376
11 Templated Systems
The constraint imposed on the nanowire dimension has such a fundamental impact
on the growth characteristics of the nanowires that even trends established for electrodeposited films become invalid. For instance, the deposition of single-crystalline
and polycrystalline Pb nanowires was successful at large and small overpotential,
respectively [76], even though the trend for open geometry is just the opposite. Similar
experience was also published for Co–Ni alloys [88], while the trend for plain films
was found to be valid for Ni nanowires [28].
Besides the growth modes, the defect structure in the nanowires also shows a
great variety. In spite of the geometrical constraints, twin boundaries perpendicular,
angled or parallel to the nanowire axis can occur [168].
Concerning the grain size of alloys, the general rule is that the larger the unit cell
size (and hence, the larger the degree of long-range ordering), the smaller the grain
size. Hence, alloy nanowires are seldom single crystalline.
Non-metallic nanowires also show a good crystallinity and a strongly preferred
orientation that can be adjusted by tuning the deposition conditions. The exceptions are ZnO for which the high crystallinity is coupled with the lack of preferred
orientation [114], and MnO 2 that can be obtained in an amorphous state [120].
Magnetization and coercivity. The magnetic studies of electrodeposited nanowires
make one of the largest portions of the field. This is why magnetic nanowires in
highly ordered templates were considered to be a prime candidate as perpendicular
recording media due to the inherent large shape anisotropy of the nanowires with
their axis perpendicular to the sample surface. For a magnetic recording media, a high
remanence and a moderately large coercivity are required so that the bit element is
sufficiently resistant to magnetization reversal and provides large enough stray field
for the read-out of the bit information.
As for the magnetization properties, the material of the nanoporous template has
no substantial impact; rather, the magnetic properties depend on the nanowire only
[59]. The decrease in diameter of homogeneous nanowires tends to increase the coercive field, the remanence and the squareness of the magnetization curve [53, 57, 58,
66], and the increase of the aspect ratio has a similar effect [64]. This is because the
shape anisotropy of the homogeneous nanowires is the dominant factor that determines the magnetization reversal properties. When the separation distance between
the neighbouring nanowires is decreased by applying a pore widening process in PAA
membranes, the coercivity is diminished, indicating the onset of the magnetostatic
interaction between the neighbouring nanowires [65]. A comparison of the magnetization behaviour of single-crystalline and polycrystalline nanowires is shown in
Fig. 11.6.
The optimization process of the deposition of Co and Ni nanowires often refers to
the achievement of the production of single-crystalline nanowires. The explanation is
that the lack of crystal boundaries that serve as domain wall pinning centres ensures
the synchronized magnetization reversal of the entire nanowire with no possibility
of counterdomain formation within the same wire. The single-crystalline nature of
magnetic nanowires, together with the coincidence of the magnetocrystalline easy
axis with the nanowire axis, is the key for both the high squareness of the magnetization curve and the high coercivity. These two parameters can often be achieved
11 Templated Systems
The constraint imposed on the nanowire dimension has such a fundamental impact
on the growth characteristics of the nanowires that even trends established for electrodeposited films become invalid. For instance, the deposition of single-crystalline
and polycrystalline Pb nanowires was successful at large and small overpotential,
respectively [76], even though the trend for open geometry is just the opposite. Similar
experience was also published for Co–Ni alloys [88], while the trend for plain films
was found to be valid for Ni nanowires [28].
Besides the growth modes, the defect structure in the nanowires also shows a
great variety. In spite of the geometrical constraints, twin boundaries perpendicular,
angled or parallel to the nanowire axis can occur [168].
Concerning the grain size of alloys, the general rule is that the larger the unit cell
size (and hence, the larger the degree of long-range ordering), the smaller the grain
size. Hence, alloy nanowires are seldom single crystalline.
Non-metallic nanowires also show a good crystallinity and a strongly preferred
orientation that can be adjusted by tuning the deposition conditions. The exceptions are ZnO for which the high crystallinity is coupled with the lack of preferred
orientation [114], and MnO 2 that can be obtained in an amorphous state [120].
Magnetization and coercivity. The magnetic studies of electrodeposited nanowires
make one of the largest portions of the field. This is why magnetic nanowires in
highly ordered templates were considered to be a prime candidate as perpendicular
recording media due to the inherent large shape anisotropy of the nanowires with
their axis perpendicular to the sample surface. For a magnetic recording media, a high
remanence and a moderately large coercivity are required so that the bit element is
sufficiently resistant to magnetization reversal and provides large enough stray field
for the read-out of the bit information.
As for the magnetization properties, the material of the nanoporous template has
no substantial impact; rather, the magnetic properties depend on the nanowire only
[59]. The decrease in diameter of homogeneous nanowires tends to increase the coercive field, the remanence and the squareness of the magnetization curve [53, 57, 58,
66], and the increase of the aspect ratio has a similar effect [64]. This is because the
shape anisotropy of the homogeneous nanowires is the dominant factor that determines the magnetization reversal properties. When the separation distance between
the neighbouring nanowires is decreased by applying a pore widening process in PAA
membranes, the coercivity is diminished, indicating the onset of the magnetostatic
interaction between the neighbouring nanowires [65]. A comparison of the magnetization behaviour of single-crystalline and polycrystalline nanowires is shown in
Fig. 11.6.
The optimization process of the deposition of Co and Ni nanowires often refers to
the achievement of the production of single-crystalline nanowires. The explanation is
that the lack of crystal boundaries that serve as domain wall pinning centres ensures
the synchronized magnetization reversal of the entire nanowire with no possibility
of counterdomain formation within the same wire. The single-crystalline nature of
magnetic nanowires, together with the coincidence of the magnetocrystalline easy
axis with the nanowire axis, is the key for both the high squareness of the magnetization curve and the high coercivity. These two parameters can often be achieved