periodical rotation of crystal orientation.MFM Measurement shows perpendicular magnetic anisotropy nature of the wires.The magnetic domains of the wires can be arranged regularly by magnetizing the wires.Single domains can be observed in some single units of the zigzag wire after magnetization.