Magnetic Multilayer Nanowires:Fabrication,Characterization At The Nanoscale And Magnetic Property

The multilayer nanowires play an important role in the research field of fundamental physics and nano-device applications due to their unique structures. Transmission electron microscopes (TEMs) have been widely used for morphological, structural and chemical characterizations of various materials at the micro/nano scale, including magnetic multilayer nanowires. The understandings of the morphology, structure and chemistry of individual multilayer nanowires are the initial point for not only exploring their electric and magnetic properties, but also applying into nanoscale electronics and sensors.In this thesis, individual NiCu/Cu, NiFe/Cu and NiCo/Cu multilayer nanowires have been successfully fabricated by a pulse electrodeposition technique into the channels of nanoporous anodic aluminum oxide (AAO) templates. Their morphologies were observed by bright field (BF), high-resolution TEM and scanning transmission electron microscope modes. Their structures were investigated by a selected-area electron diffraction (SAED) and convergent beam electron diffraction (CBED) at the micro and nano scale. Their chemical compositions, distributions were comprehensively studied by EDX spectra, line-scan and elemental mappings. Their magnetic properties were measured by a vibrating sample magnetometer (VSM).The results show that individual multilayer nanowires have a continuous morphology, smooth surface, and polycrystalline structure. The average diameters are between40nm and50nm, the same as those of AAO channels. On each multilayer nanowire, the magnetic layers are rod-or disk-like structures. It is found that the lengths of non-magnetic layers are between5and15nm, decided by the periods of the electrodeposition signals. For the NiCu/Cu multilayer nanowires whose individual non-magnetic layers are less than10nm in length, there are interlayers between NiCu magnetic layers and Cu non-magnetic layers, and the average thicknesses of these interlayers are measured to be about1.5nm.In the case of NiFe/Cu, the bundles of multilayer nanowires show that these nanowires nearly keep the embedded feature in the AAO channels where they were grown, and it is observed that neighbouring nanowires have nearly the same periodicity and layer sizes. The composition was measured to be (Ni60Fe40/Cu)n, and the aspect ratio of individual magnetic layers was1.For the NiCo/Cu multilayer nanowires, the length of Cu non-magnetic layers is approximate20nm, and the aspect ratio of magnetic layer was3. EDX measurements reveal that The composition was (Ni80Co20/Cu)n. The EDX elemental mappings prove that nickle element has a uniform distribution in the magnetic layers and cobalt element increase sharply at one end of the magnetic layer. The magnetic measurements indicate that the magnetic properties of multilayer nanowires could mainly be influenced by shape anisotropy, magneto-crystalline anisotropy, thicknesses of the non-magnetic layers, dipole-dipole interactions from adjacent magnetic layers and magnetostatic coupling between the neighboring nanowires.