# Growth and Magnetic Properties of Polycrystalline Self-Assembled Bifurcated Co Nanowires

Authors: Jesse Silverberg; Adam L. Friedman; Latika Menon
Journal: Journal of Nanomaterials
 (2008)

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## Abstract

We use anodization of aluminum foil with variable applied anodization voltage to create an alumina template with bifurcated porous structures.  The template is then used to electrodeposit Co, fabricating unique bifurcated Co nanowires.  In order to better understand the crystal structure of our new material, we then report magnetic properties of these self-assembled bifurcated Co nanowires.  Magnetic measurements of the bifurcated wires are studied as functions of branch/stem ratios, wire length, and temperature. The results are compared with those of straight Co nanowires of similar dimensions and thin film Co samples to find that a different crystal lattice structure prevails in the stems than in the branches of the wires.


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## Body

## 1. Introduction

The fabrication
of magnetic nanowires and investigation of their magnetic properties have been the focus of much research [1–4]. Particularly, the combined process of
anodization of aluminum foil to produce nanoporous alumina templates and
electrodeposition of magnetic nanowires inside such templates has attracted
significant attention for its high yield, low cost, and rapid throughput of
ordered nanowires arrays [1–5]. Interest lies
in understanding the crystalline geometry and temperature-dependent properties
due to the possible applications in magnetic recording devices and interesting
fundamental physical phenomena [1, 6–9]. However, because these wires are grown using
self-assembled techniques, their crystal structure can be somewhat questionable
[10, 11]. As-deposited ferromagnetic
wires tend to exhibit multiple magnetic domains due to graininess. And, as the wires get smaller in diameter, single domain particles are extant, manifesting in superparamagnetic
effects. Moreover, magnetocrystalline
anisotropies, due to interactions between different crystal structures in the
wire, can greatly affect the measured magnetic behavior.In this work, we show that we have
created a new type of nanostructure, bifurcated Co nanowires, using porous
alumina templates and electrodeposition. We prepared these nanowires in a unique bifurcated geometry, where the
wires consist of a single stem bifurcating into two branches with smaller
diameters which subsequently join together to form a single stem at the other
end. We report our results on the
crystal structure as probed by the magnetic properties of the Co
nanowires. We have investigated the
effects of wire length, sectional proportions, and temperature dependence. The
results are compared with straight (unbranched) nanowires and also thin film
specimens.


## 2. Fabrication

In order to
fabricate the alumina templates, a 99.997% pure Al foil of 0.125 mm thickness is
first degreased in ethanol and then loaded into an electrolytic cell. The Al foil acts as the anode, and at a
distance of 11 cm, a Pt mesh acts as the cathode. The cell is filled with a
solution of 15% sulfuric acid (v/v) and held under potentiostatic (i.e.,
controlled voltage) conditions. The
mechanics of anodic porous alumina is adequately discussed in [12, 13] and will
not be repeated here. For the straight nanowires, we first prepared a
nanoporous alumina template consisting of an array of vertically arranged
cylindrical pores. For this, a constant voltage of 10 V is applied across the
electrodes. Under these conditions, nanoporous alumina forms at the surface
with pore diameters of around 10 nm. Subsequently, Co nanowires are prepared
inside the pores by means of electrodeposition. Electrodeposition is carried
out under AC conditions at 20 V and 250 Hz with a cobalt solution containing 10 g
cobalt heptahydride and 2.5 g of boric acid dissolved in 250 mL of water and
with a Pt mesh acting as a counter electrode. In order to prepare the bifurcated nanowires, we started with a template
consisting of the desired branched pore structure. For the magnetic studies
reported here, during the anodization process, the applied voltage was
initially set at 10 V and then reduced to 7.1 V and subsequently set back at 10 V.
This results in 10 nm diameter stems at the two wire ends with 7 nm diameter
branches in the midsection. However,
more generally, the selection of bifurcation voltages is based on a simple
relation between the stem voltage VS and the branch voltage VB given by (1)VB=VSn, where n is the desired number of branches [14]. Thus, for two branches, the anodizing
voltage for the branches turns out to be 7.1 V when the anodizing voltage for
the stem is 10 V. Anodization time for each step was adjusted depending on the
desired lengths of the individual wire sections.A typical image of straight and branched pore templates is shown in Figures1 and
2. A 3/4 top-down view of a straight pore template (anodized at 40 V) obtained using SEM is shown in Figure 1
while Figure 2 shows a bifurcated template made by anodizing under 40 V/28 V/40 V conditions. Figure 3 shows a schematic and an SEM micrograph of a typical bifurcated nanowire prepared in a bifurcated template. We used larger samples for
visualization than we did for the magnetic measurements due to the extreme
difficulty in viewing the smaller size wires with SEM. However, here we are only interested in the
magnetic properties of the smaller nanowires, which, due to their small size,
provide a perfect platform for studying the effect of low magnetic domain
materials, as discussed below. For
comparison of magnetic properties of the wire samples with thin films, we
prepared Co films deposited on commercial Al foil by thermal evaporation of Co
(99.5% pure from Alfa Aesar). Using this method, we
created a polycrystalline film whose thickness was approximately 2 μm which is
comparable to the total length of straight and bifurcated nanowires.Figure 1
SEM image of straight alumina template. Anodization conditions yield 40 nm pore diameters and approximately 1μm pore lengths.Figure 2
SEM image of bifurcated alumina
template. Anodization conditions yield 40 nm branch diameters and 20 nm stem diameters. Note the region of bifurcation where the larger pores form Y-shaped
junctions.(a) Schematic representation of a
bifurcated nanowire with taxonomy (b) micrograph of a Co bifurcated nanowire
with insets on the regions of bifurcation and convergence.
(a)(b)Magnetic measurements were
subsequently taken with a Princeton Measurements Corporation AGM, for room
temperature measurements (~300 K) and a
Quantum Designs Corporation SQUID for variable temperature measurements (4–350 K).


## 3. Results and Discussion

First, we
discuss our results on the magnetization versus field curves for the nanowires
in both perpendicular and parallel orientations of the nanowires. For clarity,
we define the perpendicular alignment to be one in which the field is applied perpendicular to the
surface of the template, or parallel to the cylindrical axis of the
nanowires. The parallel alignment is
defined as one in which the field is applied parallel to the surface of the
film or perpendicular to the cylindrical axis of the nanowires.Typical
hysteresis loops are shown in Figure4 for a typical bifurcated and straight Co nanowire-array sample with similar total wire length. In the case of the
straight nanowires, the coercivity in the perpendicular direction is around
1000 Oe. In the parallel direction, the coercivity is negligible and is only
around 151 Oe. This behavior is typical
of magnetic nanowires where the magnetic properties are largely determined by
shape anisotropy effects when compared to magnetocrystalline anisotropy. Also
because of the very large aspect ratio of the nanowires (>100), shape
anisotropy effects clearly dominate magnetocrystalline anisotropy effects. For
the bifurcated nanowires, the coercivity in the perpendicular orientation is
only about 500 Oe (much lower than that for the straight nanowires). This
reduced coercivity may be related to the reduced aspect ratio of the nanowires.
The bifurcated nanowires can be thought of as consisting of four segments (two
at the midsection and one each at the two ends) with reduced aspect ratio
(~30). Here, the magnetic properties are a result of competing shape anisotropy
and magnetocrystalline anisotropy. In consistence with this explanation, it may
be noted that the coercivity in the parallel direction is reasonable, ~150 Oe,
as in the case of the straight wires. For comparison, Figure 4 also shows the
magnetization versus field curves for polycrystalline thin film samples. Here,
the coercivity in both the perpendicular and parallel direction is about 250 Oe.
In thin films, the magnetization behavior is mostly dominated by
magnetocrystalline anisotropy and multidomain effects. By comparing the thin
films to the bifurcated and straight nanowires, it is evident that the
magnetocrystalline anisotropy effects are only implicated in the reduced
coercivities of thin films and larger diameter nanowires. In straight nanowires, clearly shaped
anisotropy dominates. In the bifurcated wires, there appears to be a mixture of
shape anisotropy and multidomain effects caused by the presence of ample
defects and separate grains in the branched region of the wires (discussed
below).Hysteresis curves for perpendicular and
parallel orientations of straight, bifurcated, and thin-film samples.
(a)(b)(c)(d)(e)(f)We have studied the dependence of coercivity
as a function of total length of the nanowires and also as a function of branch/stem
length ratios of the nanowires. For this, we first prepared the bifurcated
templates with specific ratios of branch/stem and also specific total length of
the pores. The obtained templates were then completely filled with Co. The anodization times took place in ratio of
1:2:1. For example, one sample was
anodized for 2 minutes at 10 V, 4 minutes at 7.1 V, and 2 minutes at 10 V at
10/7.1/10 V (a total of 8 minutes). Each
sample underwent electrodeposition until signs of overfilling were present. We will not discuss the process of
electrodeposition here, as it is adequately discussed elsewhere in the
literature [15]. However, electrodeposition was halted before significant
portions of the template were made to contain wires linked at the top.Figure5 shows the coercivity as a function of total anodization time. We see that in
the perpendicular alignment, the coercivities tend to increase with time and in
the parallel alignment, the coercivities tend to decrease with time. The
reduced coercivity for the wires with shorter anodization time is attributed to
presence of defects and possibly uncoupled superparamagnetic components in the
wires. Thermal fluctuations can reduce the energy barrier for magnetization
reversal which in turn reduces the coercivity. Interestingly, in Figure 6, the perpendicular
coercivity is found to decrease with increased branch/stem ratio. This is
consistent with the previous explanation. Presence of superparamagnetic components is expected to be comparatively higher in the branched sections of the nanowires due to the
reduced diameter. This can lead to reduced coercivity for the wires with
largest branch lengths.Coercivity as a function
of anodization time for 1:2:1 bifurcated nanowires.
(a)(b)The anodization ratio is calculated as anodization time for the branch/anodization time for the stem.
(a)(b)Finally, we have also studied the temperature dependence of the net magnetization of the
nanowires. For each type of nanowire and
alignment, samples were made as-deposited as well under two different annealing
conditions 200°C and 400°C in air for 3 hours. Figure 7 shows the magnetization versus temperature plots for straight and bifurcated nanowires in both alignments (perpendicular and parallel to
applied field of 5000 Oe and zero-field cooled (ZFC). Field-cooled (FC) measurements were also
taken (not shown here), however were found not to deviate significantly from
the ZFC data. Interestingly, we observed
no evidence of superparamagnetism when comparing the ZFC and FC plots, which is
typically revealed in a maximum corresponding to the blocking temperature. Such a peak is expected due to the previous
explanation of lowered coercivities being caused by increased superparamagnetic
interactions. However, we expect the
ferromagnetism of the wires to dominate, which could mask the appearance of a
blocking peaking. Moreover, the observed
slight upturn the ZFC plots near low temperature can be partially attributed to
superparamagnetic domains becoming ferromagnetic, thus contributing to the overall
ferromagnetic moment.Zero field cooled
magnetization versus temperature for straight and bifurcated nanowires in
parallel and perpendicular alignments. Samples were annealed at 200°C and 400°C for comparison. Applied field was 5 kOe.
(a)(b)(c)(d)Also, the magnetization versus temperature dependence does not demonstrate the
expected Bloch law behavior. In general,
the magnetic moment of alumina (as measured in blank alumina templates) is
several orders of magnitude lower than that of the ferromagnetic wires; however,
it does seem to contribute slightly to a paramagnetic upturn at lower
temperatures. Moreover, this non-Bloch
behavior could be due to magnetoelastic effects, as described by Kumar et al.
in [16]. Expansion and contraction of the
aluminum (upon which the template sits) due to the temperature change could be altering
the orientation of the wires in the field. The expansion and contraction of aluminum under temperature change, are
much greater than that of alumina. Thus, the expansion and contraction would cause a slight curling of the sample in the field, which would alter the orientation of the wires in the applied field. This could be the cause of the slight decrease in magnetic moment as the temperature decreases (see Figure 7).Annealing may help to reduce the graininess
of the deposited Co, forming structures that are more crystalline. For the straight nanowires, the magnetization
of the as-deposited sample is seen to be significantly larger at higher
temperatures. This is not the case for the bifurcated nanowires. This may be
attributed to multiphased nature of Co. Bulk Co is typically known to exist in hcp
form at room temperature while the fcc phase is known to exist only at very
high temperatures. However, in nanoscale forms of Co, particularly when
prepared by means of electrodeposition, Co has been reported to exist in both
forms hcp and fcc. It has been observed by Kröll et al. in [17, 18], through
XPS and EXAFS studies, that in small Co nanoparticles, there is a decrease in
hcp-Co in favor of fcc-Co. Hcp-Co is known to exhibit strong magnetocrystalline
anisotropy in contrast with fcc-Co. This in turn reduces magnetocrystalline
anisotropy in smaller Co nanowires—another
indication of the strong shape anisotropy in our nanowire samples outweighing
the effects of magnetocrystalline anisotropy. Interestingly annealing the
sample appears to reduce the magnetization of the nanowires, which may be
indicative of a phase-stabilization process achieved through the annealing
process. Also for the annealed samples, the magnetization versus temperature
exhibits a more typical Bloch law kind of behavior. This is also true for the
bifurcated nanowires (both as-deposited and annealed samples) where the
behavior is of the Bloch-law type. The smaller diameters of the midsection of
the wires possibly ensure a more single-phased fcc-type Co in such small
dimension wires.


## 4. Conclusions

In conclusion, we fabricated an
interesting new geometry of nanowire using bifurcated anodic porous alumina
templates. The pores were filled with Co
using electrodeposition, and the magnetostatic properties were investigated in
order to better understand the crystal structure of the wires. The magnetic properties indicate strong shape anisotropy effects which outweigh magnetocrystalline anisotropy. Also, magnetoelastic effects are visible in temperature dependent samples. Our results may also indicate the presence of Co in fcc-form rather than the usually observed hcp-form for bulk Co.



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