Magnetic Properties of Electrodeposited NixFe1-x Nanowires Investigated by X-ray Magnetic Spectroscopy
|關鍵字:||陽極氧化鋁;奈米線;磁性材料;鎳鐵合金;AAO;nanowires;magnetic materials;NiFe alloys|
Nickel-iron(Ni-Fe) alloys have been extensively used in magnetic-shielding and spin-valve systems, and their nanostructures have become a subject of intense of research in recent days. In this thesis, Ni-Fe alloys were fabricated into nano-wire structures and the wires’ coupled magnetic, electronic and structural degrees of freedoms which determine their macroscopic magnetic behaviors, were investigated. Pulse-electrodeposition method combined with an anodic aluminum oxide template were used to fabricate highly aligned NixFe1-x nanowires (x= 0、0.3、0.5、1). To improve nanowires’ magnetic properties, rapid thermal annealing (RTA) was applied, and the annealed samples were subjected to the comparison with the as-plated ones in terms of all analyses. The samples’ crystallographic structures and magnetic properties were identified using an x-ray diffraction (XRD) facility and a vibrating sample magento-meter (VSM), respectively. To further understand the wires’ magnetism with elemental specificity, x-ray absorption spectrum (XAS) and x-ray magnetic circular dichrosim (XMCD) were employed to probe the electronic state and spin-polarization in the vicinities of Fermi-levels of Ni and Fe, at BL11A, National Synchrotron Radiation Research Center (NSRRC). By operating XAS/XMCD over the Ni and Fe L2 and L3 absorption edges and followed by a careful sum-rule estimation, we were able to explore the wires’ composition-dependent behaviors, from the viewpoints of micro-magnetism. From VSM results, we found that the magnetization was reduced upon the increase of Ni. This suggests that the role of Ni was to magnetically soften the wires. Such magnetically softening was accompanied by a structural transition starting from a BCC, then a mixture of BCC and FCC, and finally to a FCC structure, with the increase of Ni. For the annealing effect, despite the total magnetizations of the wires (x = 0.3 and 0.5) remaining almost unaltered before and after the RTA, a charge transfer from Fe to Ni was observed upon the heat treatment, as probed by XAS/XMCD, and the transferring effect of x = 0.3 was more pronounced than that of x = 0.5. For x = 0.3, this has resulted in a more oxidized state for Fe, but a more reduced state for Ni, leading to a decrease and increase in Fe and Ni local moments, respectively. The compensation between the Ni and Fe local moments that were invisible to a macroscopic approach therefore explained the nearly invariable magnetization upon the RTA, which emphasized the importance of the microscopic approach adopted in this work.