IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 6, JUNE 2009 2475
Electrodeposited CoNiP Hard Magnetic Nanowiresin Polycarbonate Membrane
V. Sudha Rani�, S. AnandaKumar�, K. W. Kim�, Seok Soo Yoon�, J.-R. Jeong�, and CheolGi Kim�
Department of Materials Science and Engineering, Chungnam National University, Daejeon 305-764, KoreaDepartment of Physics, Andong National University, Andong 760-749, Korea
An array of CoNiP magnetic nanowires were grown in polycarbonate membrane using potentiostatic electrodeposition techniqueunder three electrodes configuration. The commercially available track etched polycarbonate membranes of thickness 6 m with poresize of 50 nm diameter were used in these experiments. The electrolyte bath consists of NiCl�-6.81 g/l, CoCl�-2.76 g/l, NaH�PO�-2.59 g/l,H�BO�-2.49 g/l, NaCl-2.20 g/l, Saccharin-0.8 g/l was used for deposition of CoNiP magnetic nanowires. The main aim of this work fo-cuses on growth conditions, structural and magnetic properties of the CoNiP nanowires. In this context first we observed three differentgrowths of nanowire lengths 1.21 m, 4.31 m and 6 m at three different deposition times 30 min, 60 min, and 90 min, respectively.The X-ray diffraction patterns of CoNiP nanowires have shown the intermixture of fcc and hcp phases. The structural properties of theCoNiP nanowires were observed using scanning electron microscope (SEM). The magnetic properties of the CoNiP nanowires were ob-served using vibrating sample magnetometer (VSM), which show hard magnetic properties with no preferential magnetization directionof the nanowires having high coercivity values around 500 Oe.
Index Terms—Electrodeposition, magnetic properties, nanoporous templates, nanowires.
I. INTRODUCTION
H ARD magnetic nanowires have been drawn muchattention due to their potential in biosensing applica-
tions, nanowire sensors, high density perpendicular magneticrecording media and also their additional capability in MEMSdevices[1]–[6]. Especially, CoNiP magnetic nanowires aremuch interested because of its larger coercivity and highersaturation magnetization [7]. However, there are some reportson Co and Ni based electrodeposited nanowires for numerousapplications [8]–[11]. The CoNiP ternary alloy based nanowiresare having hard magnetic properties with much higher coer-civity than individual Co and Ni nanowires.
The electrodeposition method is mostly used for processingnanostructured materials that require specific physical andchemical properties [12], [13]. It can be considered as an alter-native approach to conventional lithography methods becauseit is inexpensive, simple and high throughput technique formass production. Recently, using electrodeposition techniquea significant progress has been made in fabricating an arrayof nanowires by means of nanoporous templates. Amongvarious nanoporous templates, the track etched polycarbonatemembranes are suitable candidates for growing any kind ofmagnetic nanowires, since they are cost effective, and availablewith different thickness and pore sizes. The properties ofnanowire arrays are directly related to the properties of thenanoporous templates such as the relative membrane thickness,pore size and spatial distribution [14]. The nanowires are notapplicable directly in devices when they are embedded inthe polycarbonate templatesembedded in template cannot beintegrated directly into conventional devices [13]. Hence, thetemplates must be dissolved after electrodeposition.
Manuscript received October 17, 2008. Current version published May 20,2009. Corresponding author: C. Kim (e-mail: [email protected]).
Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TMAG.2009.2018657
In this study, potentiostatic electrodeposition technique underthree electrodes configuration was employed for growing theCoNiP nanowires into polycarbonate membranes. In this com-munication we have reported the structural and magnetic prop-erties of CoNiP hard magnetic nanowires deposited at three dif-ferent deposition times.
II. EXPERIMENTAL PROCEDURE
In the three electrodes potentiostatic configuration an 8-cmthin platinum sheet and Ag/AgCl were used as counter andreference electrodes, respectively. A metallic Au film of200 nm was sputtered on one side of a commercially avail-able track etched polycarbonate membranes (thickness 6 m,with pore size of 50 nm diameter) which is serving as aworking electrode. The room temperature electrolyte bathconsists of NiCl -6.81 g/l, CoCl -2.76 g/l, NaH PO -2.59 g/l,H BO -2.49 g/l, NaCl-2.20 g/l, Sachharin-0.8 g/l is preparedfor deposition of the nanowires [15]. The pH value of the elec-trolyte bath was maintained around at 3.2 during the deposition.
Prior to the deposition the membrane was placed in the depo-sition cell filled with distilled water for several hours to make thepores hydrophilic. After the wetting process, the distilled waterwas replaced with the electrolyte bath just before starting thedeposition process. The deposition was carried out at a constantpotential of V with respect to the Ag/AgCl reference elec-trode for the reduction of the metallic ions from the electrolytebath. During the deposition process current-time profiles wererecorded to understand the growth rate of the nanowires. Theconsecutive experiments were carried out by increasing the de-position time to estimate the growth rate of the nanowires.
Crystallization properties of the CoNiP nanowires were ex-amined by X-ray diffractometer using Cu-K radiation. Thestructural properties of the deposited CoNiP nanowires were ob-served using scanning electron microscope. The magnetic prop-erties of the nanowires were investigated using Lake-Shore 7400series vibrating sample magnetometer before removal of thetemplate. After the measurement, the template was dissolved
2476 IEEE TRANSACTIONS ON MAGNETICS, VOL. 45, NO. 6, JUNE 2009
Fig. 1. SEM images for the cross sectional view of electrodeposited nanowiresarray of CoNiP in polycarbonate membranes with different electrodepositiontimes of (a) 30 min, (b) 60 min, and (c) 90 min.
in dichloromethane, the nanowires were then spin coated onthe on the Si substrate and rinsed with DI water to remove theresidual bases and acids. Finally, nanowires were collected andpreserved in ethanol for further use [16].
III. RESULTS AND DISCUSSION
Fig. 1 shows the cross sectional SEM images of the electrode-posited CoNiP nanowires with three different deposition times.It is shown that the length of the nanowires increases with in-creasing deposition time. Fig. 1(a) shows the short nanowireswith 1.21 m lengths for the deposition time of 30 min. Fig. 1(b)shows the moderate growth with 4.31 m lengths for the depo-sition time of 60 min and Fig. 1(c) shows the full length of thenanowire equal to the membrane thickness 6 m for the depo-sition time of 90 min.
Fig. 2 shows the current-time profile recorded during the de-position of CoNiP nanowires into the polycarbonate membraneat a constant potential of V with respect to the Ag/AgClreference electrode. The insets in Fig. 2 schematically displaypore filling levels for corresponding deposition times. Thecurve shows that the current increases with time and approachan asymptotic value that is the electrodeposition time at 90 min
Fig. 2. Current-time profile recorded during the electrodeposition of CoNiPnanowires.
Fig. 3. X-ray diffraction patterns of CoNiP nanowire arrays.
[14]. The reduction current is mainly determined by appliedreduction potential and mass transport of ions by diffusion[17], [18]. As wires approach the top surface of membraneby progressive growth, the actual applied reduction potentialrelative to the reference electrode located at the top surfaceincreases and diffusion current also increases due to increasingconcentration gradient. Then the reduction current increaseswith time for wire growth stage. The current monitoring isuseful to determine maximum electrodeposition time to avoidthe film growth after complete filling of pores.
Fig. 3 shows the X-ray diffraction patterns of the electrode-posited CoNiP nanowires array. The diffraction peaks at 41.7shows the mixture of Co or NiP phases which is attributed to theintermixing of the NaH PO in the electrolyte bath. The peaksat 44.5 and 48.7 corresponding to the Co-fcc (111) and theCo-hcp (101) phases, respectively. The peak at 38.2 representsthe Au-fcc (111) orientation. The mixture of fcc (111) and hcp(101) phases is may be due to the pH value of the electrolyte.Fashen Li et al., [19], [20]reported earlier that for the Co basednanowires, the single crystalline fcc phase occurs at pH valueless than 2.7 and the hcp polycrystalline growth occurs at pH
RANI et al.: ELECTRODEPOSITED CONIP HARD MAGNETIC NANOWIRESIN POLYCARBONATE MEMBRANE 2477
Fig. 4. Hysteresis loops of the electrodeposited CoNiP nanowires in paralleland perpendicular directions.
Fig. 5. SEM images of CoNiP nanowires after the removal of the membrane.
value more than 5.0. But at pH value around 3.2 the structure iscomplicated and consists of mixture of fcc and hcp phases.
Fig. 4 shows the magnetization curves of the CoNiPnanowires parallel and perpendicular directions of thenanowires axes. It is shown that there is no preferential easymagnetization direction which may be due to the intermixing ofCo, NiP, Co fcc (111) and hcp (111) phases. It is observed fromthe hysteresis loops larger coercivity values around 500 Oereveals the hard magnetic properties of the CoNiP nanowiresfor parallel and perpendicular directions of the nanowires.
Fig. 5 shows the SEM image of the CoNiP nanowires spreadon the Si substrate after removal of the membrane. It is shownthat these nanowires were grown uniformly when compared tothe membrane thickness and pore diameter. We have made hardmagnetic nanowires suspension by collecting and preserving thewires in the ethanol.
IV. CONCLUSION
In summary, we have fabricated CoNiP nanowires using elec-trodeposition technique with track etched polycarbonate mem-branes. We have precisely controlled the nanowire length by
adjusting the deposition time and by monitoring current-timecurve. The CoNiP nanowires showed hard magnetic propertieswith coercivity values around 500 Oe.
ACKNOWLEDGMENT
This work was supported in part through a fundamental R&Dprogram for core technology of materials funded by the Ministryof Knowledge Economy, Republic of Korea, and also by a grantfrom KOSEF under project number M10803001427-08M0300-42710.
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