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RESEARCHARTICLE

Copyright 2012 American Scientic PublishersAll rights reservedPrinted in the United States of America

Journal ofSpintronics and Magnetic Nanomaterials

Vol. 1, 2327, 2012

AC Electrodeposition of Amorphous CoP NanowiresEmbedded in an Alumina Template

F. Nasirpouri1, S. M. Peighambari1, S. J. Bending2, E. V. Sukovatitsina3, and A. S. Samardak31Department of Materials Engineering, Sahand University of Technology, Tabriz 51335-1996, Iran

2Department of Physics, University of Bath, Bath BA2 7AY, UK3Laboratory of Thin Film Technologies, School of Natural Sciences, Far Eastern Federal University, Vladivostok 690950, Russia

In this paper we report on the fabrication of amorphous CoP alloy nanowires by means of alter-nating current (ac) electrodeposition in an highly ordered anodic aluminum oxide (AAO) template.An ac voltage with sinusoidal waveform at a xed frequency of 400 Hz was applied to elec-trodeposit nanowires from aqueous solutions with different phosphorous content at room temper-ature. Current transients demonstrate the general four-stage nucleation and growth behavior ofnanowires in nanoporous templates and scanning electron microscopy conrms the formation ofCoP nanowires in AAO. Results show that electrolyte phosphorous content inuences the growth,microstructure and magnetic properties of the nanowires. An increase of phosphorus contentreduces the growth rate of nanowires. The crystalline structure of cobalt nanowires electrodepositedin AAO changes signicantly to amorphous by the incorporation of phosphorous as an alloyingelement in the structure, as X-ray diffraction patterns show. Furthermore, the coercivity of the CoPnanowires decreases when the electrolyte phosphorous content increases.

Keywords: CoP, Nanowires, AAO, Electrodeposition, Amorphous.

1. INTRODUCTION

The controlled production of magnetic nanowire arrayswith outstanding characteristics is attracting much interestrecently owing to their applications in emerging technolo-gies related with magnetic information storage, high sensi-tivity GMR sensor devices, thermoelectric cooling systemand photonic crystals.12

Patterning materials using templating is a very cheapand efcient synthesis technique. Templating is not anestablished technique for magnetic media at the micro-scale, but interest in using this technique is growingrapidly due to the ease of fabrication of materials inthe nanoscale range in an efcient and cost effectiveway. There are several ways to ll the nanopores withmetals or other materials to form nanowires, but theelectrochemical deposition method is a general and versa-tile method which has been successfully used for creatingnanowires of magnetic, semiconductor, and superconduc-tor materials.35 Various templates have been used to elec-trodeposit nanowires, but the anodic aluminium oxide(AAO) template method has been applied widely due to its

Author to whom correspondence should be addressed.

self-oraganized nanopore structure, convenience and exi-bility of fabrication.67

There are two main factors determining the mag-netic properties of the nanowire arrays: (1) the magneticcharacter of individual nanowires, which is determinedby its magnetic anisotropy, such as magnetocrystallineanisotropy and shape anisotropy, and (2) the periodicityor the symmetry of nanowire arrays, which determinesthe strength of the magnetostatic interaction betweennanowires.8 If the microstructure is amorphous in nature,then the magnetocrystalline anisotropy can be neglected.Electrodeposition has been widely used for the syn-

thesis of amorphous alloys including binary transition-metalmetalloid glasses such as NiP, CoP, and FeP.The microstructure and magnetic properties of the mate-rials depends on the composition, and can be controlledby varying the electrodeposition conditions such as theelectrolyte and the current density.9 Arrays of amorphousNiP and CoP nanowires have been dc electrodepositedinto polymeric and AAO templates. The microstructureand magnetic properties of the nanowire arrays weredemonstrated to strongly depend upon the composition,i.e., content of phosphorous (%P) and could be controlledby varying the composition, pH and the temperature of theelectrodeposition bath. It was implied that the replacement

J. Spintron. Magn. Nanomater. 2012, Vol. 1, No. 1 2158-866X/2012/1/023/005 doi:10.1166/jsm.2012.1008 23

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RESEARCHARTICLE

AC Electrodeposition of Amorphous CoP Nanowires Embedded in an Alumina Template Nasirpouri et al.

of Ni with Co results in both a decrease of the coerciveeld and an increase of the Curie temperature.10 However,little research has been carried out on the ac electrodepo-sition of amorphous magnetic nanowires. More recently,CoP and FeCoP nanowires were synthesized in AAO by acelectrodeposition1112 showing the existence of a relation-ship between bath composition, i.e., sodium hypophosphiteconcentration and the P content of nanowires, which werefound to be amorphous in structure. The magnetic hystere-sis loops showed the easy magnetizing axis is parallel tothe nanowires, suggesting that the nanowire arrays havestrong shape anisotropy.In this paper, we aim to present our experimental results

on the effect of ac electrodeposition of CoP alloy nanowirearrays embedded in AAO templates and demonstrate theformation of an amorphous structure under different elec-trodeposition parameters.

2. EXPERIMENTAL DETAILS

High purity Al foils (99.999%) were used as a substrateto fabricate highly ordered AAO templates via a double-anodization process. The Al foil was degreased, etched innitric acid, and electropolished in a mixture of perchloricacid (60%) and ethanol (1:4 in volume) under 16 V below5 C for approximately 4 min. Anodization was conductedunder a constant cell potential in a 0.3 M oxalic acidelectrolyte. The temperature of the electrolyte was main-tained at 0 C (between 2 and +2 C) during anodizationusing a cooling system. The solution was stirred vigor-ously in order to accelerate the dispersion of the heat thatevolved from the samples. The rst and second anodiza-tion steps were conducted under the same conditions men-tioned above. Meanwhile, the oxide layer formed in therst step was removed by wet chemical dissolution in amixture of 0.2 M chromic acid and 0.4 M phosphoric acidat 60 C for an appropriate time depending on the anodiz-ing time. The thickness of the barrier layer of oxide lmwas reduced by decreasing the anodizing voltage at theend of the second step of anodization. The voltage waslowered at 2 V min1 down to 20 V and then 1 V min1 to13 V. In the next step the anodization voltage was reducedto 12 V in a rate of 0.5 V min1. Anodization was thencontinued for 8 min at this nal voltage.CoP nanowires were then ac electrodeposited into the

AAO template with stainless steel as a counter-electrodeat room temperature. Before electrodeposition, the AAOtemplate was sonicated for 10 min in the electrolyte withan ultrasonic probe to facilitate wetting of the nanopores.The electrolyte solution consisted of 0.1 M CoSO4

7H2O, 0.5 M boric acid and 0, 5, 15 and 25 g/litreNaH2PO2 at a pH value of about 4. The root mean square(RMS) voltage used in the ac electrodeposition was 12 and15 V at a frequency of 400 Hz with a sinusoidal waveform.Current transients were recorded during electrodeposition

using a computer controlled A/D data acquisition system.The deposition time was determined depending on the ll-ing time of the pores.Field emission scanning electron microscopy was used

to conrm the morphology of the samples. Energy disper-sive spectroscopy (EDX) (Oxford Instruments) was usedto determine the chemical composition of nanowires. Thecrystal structure of the Co nanowires arrays were exam-ined by X-ray diffraction. A Bruker D8 Advanced X-raydiffractometer was used, which utilizes a standard Cu tubesource run at a voltage of 40 kV and lament current of40 mA. Cu (K radiation of wavelength 0.1540496 nm isproduced by this system. All 2 scans were made fromincident beam angles of 35 to around 80 of the surfacewith detector increments of 0.05 degree every second. Themagnetic properties of nanowires were investigated usinghomemade vibrating sample and magneto-optical Kerreffect magnetometers. The possible maximum applied eldwas 2000 Oe.

3. RESULTS AND DISCUSSION

Electrodeposition of CoP nanowire arrays into highlyordered AAO template was controlled using the currenttransients. It is clear from the literature that nucleationand growth of nanowires takes place in nanoporous tem-plate through a four-stage process which is exploited bythe current-time curves including(1) nucleation of nanowires at the pore bottoms,(2) growth of wires within the pores,(3) pore llings and(4) overgrowth, as reported by Whitney et al.13

Figure 1(a) illustrates current transients recorded duringelectrodeposition of CoP nanowires under a sinusoidalwaveform with a frequency of 400 Hz from different solu-tions containing 0, 5, 15 and 25 g/litre sodium hypophos-phate. The four-stage nucleation and growth mechanism isclearly observed for the electrodeposition of nanowires inAAO. However, the transients show slight differences inregard to the position of different stages and also the depo-sition current and its trend during the deposition. Whenthe phosphorous does not incorporate in the reductionreaction, i.e., pure cobalt nanowires, the lling time islonger with a sharp increase exhibiting a uniform lling ofnanopores.5 However, looking at the current transients ofthe electrodeposition from the solutions containing sodiumhypophosphate reveals that stage (3) starts more quicklyas long as the variation has a shallow slope during stages(2) and (3). These effects become clearer when the con-centration of the phosphorous in the solution is higher.Another feature is that the deposition current decreaseswith increasing phosphorous content of the electrolyte.This shows that the nanopores are lled uniformly acrossthe overall template area. A probable explanation for theincreasing current during stages (2) and (3) is that poor

24 J. Spintron. Magn. Nanomater. 1, 2327, 2012

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RESEARCHARTICLE

Nasirpouri et al. AC Electrodeposition of Amorphous CoP Nanowires Embedded in an Alumina Template

0 300 600 900 1200 1500

15

20

25

30

0 100 200 300 400 500

0

10

20

30

40

(a)I (m

A/cm

2 )

time (s)

5 g/litre15 g/litre25 g/litre0 g/litre

(b)

time (s)

I (mA/

cm2 )

15 Vrms12 Vrms

Fig. 1. Current transients recorded during electrodeposition of CoPnanowires in AAO with an applied sinusoidal waveform with a frequencyof 400 Hz (a) from different solutions containing 0, 5, 15 and 25 g/litresodium hypophosphate at 15 Vrms and (b) from a solution containing5 g/litre sodium hypophosphate at 12 and 15 Vrms.

wetting leads to a delay in nucleation in some pores sothat the number of pores in which growth takes place andtherefore also the current, rise gradually.14 The increasein current during stage (2) and its duration are signi-cantly greater for the polycarbonate membranes than forthe polyester ones, consistent with poorer wetting in theformer case. According to Fokkink et al.14 poor pore-wetting also leads to a smearing out of the transition tobulk growth and it is noticeable that stage (3) is muchlonger for the polycarbonate than for the polyester mem-branes.The effect of deposition potential on the growth of CoP

nanowires has also been studied. Figure 1(b) shows thecurrent transients recorded during electrodeposition at 12and 15 Vrms with a sinusoidal frequency of 400 Hz. Thesole difference is the reduction of growth rate as the depo-sition potential decreases. This is evident from the cross-sectional SEM image of the CoP nanowires grown in theAAO template. Figure 2 shows SEM images and a typi-cal EDX spectrum of CoP nanowires electrodeposited at12 Vrms. The SEM image reveals that the pores are incom-pletely lled with CoP nanowires after electrodepositionunder 12 Vrms for 500 s.The crystalline structure of CoP nanowires electrode-

posited at 15 Vrms with a frequency of 400 Hz was stud-ied using XRD. X-ray diffraction patterns obtained from

Fig. 2. (a) SEM image and (b) a typical EDX spectrum of CoPnanowires ac electrodeposited at 12 Vrms and 400 Hz for 500 s in AAO.

CoP nanowires electrodeposited in AAO from solutionscontaining 0, 5, 15 and 25 g/litre sodium hypophosphateare shown in Figure 3. For Co nanowires, it is observedthat the microstructure is HCP with (001), (200) and (110)Bragg diffraction lines. The electrodeposited nanowirestend to lose crystallinity when phosphorous is added to thecomposition, since the Bragg diffraction peaks of (002)and (001) disappear. However, the phosphorous contentof the electrodeposition bath directly inuences the crys-talline structure as the higher the P content in the bath, thelower the XRD peak intensity. This is shown in Figure 4for the two Bragg diffraction peaks.The magnetic properties of CoP nanowires electrode-

posited in AAO reveal a strong shape anisotropy alongthe wires long axis and also the effect of the phos-phorous content. Figure 5(a) indicate that an easy axisof magnetization is achieves along the CoP nanowires.Also, Figure 5(b) shows magnetization curves for CoPnanowires arrays electrodeposited at 15 Vrms with a fre-quency of 400 Hz from different solutions, when theexternal eld is applied in the plane of template, i.e.,perpendicular to the long axis of the wires. The main inu-ence of the P is seen in either on the magnetic moment orthe coercivity, both of which decrease with increasing P

J. Spintron. Magn. Nanomater. 1, 2327, 2012 25

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RESEARCHARTICLE

AC Electrodeposition of Amorphous CoP Nanowires Embedded in an Alumina Template Nasirpouri et al.

30 40 50 60 70 80 901

10100

100010000

30 40 50 60 70 80 901

10100

100010000

30 40 50 60 70 80 901

10100

100010000

Co (110)Co (100) Co (002)

2 theta

0 g/litre

Inte

nsity

(CPS

)

5 g/litre

Al (0

22)

Al (1

13)

15 g/litre

Fig. 3. X-ray diffraction patterns obtained from CoP nanowires elec-trodeposited in AAO from solutions containing 0, 5, 15 and 25 g/litresodium hypophosphate at 15 Vrms with a sinusoidal waveform with a fre-quency of 400 Hz. It should be noted that Al peaks are not seen in bottompanel due to annealing of Al substrate before anodization.

content in the electrolyte. For coercivity change, it can beexplained with the domain structure in amorphous alloys.Because, the domain walls are wide in amorphous alloysand the defects are narrow, there is little pinning of domain

44.0 44.5 45.0 45.5 46.0

0

2000

4000

6000

41.0 41.5 42.0 42.5 43.0

0

100

200

300

400

(a)

Inte

nsity

(CPS

)

2 theta

5 g/litre15 g/litre25 g/litre0 g/litre

(b)5 g/litre15 g/litre25 g/litre0 g/litre

Inte

nsity

(CPS

)

2 theta

Fig. 4. Annilihitaion of Bragg diffraction peaks (a) Co (002) and (b)Co (001) for CoP nanowires electrodeposited in AAO from solutionscontaining 0, 5, 15 and 25 g/litre sodium hypophosphate at 15 Vrms witha sinusoidal waveform with a frequency of 400 Hz.

Fig. 5. Magnetization curves of CoP nanowires arrays electrodepositedat 15 Vrms with a frequency of 400 Hz from (a) a solution containg5 g/litre phosphorous content in two congurations; in-plane and out ofplane of AAO template, and (b) different solutions with different phos-phorous contents with an applied eld in plane of AAO template. Ms isthe magnetization at the possible maximum applied eld.

walls on defects in amorphous materials and coercivitydecreases.15

4. CONCLUSION

We have demonstrated the electrodeposition of amorphousCoP nanowires using an ac sinusoidal waveform with afrequency of 400 Hz under potentiostatic conditions fromaqueous solution. The incorporation of phosphorus intothe cobalt nanowires takes place during electrodeposition,making the microstructure amorphous. The Phosphorouscontent in the electrolyte inuence the nucleation, crystal-lographic structure and coercivity of CoP nanowires elec-trodeposited in the AAO template.

References and Notes

1. H. Zeng, R. Skomski, L. Menon, Y. Liu, S. Bandyopadhyay, andD. J. Sellmyer, Phys. Rev. B 65, 134426 (2000).

2. C. A. ross, Annu. Rev. Mater. Res. 15, R841 (2001).3. F. Nasirpouri, Recent developments in electrodeposition and pitting

research, edited by A. El-Nemr, Research Signpost Publications,India (2007), pp. 5193.

4. L. Pter and I. Bakonyi, Nanomagnetism and spintronics, editedby F. Nasirpouri and A. Nogaret, World Scientic Publishing Co,Singapore (2010), pp. 89120.

5. F. Nasirpouri, P. Southern, M. Ghorbani, A. Irajizad, andW. Schwarzacher, J. Magn. Magn. Mater. 308, 35 (2007).

26 J. Spintron. Magn. Nanomater. 1, 2327, 2012

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RESEARCHARTICLE

Nasirpouri et al. AC Electrodeposition of Amorphous CoP Nanowires Embedded in an Alumina Template

6. M. Ghorbani, F. Nasirpouri, A. Irajizad, and A. Saedi, Mater. Des.27, 983 (2006).

7. F. Nasirpouri, M. Abdollahzadeh, N. Parvini, and M. Almasi, Cur-rent Applied Physics 9, 91S (2009).

8. M. Hernandez-Velez, Thin Solid Films 495, 51 (2006).9. T. Watanabe, Nano-plating, Elsevier, UK (2004).

10. H. Chiriac, A. E. Moga, M. Urse, I. Paduraru, and N. Lupu, J. Magn.Magn. Mater. 272, 1678 (2004).

11. J. Xu and Y. Xu, Mater. Lett. 60, 2069 (2006).

12. D. S. Xue, J. L. Fu, and H. G. Shi, J. Magn. Magn. Mater. 308, 1(2007).

13. T. M. Whitney, J. S. Jiang, P. C. Searson, and C. L. Chien, Science261, 1316 (1993).

14. C. Schnenberger, B. M. I. van der Zande, L. G. J. Fokkink,M. Henny, C. Schmid, M. Krger, A. Bachtold, R. Huber, H. Birk,and U. Staufer, J. Phys. Chem. B 101, 5497 (1997).

15. R. C. OHandley, Modern Magnetic Materials, John Wiley and Sons,USA (2000).

Received: 26 October 2011. Accepted: 26 November 2011.

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