Off-Axis Electron Holography of Isolated Ferromagnetic Nanowires

The investigations carried out in this thesis involved the nanoscale characterization of isolated ferromagnetic nanowires (NWs) using off-axis electron holography (EH), high-resolution transmission electron microscopy, scanning transmission electron microscopy and energy dispersive spectroscopy. The research focused on two categories of NWs, single phase CoFeB NWs and multilayer CoFeB/Cu NWs, which were fabricated by pulsed-current electrodeposition in nanoporous alumina membranes as an array of NWs. EH has been used to investigate the local magnetic behavior of the isolated NWs in their remanent state. In addition, the uniformity in diameter, composition, crystal structure of individual NWs were investigated. Single phase CoFeB ferromagnetic NWs, with diameters ranging between 20 to 170 nm, were studied. Electron diffraction patterns indicated that the NWs were nanocrystalline, BCC CoFeB, with grain sizes up to 20 nm × 20 nm. Holograms from EH showed that the magnetization inside the NWs was uniform over most of their length, except at their edges. Since the NWs consisted of soft magnetic nanocrystals, the magnetic anisotropy was likely dominated by the shape anisotropy. Numerical simulations suggested that the stray field at the tips of the NWs was well reproduced by a truncated cone model, rather than a cylinder. The average magnetic induction was 1.4 ± 0.3 T. Multilayer NWs consisted of periodic magnetic layers of CoFeB alloys and non-magnetic layers of Cu. Individual NW compositions, crystallinity, and layer thicknesses were calibrated using scanning transmission electron microscopy and energy dispersive spectroscopy. These properties were found to be significantly different from their expected nominal values assumed for the arrays, based on single-phase growth rates. Diffraction patterns obtained from the NWs again showed that both the CoFeB and Cu layers were nanocrystalline (BCC CoFeB, FCC Cu) but that the CoFeB layers had a significant atomic fraction of Cu, despite the small concentration of Cu used in the electrolyte. Nevertheless, the average magnetic induction of individual CoFeB layers ranged between 0.5 and 1.5 T, depending upon the thickness of the layer, from 50 nm to 250 nm, and the direction of an external magnetic field applied in situ. The magnetization was axial for all external field directions when the CoFeB layer was thicker than the diameter (45 nm), while for thin CoFeB and Cu layers (< 10 nm), magnetic vortices were detected, associated with opposing magnetization in neighbouring layers. These observations provided important insight for the interpretation of previously reported effective-anisotropy fields of similar NW arrays.