Physica E 50 (2013) 17–21

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Physica E

1386-94

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n Corr

E-m

journal homepage: www.elsevier.com/locate/physe

Effect of length on the magnetic properties of Ni 300 nm wide nanowires

Lirong Qin, Jianwei Zhao n, Qing Guo, Zhongke Yan, Fan Mu, Peng Chen, Guoqing Li

School of Physical Science and Technology, Southwest University, Chongqing 400715, PR China

H I G H L I G H T S

G R A P H I C A L A

c The Ni 300 nm wide nanowireswere fabricated by templatemethod.

c The magnetic properties of Ni nano-wires with various lengths havebeen comparatively studied.

c The shorter Ni nanowires show thespecial magnetization behaviorsdue to their strong magnetostaticcoupling.

77/$ - see front matter & 2013 Elsevier B.V. A

x.doi.org/10.1016/j.physe.2013.02.016

esponding author. Tel.: þ86 2368252355; fa

ail address: zhaojw@swu.edu.cn (J. Zhao).

B S T R A C T

a r t i c l e i n f o

Article history:

Received 30 September 2012

Received in revised form

29 January 2013

Accepted 18 February 2013Available online 26 February 2013

a b s t r a c t

The Ni 300 nm wide nanowires were fabricated inside the as-synthesized nanochannels of anodic

aluminum oxide (AAO) template by electrochemical deposition method. The angular dependence of

coercivity and remanence of Ni nanowire arrays with various lengths have been comparatively studied.

Investigation results demonstrate that the easy axis is along the wire axis for the longer nanowires of

4 mm or 9 mm in length due to their large shape anisotropy. However, the magnetostatic coupling is

dominant for the shorter nanowires of 0.9 mm in length resulting in the change of magnetization

behaviors, including coercivity and easy axis. A further explanation was also given by simple

calculation in the paper.

& 2013 Elsevier B.V. All rights reserved.

1. Introduction

In recent years, there has been increasing interest in thefabrication of ferromagnetic nanowires due to their specialproperties that might be used in spintronic devices and morespecifically in non-volatile memory, magnetic logic devices andlow Ohmic loss devices [1–4]. For an individual long wire, thewire is predominantly magnetized along its length because of theshape anisotropy. However, for a wire array, the magnetostaticinteraction among wires may greatly change the overall magneticproperties of the arrays [5,6]. At present, two reversal modes havebeen suggested as being important: curling and the coherentrotation [4,7]. As different magnetization reversal mechanismswould give a different angular dependence of the magnetic

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properties, the measurement of magnetic properties would pro-vide helpful information on the rotation mechanisms.

Moreover, different groups have investigated the influence of thesize of nanowires on the magnetic properties [7–9]. Generally, thewire diameter and the inter wire spacing should be as small aspossible to increase the recording density for magnetic recordingmedia applications. However, for other possible magnetic applica-tions, such as magneto-optical or microwave devices, larger wirediameters may be more appealing [10]. As one of the importantferromagnetic materials, Ni nanowires have been intensively studied[11–14]. Especially, Qin et al. [10] and Han et al. [6] reported thesynthesis of the Ni nanowires with larger diameters and presentedthe angular dependence of magnetic properties. Nevertheless, con-siderable attention was paid to the magnetic properties of largeaspect ratio (length to diameter) nanowire arrays, not enough waspaid to that of small aspect ratio (e.g., o10) nanowire arrays. In thiswork, we studied the angular dependence of coercivity and rema-nence of Ni 300 nm wide nanowires with various lengths. Investiga-tion results demonstrate that the small aspect ratio Ni nanowire

L. Qin et al. / Physica E 50 (2013) 17–2118

arrays show the special magnetization behaviors, including theangular dependence of coercivity and remanence due to their strongmagnetostatic coupling among wires.

2. Experimental

The AAO templates were homemade by a two-step anodiza-tion process as described in previous reports [11,13]. In brief, thecleaned high-purity aluminum sheets (99.999%) were annealed at450 1C in vacuo for 5 h in order to release mechanical stress andsubsequently electropolished in a 1:9 volume mixture of per-chloric acid and ethanol at 25 V for 5 min to a mirror finish. Thefirst step of anodization was carried out at 165 V for 6 h in a 0.1 Moxalic acid solution at 4 1C. Then, the original film was removedchemically in a mixed solution of phosphoric acid (6 wt%) andchromic acid (1.8 wt%) at 60 1C for 9 h. After that, the secondanodization was performed under same conditions for 6 h.Finally, the remaining Al was removed in a saturated SnCl4

solution. The barrier layer was removed completely by immersingthe template in a 6 wt% phosphoric acid solution at 30 1C and thenanochannels were also widened homogeneously. Before electro-chemical deposition, a thin gold layer was sputtered onto thebottom side of the AAO template to cover the pores completelyand to serve as the working electrode. The direct current electro-deposition was carried out at a constant current of 1.2 mA in theelectrolyte with a mixture of 80 g/L NiSO4 �6H2O, 20 g/L �H3BO3

and 1.5 g/L C6H8O7 �H2O at room temperature. The pH value wasmaintained at 4.0. The length of the Ni nanowires can becontrolled by simply changing the electrodeposition time. In ourexperiments, the electrodeposition times of three samples were2 h, 5 h and 9 h, respectively.

The Ni nanowires embedded in AAO template were character-ized by X-ray diffraction (XRD, Pgeneral XD-3). After dissolving theAAO template in a 10 wt% NaOH solution, the Ni nanowires werecharacterized by scanning electron microscopy (SEM, HitachiS-4800). The angular dependence of magnetization measurementswas performed with vibrating sample magnetometer (VSM, ADEEV11) at room temperature.

Fig. 1. The SEM images of Ni nanowires with length of (a

3. Results and discussion

After etching the templates, the morphologies of the productswith different electrodeposition time were investigated usingSEM. As shown in Fig. 1a–c, the low-magnification overhead viewSEM images reveal that large quantities of well-aligned nanowireshave been obtained. Fig. 1d–f is the side views of the Ninanowires. It can be seen that every nanowire has awell-faceted head, smooth surface and uniform diameter alongits axis. The diameters of the nanowires are all about 300 nm andthe distance between nanowires are about 450 nm because of thefinite effects of the pores of the AAO templates. But clearly, due tothe different electrodeposition time, the lengths of the nanowiresare about 0.9 mm, 4 mm, and 9 mm, respectively.

XRD patterns taken from the nanowire arrays embedded inAAO templates are shown in Fig. 2. The results clearly showthat the three samples have similar curves despite their differentlengths. The diffraction peaks are all corresponding toface-centered cubic (fcc) structure of metal Ni (JCPDS card no.65-0380). The grain sizes estimated from Scherrer’s formula are16.8 nm, 17.2 nm, and 17.9 nm for Ni nanowires with length of0.9 mm, 4 mm, and 9 mm, respectively. Obviously, the grain sizesare almost the same in spite of the different length of wires. It alsocan be concluded that the synthesized Ni nanowires were poly-crystalline in agreement with Qin’s result [10]. The wires arepolycrystalline may be related to the wider diameter of the poresor the electrodeposition process.

The magnetic hysteresis loops of Ni nanowires with variouslengths embedded in the channels of AAO template were mea-sured for applied field angles of 0–901 (the angle between themagnetic field and the axis of the nanowires). Based on the resultsmeasured (see Supplementary material available with this articleonline), the angular dependence of coercivity and the remanenceratio for the three systems are obtained and shown in Fig. 3. Onthe whole, the angular dependence of coercivity (Fig. 3b and c)exhibits the same variation behavior as for wire length of 4 mmand 9 mm, decreasing with increasing angle between the magneticfield and the axis of the nanowires, similar to previous reports ofboth wider and thinner Ni nanowires [15,16]. Contrarily, the

) and (d) 0.9 mm, (b) and (e) 4 mm, (c) and (f) 9 mm.

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Fig. 3. Angular dependence of the coercive field for the wire length of (a) 0.9 mm,

(b) 4 mm, and (c) 9 mm.

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Fig. 2. XRD patterns of the Ni nanowires with different length.

L. Qin et al. / Physica E 50 (2013) 17–21 19

angular dependence of coercivity (Fig. 3a) for Ni nanowires withlength of 0.9 mm displays a nonmonotonic behavior. Namely, thecoercivity rapidly increases with increasing angle, reaching amaximum for angles between 15–301 and dropping smoothly toits minimum value. These results imply that there is a transitionof magnetization reversal mechanisms. Han et al. [6] haveobserved the similar magnetic behavior and pointed out that, atlarger angle, coherent rotation is dominant, while the curlinghappens for small angles. However, these two reversal modesoccur in homogeneous ellipsoids and extend throughout thevolume of the sample [17]. Since the present nanowire samplesare polycrystalline, it seems likely that the above two reversalmodes become localized, and nucleation preferentially takes placeat lattice defects, so that domain-wall depinning becomes thedominant reversal mechanism [18].

To find the origin of this different behavior, we will analyze theangular dependence of the remanence ratio, which has notreceived sufficient attention. It can be seen from Fig. 4b and c(All of the curves shown in Fig. 4 have been normalized) that theremanence ratio decreases with increasing angle for wire lengthof 4 mm and 9 mm, indicating that the easy magnetization axes arealso parallel to the wires. However, for the Ni nanowires withlength of 0.9 mm, the remanence ratio (Fig. 4a) increases withincreasing angle. It means that the easy magnetization axes of theshorter Ni nanowires are perpendicular with the wires. Therotation of the easy magnetization axes reflects the change ofmagnetostatic interaction among wires. To further exemplify ourconclusion, we synthesized and examined the Ni nanotube arrayswith a small aspect ratio. The corresponding SEM image (Fig. 5a)shows clearly that the Ni nanotubes stand up vertically on thesubstrate surface. The average tube diameter and length are about300 nm and 0.9 mm, respectively. Fig. 5b reveals the angulardependence of coercivity and the remanence ratio for Ni nano-tubes, which is similar to that of Ni nanowires with the length of0.9 mm. This result illustrates again the special magnetic proper-ties of Ni one-dimension nanostructures with the smallaspect ratio.

It has been investigated that the overall magnetic anisotropyof the nanowire array is mainly determined by following three

contributions [6]: (1) the shape anisotropy results from the formeffect of the individual wires, which will induce a magnetic easyaxis parallel with the wire axis; (2) magnetostatic couplingamong the wires, which tends to develop a magnetic easy axisperpendicular with the wire axis; and (3) the magnetocrystallineanisotropy field. Among them, the average magnetocrystallineanisotropy can be neglected because the Ni nanowires are poly-crystalline. Hence, the magnetic anisotropy in our case is mainlydetermined by the shape anisotropy or magnetostatic coupling. Itis noteworthy that the easy magnetization axes for the wirelength of 0.9 mm are perpendicular to the wires. This means thatthe magnetostatic coupling dominates over the shape anisotropyfor the nanowire arrays with small aspect ratio. In this way, theresults for the wire length of 4 mm and 9 mm can be understood asthe dominant shape anisotropy induces the easy axes parallelingto the wires. On the other hand, when the magnetostatic inter-action among wires dominates over the shape anisotropy field, italso can influence the magnetization behavior of the wires, e.g.,coercivity and its angular dependence. As a result, the angulardependence of coercivity for the wire length of 0.9 mm displays adifferent behavior compared to the longer wires.

In the past, magnetic nanowire arrays including Fe, Co, and Nimaterials have been studied widely. Some reports such as [6,16]

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Fig. 4. Angular dependence of the normalized remanence ratio for the wire length

of (a) 0.9 mm, (b) 4 mm, and (c) 9 mm.

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Fig. 5. (a) The overhead view SEM image of the Ni nanotubes, Inset is a side view

SEM image, (b) angular dependence of the normalized coercive field and

remanence ratio for the Ni nanotubes.

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pointed out that in large aspect ratio nanowires, the overallmagnetostatic interactions are stronger, enough to overcomethe effect of shape anisotropy thus resulting in the easy magneticaxis becoming perpendicular to the wire axis. This effect willprobably be most significant for Ni, which has the smallestanisotropy field. Our result shows that the magnetostatic inter-actions are also stronger than the shape anisotropy field for thesmall aspect ratio nanowires. Qin et al. [10] had introduced anequation to estimate the dipolar field in first approximation, thatis: moHd¼mom/[s2

þ(l2/4)]3/2, where l is the length, s the interwiredistance, and m the magnetic moment of the wire, which ism¼MSplr2. Taking into account that each wire has six neighborsand that the magnetostatic interaction is long ranged by thedipolar field, so moHd,tot¼9moHd. Using the values of our Ninanowires, we obtain that the moHd,tot is 1.36 T, 0.18 T, and0.04 T for the wire length of 0.9 mm, 4 mm, and 9 mm, respectively.In the meantime, the shape anisotropy field can be estimated byan equation [10] of moHk¼moMs/2 when the aspect ratio is large.As a result, moHk¼0.3 T for the wire length of 4 mm and 9 mm andmoHko0.3 T for the wire length of 0.9 mm. Compared with thevalues of the magnetostatic interaction and the shape anisotropyfield, it can be found that the magnetostatic coupling dominatesover the shape anisotropy only for the nanowire arrays with smallaspect ratio. This is in a good agreement with our experimentalresult. However, Qin’s equation shows that the magnetostatic

interaction would decrease with the increased length of nano-wires, which is inconsistent with some reports [6,16]. We con-sider that the difference in dimension, crystal structure, anddefects can influence the magnetic nature of the nanowires array[19,20]. Hence, no simple magnetization reversal mode or equa-tion could account for all of magnetic nanowires. We expect thefurther measurements of susceptibility in ZFC and FC modescould give more insight into the magnetic properties of Ninanowires and the corresponding work is already underway.

4. Conclusion

The Ni 300 nm wide nanowires were synthesized inside thenanochannels of AAO template by electrochemical depositionmethod. The angular dependence of coercivity and remanenceof Ni nanowire arrays with different lengths have been system-atically studied. Investigation results demonstrate that the mag-netic crossover effect is dominant for the wire length of 0.9 mm.the strong magnetostatic coupling among wires not only causedthe over easy axis change from parallel to perpendicular to thewire axis, but also had a great influence on the magetizationreversal mechanism. Our results would help to have a deeperunderstanding of the magnetic nanowire arrays.

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Acknowledgment

This work was supported by The National Natural ScienceFoundation of China (Grant no. 51101129 and 11204246), and theNatural Science Foundation Project of CQ CSTC (Grant no.2009BB4304).

Appendix A. Supporting information

Supplementary data associated with this article can be foundin the online version at http://dx.doi.org/10.1016/j.physe.2013.02.016.

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