Electrochemistry Communications 36 (2013) 96–98

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Electrochemistry Communications

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Short communication

Fabrication of multisegmented magnetic wires with micron-lengthcopper spacers

Lorena M.A. Monzon 1,⁎, Katie O’Neill 1, Yash Sheth 1, M. Venkatesan 1, J.M.D. Coey 1

School of Physics, SNIAMS building, Trinity College Dublin, Dublin 2, Ireland

⁎ Corresponding author.E-mail address: aranzazl@tcd.ie (L.M.A. Monzon).

1 Tel.: +353 1 8962171.

1388-2481/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.elecom.2013.09.023

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 August 2013Received in revised form 20 September 2013Accepted 23 September 2013Available online 30 September 2013

Keywords:Multisegmented nanowiresShape anisotropyCopper segmentMagnetization

Segmented magnetic nanowires have a distinctive magnetic signature which can be exploited in magneticbiosensors. We report on the fabrication of magnetic segmented nanowires with copper spacers a few micronslong. The key to avoid dissolution of the magnetic segment by an electroless reaction with the Cu bath is toinclude an interlayer, which protects the magnetic segments from dissolution with concentrated Cu2+ electro-lytes. Ni alloyed with Co makes it magnetically soft and for a Co80Ni20 composition, the specific magnetizationis reduced by only 15%, compared to pure Co.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

In the last decade, the use of ferromagnetic nanowires for cell separa-tion and manipulation has been proposed as a newway to detect certaintypes of cells, through surface funtionalization of the wires with antibod-ies [1]. Currently, signal detection is based on the fluorescence propertiesof probes attached to the wire surface.

Diamagnetic segments are incorporated into ferromagnetic nano-wires to create magnetic barcodes [2–5]. Magnetic sensors coupled toa fluidic channel can be used as a signal detection system [6,7]. As thewires pass down the channel, alternating magnetic signals arise fromthe different segments. The dimensions of the fluidic channel limitflow rate, but the closer the wires pass by the sensor, the stronger thesignal. This determines the best wire dimensions, which is 5–20 μmlong, with the length of each segment, 1–3 μm.

So far, micron-length segments of gold have been employed as thespacer material [8,9]. The preparation consists in alternating the bathsduring electrodeposition to create a multi-layered structure, each afew microns thick, using a porous template. Copper has also beenused as a spacer but the synthesis of these wires and the dimensionsof the segments are quite different. Pulse deposition from a singlecobalt–copper bath with alternating potential gives rise to nanometerthick CoCu/Cu segments [3,4,10,11]. Themagnetic segments are alloyedwith Cu, which dilutes the magnetization of Co. Furthermore, since thedeposition of copper is performed at a high over-potential, the Cudeposits soon result in rough layers given that the deposition of copper

ghts reserved.

is diffusion limited and from a dilute Cu2+ bath. For this reason, thesegmented structure is limited to nanometer-thick layers. These wireswere developed for giant magnetoresistance (GMR) signals, whichbenefit from very thin magnetic layers and spacers.

For magnetic signal detection purposes, magnetic segments with ahigh saturation magnetization (Ms) and zero remanence ratio (MR/Ms)are desirable; the latter prevents agglomeration due to magnetic attrac-tion (MR is remanent magnetization). FeCo has the highest Ms value,although it is not a suitable material as it tends to oxidize, losing itsmagnetic properties. Co is a good candidate as it has a high Ms and itforms a passivating oxide layer of just a few nanometers thick, does Ni,although pure Ni has a much lower Ms (1.42 MA m−1 for Co vs0.43 MA m−1 for Ni).

Here, we have developed a method to fabricate micron-length seg-mented wires with Cu spacers, including magnetic segments withoutCu. For the magnetic segments we have chosen a CoNi alloy.

2. Materials and methods

The wires were electrodeposited in gold-coated alumina membranes(Whatman anodisc 25, 200 nmpore diameter). Magnetic segmentsweredeposited potentiostatically at −0.90 V vs Ag/AgCl, from the electrolytesolution: 0.1 M boric acid, 80 mM NaCl, 0.6 M CoSO4 + 0.4 M NiSO4;pH = 3.5. The interlayer is deposited via short pulses at −0.70 V and−0.60 V, during 5 s each. The interlayer solution has the same composi-tion as that of the Co:Ni solution plus 30 mMCuSO4. The copper segmentis then deposited from a 1 M CuSO4 solution, first at −0.4 V during 3 sand then at−0.2 V until the desired charge is accumulated. These pulseddepositions are performed to avoid dendritic growth within the wires.The cell was rinsed before changing from the interlayer solution to Cu

97L.M.A. Monzon et al. / Electrochemistry Communications 36 (2013) 96–98

bath and from this to CoNi solution. From CoNi to interlayer the cellwas not rinsed. For a 3–2 configuration (3 CoNi segments with 2 Cuspacers) the charge accumulated was 11 for the first CoNi segment, and6 C for the other two. For Cu segments, it was 4 C each. Given the poresof the template are still open after gold-coating; a thin CoNi film is depos-ited before the pores are filled. Before alumina removal, the film ismechanically polished. A 1 M NaOH is used to dissolve the template, inan ultrasonic bath for about 15 min. Morphology and composition wereexamined by scanning electronmicroscopy (SEM) and energy dispersiveX-ray analysis (EDX) was employed to identify the elemental content ofeach segment. Magnetization measurements were carried out in a 5 TQuantum Design SQUID magnetometer at room temperature.

3. Discussion

To electrodeposit multilayers of different metals, the regions of po-tential where the metals are stable have to be taken into account.Fig. 1a shows the cyclic voltammograms corresponding to the electrode-position of CoNi and Cu, on a gold substrate. It can be seen that the directelectrodeposition of Cu on a CoNi layer can only be performed atpotentials more negative than −0.5 V, given that lower overpotentialswould result in the electrodissolution of the magnetic layer. This is thereason why the Cu segments of the CoCu/Cu wires grown from a singleelectrochemical bath are deposited at −0.5 V or higher [3,4,10,11].More important is the fact that once a Cu2+ solution is put in contactwith the magnetic layer, an electroless reaction takes place, resulting ina fast dissolution of the CoNi alloy. This reaction is revealed by the opencircuit potential moving toward positive values. Therefore, an interlayeris deposited to minimize the electroless reaction. Its deposition is carriedout at two potential values:−0.7 V and−0.6 V respectively, for onlya few seconds each; and from a solution containing Co, Ni and Cu, withapproximately ten times more Co and Ni than Cu. As a result, thisCu-rich and/or pure Cu capping layer, is more stable at potential valuesless negative than −0.5 V. Hence, the deposition of the copper seg-ment can then be carried out at much lower potentials, where the de-position is in the mixed-controlled regime, leading to compact metallicCu segments.

Fig. 1b displays the current transients of the CoNi segments, violetones obtained during the growth of the first magnetic layer and greenones for the second and third CoNi segments. They all have plateauvalues ~2 mA cm−2 (normalized by the area of the membrane). Theonly difference is that during the first 400–600 s the current for thefirst segments is much higher than for the rest, in agreement with a

-1.0 -0.8 -0.6 -0.4 -0.2 0.0

-6

-4

-2

0

2

4 electrodissolution

i / m

A.c

m-2

E / V

CoNi Cu

metal deposition

a

Fig. 1. a. Cyclic voltammograms of the electrodeposition of CoNi (dark green) and Cu (red). Gotransients obtained when depositing CoNi layers: 1st (violet) and successive segments (green)view (bottom). (For interpretation of the references to color in this figure legend, the reader is

larger area corresponding to the formation of a film. Once the poresstart to be filled, both set of curves overlap. We have observed thathigher overpotentials (or current densities) lead to the formation ofmore porous wires of poor quality, which are prone to oxidation (seeinset Fig. 1b). This rich oxide wires develop a bulky hairy surface. It isimportant to avoid this oxidation as there would be a loss in the totalmagnetic mass.

Close examination of the surface of the wires reveals good adhe-sion between the different segments (Fig. 2, left). After an hour ofsonicating the wires in water, they remain intact with the segmentsconnected, revealing that the structure is robust structure. Given themetals have similar atomic weights, the SEM images do not differen-tiate the segments with different contrasts. Only their textures aredifferent, a fact we believe arises from the etching effect of the solu-tion used to remove the wires from the alumina membrane on theCu segments. Fig. 2, right, corresponds to an EDX elemental line pro-file of a bunch of wires clustered together, where a clear transitionfrom CoNi to Cu is observed. A 80:20 Co:Ni ratio was determined.From SEM images of individual wires, we have found that the lengthof the wires varies: For the conditions employed, they are between12 and 17 μm, with CoNi segments ranging from 2.3 to 3.5 μm, andCu segments from 2.6 to 3.3 μm. The deposition rate of the wires isdifferent when compared to one another. The longer the wire, thelonger are the segments, and the disparity in size increases as the wiresget longer (Fig. 2).

Fig. 3 displays the normalized magnetization loops of segmentedwires with CoNi in a 3–2 configuration, with the magnetic field appliedperpendicular and parallel to the wire axis. The coercivity values, μ0H0,of 20 and 13 mT and the slight anisotropymanifest in small differencesof both hysteresis loops suggest that the CoNi alloy has some contribu-tion from its hexagonal hcp phase, which appears at a Co content higherthan 70% [12]. The Co:Ni ratio in the alloy is determined by thecomposition of the bath, applied voltage and type of membrane used[13,14]. Pure Co electrodeposits tend to crystallize in the hcp phase, itsmost stable form in ambient conditions. Given its magnetocrystallineanisotropy, K1 = 500 kJ m−3 [15] which favors the orientation of themagnetic moment along the c-axis, wires made of pure Co tend tohave non-zero coercivity and remanence ratios ranging from 20% upto 90%, depending on the wire diameter and crystalline texture, whichis controlled by the pH of the electrolytic bath [16]. The incorporationof Ni in the alloy minimizes the anisotropy, stabilizing the cubic phase,reflected by the μ0H0 values. For a 20% of Ni in the alloy, the remanenceratio is less than 4% and the expected magnetization is 1.21 MA m−1,

0 200 400 600 800 1000 1200-5

-4

-3

-2

1st successive

t / s

CoNi segments

b

500 nm

~ 2 mA.cm-2

> 3 mA .cm-2

200 nm

ld substrate, scan rate 50 mVs−1. Solutions: see Materials and methods section. b. Current. Inset: SEM image of set of wires deposited at−0.90 V and −1.0 V: side view (top) topreferred to the web version of this article.)

0 5 10 15 200

25

50

75

100

125

175

com

posi

tion

/ ato

m %

length /µm

O Co Ni Cu

150

1 µm

Fig. 2. SEM images of the nanowires taken at 3 kV, to examine the true surface morphology (left) and EDX elemental linear profile of wires, 15 kV (right). The oxygen content does notcome exclusively from the wires as the interaction volume is much larger than the one of the sample. C and Al signals are not included.

98 L.M.A. Monzon et al. / Electrochemistry Communications 36 (2013) 96–98

only ~15% less than pure Co. This represents a good compromisebetween high specific magnetization and lack of agglomeration of thewires due to magnetic attraction.

4. Conclusions

A method to grow segmented magnetic wires with micron-lengthcopper as a spacer was presented. To avoid the dissolution of theelectrodeposited magnetic segments, an interlayer is deposited from adiluted copper solution, at high over-potentials and for a short periodof time, followed by deposition of Cu from a concentrated solution, atmuch lower overpotentials.

This ‘multipot’ synthesis is required when long Cu segments arerequired, as the high overpotential necessary to produce Cu segments

-2.0 -1.5 -1.0 -0.5 0.0 0.5 1.0 1.5 2.0

-1.0

-0.5

0.0

0.5

1.0

-0.05 0.00 0.05

-0.05

0.00

0.05

M /

Ms

B B

Ms

MR

µ0H (T)

Fig. 3. Room temperaturemagnetization loops of segmented wires in the template with a3–2 configuration. The inset shows a magnification of the hysteresis loop near zero field.Average length of the segments is about 3 μm in all cases.

with the ‘single pot’ approach would lead to rough Cu segment andthe Cu solution will be rapidly depleted. Also, Cu would be alloyedwith the magnetic segments, decreasing their Ms value, and as a conse-quence, the stray field of the magnetized barcodes.

Someof the advantages of choosing Cu over Au to fabricatemagneticbarcodes include the price of the salts (copper salt is 50 times cheaper),the growth rate of the segments which is ten times faster than for gold.

Overpotential values deeply affect themorphology of thewires, withonly 100 mV extra increasing the porosity of the wires and makingthem more prone to oxidation.

Softmagnetic segments containing only 20%Ni and 80%Co are prov-en to be very suitable for use in magnetic barcodes. Another possibilityto fabricate soft magnetic wires would be to drive the electrodepositionof Co towards its fcc phase by including additiveswith carboxylic groupsto the electrolyte [17,18].

Polycarbonate membranes are an alternative to aluminamembranes,since the dissolution does not require an alkaline solution.

Acknowledgments

This work was supported by Science Foundation Ireland, as part ofERC support program 13/ERC/I2561 and by the EU as part of theNAMDIATREAM FP7 project.

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