Synthesis And The Magnetism Study Of One-dimensional Nanomaterials

In recent decades, the magnetic properties of one dimensional nano-materials has attracted more attentions and interests due to its potential applications in magnetic memory device,spin electronic device, and so on. At the same time, one dimensional nano-materials make the important sense in scientific research due to its easy manipulation and easy theoretical modeling. Furthermore, with the rapid development of fabrication techniques, various preparation methods have been explored for one dimensional nano-structure, such as self-assembling, electron beam lithography, ion beam etching, etc.In this thesis, for two magnetic one-dimensional nano-structure including Ni80Fe20 constricted nano-wire and Fe3O4 nano-particle chains, the magnetic transport properties are investigated systematically by experimental and theoretical simulation methods. The main contents of the research are as follows:1、Focusing on the new topic of spintronics of current induced domain wall motion and ballistic magnetorsistance effect, selecting permalloy constricted nano-wire device, we studied the spin distribution and domain wall formation in the constricted nano contact device.(1) Well-defined constricted permalloy nanowire device is fabricated successfully by electron beam lithography (EBL) and double exposure techniques. Different width of nano point contact are achieved by designing size and exposure values for avoiding neighboring effect.(2) It is found that the formation of magnetic domain wall refer to the width of point contact structure. The simple and narrow magnetic domain wall are observed in the point contact width of 80 nm and change to the single domain structure when the width is 260nm.(3) By micromagnetic simulation, the spin distribution and domain wall formation at the center of Ni constricted nano-wire devices to finding the geometrical sizes for trapping the magnetic domain wall, which provides the instruction for future experiment realization.2、By applying a induced magnetic field of various strength during the synthesis of Fe3O4 particles and the Fe3O4 nanometer spherical chain structure are fabricated self-assembly. The various length-diameter ratio are effectively adjusted by changing the intensity of synthesizing magnetic field and the addition of glycol in the reaction solvent. Under an applied magnetic field, the chains depositing on a silicon substrate could form a well-ordered structure for further microstructure and magnetic investigations. Some interesting results of structure and magnetic properties of Fe3O4 particle chains of various length-diameter ratio have been obtained.(1) According to the results of XRD and Raman, the prepared sample are comfirmed to be a pure single spinel phase. By the measurements of hysteresis loops, the average length-diameter ratio of the chains increases with increasing synthesizing magnetic field, which causes the increase of uniaxial shape magnetic shape anisotropy. The results of XMCD spectra indicates that the distribution of Fe3+ and Fe2+ changes with the different external magnetic fields.(2) Single Fe3O4 particle chain device is prepared successfully and its transport property has been studied. It is found that the resistivity dramatically reduced to 0.96±10-4Ωm from 1.29±10-4Ω m at 210μA, which could be illustrated as the presence of domain wall between particlesin the particle chain, and current driving the motion of domain wall from spin transfer torque effect. The presence of magnetic domain wall is comfirmed by measurement of XPEEM and the micromagnetic simulations.(3) The XPEEM results reveal that the spin direction of particles in the particle chains are alternated arrangement. The domain wall could be existed between two neighboring particles with opposite spin direction and the site with damain wall are random. For the two columns of paratactic chains, it is interesting to find that the spin orientation in the closed particles in two columns are in the opposite direction by magnetic interaction between chains.(4) The magnetization distribution and the formation of magnetic domain wall in single/double one-dimensional Fe3O4 particle chains with 200nm of diameter nano-particles in initial and remanent state have been simulated by using the micromagnetic simulation, the probabilities for trapping domain wall have been counted, which confirmed that the domain wall could be trapped between the particles in particle chain. The simulation results are fitted well with our experiment results.