(Color online) (a) SEM image of free standing cobalt iron oxide nanotubes, demonstrating the applicability of the cobalt iron oxide ALD reaction to structures of high aspect ratio. The inset is the magnified image of the nanotubes. (b) EDX results showing the atomic percentage of the elements Fe and Co at different regions along the tube length. The ratio between Fe and Co remains almost constant along the tube length, whereas the total amount of Fe and Co decreases significantly from the top to the bottom of the pores.
(Color online) (a) TEM image of an isolated cobalt iron oxide nanotube with d w ≈21 nm showing a homogeneous thickness along the tube length. Inset: magnified part of the nanotube. The nanotube has a granular structure. (b) SAED pattern taken from the nanotube, which exhibits polycrystalline rings and can be indexed to the spinel phase.
TEM dark field images taken from the (311) plane of nanotubes with various thicknesses: (a) 6 nm; (b) 9 nm; (c) 12 nm; (d) 15 nm; and (e) 21 nm. The images evidence that the grain size distribution and its mean value increase with the wall thickness of the nanotube.
(Color online) Magnetization isotherms of a sample with d w = 21 nm obtained with magnetic field parallel and perpendicular to the sample at 300 K and at 5 K. At each temperature, the measurements carried out in both configurations have minor difference in remanence and no differences in their coercivity, suggesting that shape anisotropy is negligible.
(Color online) Graph showing relative remanence and coercive field of cobalt iron oxide nanotube arrays with various tube wall thicknesses obtained at 5 K and at 300 K. Both, remanence and coercive field, remain almost constant with wall thickness at 5 K, whereas both values increase with wall thickness at 300 K.
(Color online) (a) Field-cooled and zero-field-cooled curves of the 6-nm sample obtained under various magnetic fields. The shape of the curves shows that the nanotubes have a superparamagnetic behaviour with blocking temperature well below 300 K. (b) Graph showing the dependence of the blocking temperature (maxima of ZFC curve) on the applied field for the same sample. The blocking temperature first increases at small applied fields and then decreases linearly at larger fields.
(Color online) Field-cooled and zero-field-cooled curves of 6-, 9-, and 12-nm samples obtained under (a) 10 mT and (b) 500 mT. The curves obtained at 10 mT indicate that the blocking temperatures (maxima of ZFC curve) of the 9- and 12-nm samples are higher than 350 K. The curves obtained at 500 mT shows that the blocking temperature increases with wall thickness.
Grain size distribution and its mean value for nanotubes with various wall thicknesses. Both the distribution and mean grain size increase with wall thickness. The distributions were obtained by processing the corresponding TEM images with Image J.
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