Effects of pH and annealing on microstructure and magnetic properties of fabricated Co100-xWx nanowire arrays by AC electrodeposition

Effects of pH and annealing on microstructure and magnetic properties of fabricated Co100-xWx nanowire arrays by AC electrodeposition

Journal of Magnetism and Magnetic Materials 498 (2020) 166245 Contents lists available at ScienceDirect Journal of Magnetism and Magnetic Materials ...

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Journal of Magnetism and Magnetic Materials 498 (2020) 166245

Contents lists available at ScienceDirect

Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm

Research articles

Effects of pH and annealing on microstructure and magnetic properties of fabricated Co100-xWx nanowire arrays by AC electrodeposition


Bandar Astinchapa,b, , Zahra Alemipoura,b, Mohammad Jamil Farajia a b

Physics Department, Faculty of Science, University of Kurdistan, 66177-15175 Sanandaj, Iran Research Center for Nanotechnology, University of Kurdistan, 66177-15175 Sanandaj, Iran



Keywords: Coercivity Magnetic properties Cobalt nanowires Electrodeposition

Co100-XWX (0 ≤ Χ ≤ 8) nanowire arrays with mean diameters of ~33 nm were synthesized by alternating current electrodeposition of Cobalt and Tungsten into template of anodic aluminum oxide. Effects of W concentration, pH and annealing process on magnetic properties and crystal structure of the fabricated nanowires were studied using Scanning electron microscopy, Energy dispersive X-ray spectroscopy, X-ray diffraction, and alternating gradient force magnetometer. The obtained results demonstrated that the crystal structure of prepared nanowires was hcp and fcc and a mixture of both, and the magnetic parameters such as coercivity and squareness (Sq = Mr/Ms) of nanowire arrays decreased with increasing the tungsten content and pH. For the study the effect of annealing temperature on the nanowire magnetic properties, the fabricated nanowires were annealed at different temperature (300 °C–600 °C). It is observed that the coercivity for all nanowire arrays with the different value of W content and pH increases with annealing temperature and the maximum variation is related to Co97W3 nanowire at pH = 3, which is explained by shape and magnetocrystelline anisotropy that is discussed in detail.

1. Introduction Recently, production of ferromagnetic nanowire arrays has attracted much notice due to their potential technological utilizations in recording devices [1,2], giant magnetoresistance materials [3], optics [4], nanosensors [5,6], and interfacial microrheology [7]. The magnetic nanowires are synthesized in various methods, such as electrodeposition, sol–gel, and chemical vapor deposition (CVD) [8–10]. Among the various methods to the fabrication of nanowires, electrodeposition elements in anodic aluminum oxide (AAO) membranes expanded by two-step anodization are a quicker and useful method to fabricate an array of uniform nanowires [11]. Electrodeposition of magnetic elements into AAO membranes has been already used to prepare the onedimensional nanostructure alloys by many groups [12–18]. The formerly investigations show that the prepared magnetic nanowires by filling AAO template pores make them more suitable as a candidate for use in magnetic recording instruments because of the ability to store high density data [19–21]. Recently, magnetic Co and Co-base nanowires have attracted special attention [11,13,14,22–25,27]. The interesting changes in magnetic behavior can be expected to occur by alloying the ferromagnetic with nonmagnetic through electrodeposition in a non-magnetic template like AAO [26]. A useful manner to regulate

the magnetic property of the nanowire arrays is adding nonmagnetic substance in a magnetic array [27–35]. Notwithstanding, Co-W nanowires are a candidate for magnetic recording instrument, there are several works for thin films preparation of Co-W alloys [36–41]. But up to now, there are two reports about fabrication of Co-W nanowires and mesowires, as follow: Vernickaite et al. fabricated Co-W nanowires by pulse current (PC) electrodeposition with a citrate-glycine electrolyte into nanoporous polycarbonate membranes. They added gold nanoparticles (with the size of 50 nm) to the electrolyte during electrodeposition of the Co-W alloy nanowires [42]. Tsyntsaru et al. electrodeposited Co-W alloys by direct current (DC) and pulse current (PC) methods, into AAO mesoporous (with an average diameter of 240 nm) from ammonia-free solutions. They studied the effect of nanowires length (with the diameter about of 240 nm) on magnetic properties of Co-W mesowires [43]. In this work, we studied the crystal structure and magnetic properties of fabricated Co nanowires doped with Tungsten (W) by alternating current (AC) electrodeposition, using the AAO template. We also investigated the effects of electrolyte solution pH, annealing process and W concentration on the crystal structure and magnetic properties of Co-W nanowires. We used an alternating gradient force magnetometer (AGFM), conventional X-ray diffraction (XRD), scanning electron

Corresponding author at: Physics Department, Faculty of Science, University of Kurdistan, 66177-15175 Sanandaj, Iran. E-mail address: [email protected] (B. Astinchap).

https://doi.org/10.1016/j.jmmm.2019.166245 Received 6 November 2018; Received in revised form 6 November 2019; Accepted 29 November 2019 Available online 02 December 2019 0304-8853/ © 2019 Elsevier B.V. All rights reserved.

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microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX) to study the magnetic properties of the nanowires, crystal structure, morphology and chemical composition of fabricated nanowires, respectively.

scanning electronic microscope (SEM) that microscope operated at 30 KeV and also it used to determining the chemical composition of samples by an energy dispersive X-ray spectroscopy (EDX) (TSCAN, model: MAIA3).

2. Material and methods

3. Results and discussions

Co100-XWX (0 ≤ Χ ≤ 8) nanowires were fabricated by electrodeposition in AAO templates. The aluminum foil was used to fabricate AAO templates by two-step anodization [44,45]. To prepare templates, high purity aluminum sheet (99.999% purity and 0.3 mm thickness) was cut into specific size and annealed at 450 °C for 20 min in Ar atmosphere to obtain homogenous microstructure. Then, the first anodization was carried out in electrolyte of oxalic acid (0.3 M) with the applying voltage of 40 V at 15 °C for 15 h. After the first anodized, the oxide layer was removed by the chromic and phosphoric acid solution, and then the samples were anodized for the second time in the same condition as the first anodization just for one hour. Afterward, the anodization voltage was reduced from 40 V to 8 V in four steps for thinning barrier layer on the end of the AAO template holes [44]. First, the voltage was reduced by (2 V/30 Sec) from 40 V to 20 V and in the second step the voltage reduced from 20 V to 10 V (with 1 V/30 Sec) and then decreasing to 8 V (with 0.5 V/30 Sec). At the end voltage was held at 8 V for 3 min until acquires a homogenous barrier layer. The current during the thinning process versus time is shown in Fig. 1. The prepared AAO templates were used for fabrication of Cobalt nanowire with the impurity of Tungsten. The nanowires were synthesized with AC voltage of 30 V and 200 Hz frequency at 15 °C for 4 min. Aqueous solutions (with 1 M concentration) were containing (100-X) percentage of CoSO4·7H2O, X percentage of Na2WO4·2H2O, 30 g/l boric acid and 4 g/l sodium gluconate acid with a pH value of 3.5, was used as the electrolyte. To investigate the effect of Tungsten concentration as an impurity, the W content of the nanowires was adjusted by varying X (0, 0.25, 0.5, 0.75, 1, 2, 3, 4, 5 and 8). Also, in order to study the effect of solution pH as other effective parameter, the value of pH was changed (2, 3 and 3.5). For this purpose, the NaOH and HCl solutions were used for the pH adjustment. The fabricated nanowires have been annealed at different temperatures (300, 400,500, 550 and 600 °C) in the Ar for 20 min and then slowly cooled down in the same conditions at room temperature. The magnetic properties of all samples were measured by alternating gradient force magnetometer (AGFM) at room temperature (made by MPD Company in Iran), where the maximum values of applied magnetic field was 5000 Oe for parallel (||) to axis of nanowires. For study crystalline structure of samples the X-ray diffraction (XRD) has been applied (used Cu Kα radiation wavelength was 0.15405 nm) (PHILIPS, model: PW1730), the morphology of NWs was surveyed by

3.1. Morphology The SEM images of the fabricated AAO template before electrodeposition of nanowires and the nanowires after releasing from the AAO template is shown in Fig. 2. To release the nanowires from the AAO templates, the electrodeposited membranes were dissolved in the solution containing 1 M NaOH and then the released nanowires washed several time by distilled water and dispersed in methanol. The length and diameter of as-deposited nanowires are almost 3.3 µm and from 33 nm, respectively. We have found that the diameter range of nanowires is between 33 and 50 nm, which is maybe because of not well removing AAO template from nanowires side. Therefore, the samples have an aspect ratio of about 100, which means they have shape anisotropy along their axis. A closer look at the end of the nanowires in this image shows that nanowires branching at the end and Y-barnched nanowire are formed. The reason for this is the voltage reduction in four steps in second anodization, which make Y-branched pore structures in AAO template as previously reported [46,47]. 3.2. Effect of W concentration The hysteresis loops of the fabricated samples with the various molar percentage of W in electrolyte solution are shown in Fig. 3 for applied external magnetic field parallel (||) to axis of nanowires. Fig. 3 shows the hysteresis curves have become narrowed with increasing in W concentration and remanent magnetization (Mr) and Saturation magnetization (Ms) decreased. The variation of Sq and coercivity field (HC) of the Co100-XWX nanowires versus W concentration in the electrolyte (the value of X varies from 0% to 8%) are illustrated in Fig. 4. The results indicate that at the first HC decreased rapidly with increasing Tungsten concentration in the electrolyte (from 1305 Oe for Co100 to 800 Oe for Co99W1) and then remained almost constant with slight changes. As seen in Fig. 4, at the first the Sq decreased with W concentration which has changes similar to HC, and then Sq is increased from 0.61 to 0.71 and after that its decreases again. The interplay between magnetocrystelline anisotropy, shape anisotropy and magnetostatic interactions (dipolar interactions) are main factors that affect HC and Sq parameters of magnetic properties of the nanowires. So that, the shape anisotropy tries to align magnetic easy axis parallel to the nanowires axis, but the magnetostatic interaction tries to rotate magnetic easy axis in the direction of perpendicular to the wire axis. The placement of magnetic easy axis along the crystalline easy axis is the magnetocrystelline anisotropy effect. According to SEM images, the length to diameter ratio (aspect ratio) of the nanowires is about 100. Therefore, the shape anisotropy has an identical effect on the coercivity field and squareness of the samples. Since the same AAO template is used to prepare nanowires, so the pores diameter, interpore distance, and length were identical for all the nanowires. Therefore, the magnetostatic interactions between nanowires and shape anisotropy have the same effect on all nanowires with different W concentration. So, the parameter that could change the magnetic properties of fabricated nanowires with different concentration of W is the magnetocrystalline anisotropy. This can be explained by changing crystal structure of the samples due to the increases of W content in the Co nanowires, probably the nanowires structure changes from crystal to amorphous phase. 3.2.1. Effect of annealing To study annealing effects on magnetic properties of the fabricated

Fig. 1. The variation of current vs. time during the thinning of barrier layer. 2

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Fig. 2. SEM image of (a) the fabricated AAO template (b) the fabricated nanowires.

Fig. 4. The dependence of HC and Sq on W% content in Co-W nanowires.

samples, the Co-W nanowires were annealed for 20 min at different temperatures include 300, 400, 500, 550, and 600 °C in Ar atmosphere and then they were cooled slowly. Fig. 5 shows effects of annealing temperature on HC parameter of the samples. The obtained results indicate that HC increases with annealing temperature for all fabricated

Fig. 3. The Measured hysteresis curves of fabricated nanowires with different W contents.


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Fig. 5. The variation of HC vs. W% contents at different annealing temperatures for fabricated nanowires.

Fig. 7. The Sq variation of the Co-W nanowires versus W% in the electrolyte at different annealing temperatures.

magnetocrystelline anisotropy changes. A change in crystalline anisotropy of samples is due to the annealing temperature. Also, the annealing process can be reduced intrinsic stress and structural defects of nanowires structure that have arisen because of fast AC electrodeposition of ions in the AAO pores of the template, and they have an effect on magnetic properties. Therefore, in order to study the effects of annealing on microstructure and magnetocrystelline anisotropy of samples and also to investigate the cause of the magnetic properties improvement, the fabricated nanowires were analyzed by X-ray diffractometer. 3.2.2. Effects of annealing and W content on the crystalline structure XRD patterns of the as-deposited Co97W3 nanowires and annealed Co97W3 and Co95W5 nanowires at 600 °C are shown in Fig. 8. As specified in the Figure, the peaks of the Cobalt crystalline planes appear in the XRD pattern, which is related to Cobalt with fcc and hcp structures. Therefore, the XRD patterns confirm that the crystal structure of these nanowires are mixture of two structures: fcc and hcp, and restructuring has not taken place after annealing. This result is in well agreement with the Li’s result that obtained the Cobalt structure is a mixture structure of hcp and fcc at pH = 3.5 [48]. The presence of Al peaks with

Fig. 6. The hysteresis loops of Co97W3 nanowires at different annealing temperatures.

nanowires. It is observed that variation of HC is slight for the samples with lower W content (0, 0.25, 0.50 and 0.75), but for samples with higher W%, especially 2 and 3, there is an obvious change in HC compared with as-deposited nanowires. The fabricated sample with W concentration of 3% has maximum HC variation after annealing at 600 °C. Its coercivity is changed from 850 Oe to 1340 Oe with increases annealing temperature from room temperature to 600 °C (ΔHC = 490 Oe), and its hysteresis loops at different annealing temperature are illustrated in Fig. 6. In addition to HC, the Sq for the prepared samples also changed with increasing annealing temperature. Fig. 7 represents the variation of Sq for as-deposited and annealed samples at different annealing temperatures versus W percentage. Similar to the HC, the Sq variations are negligible for nanowires with a lower W content of 1%, and significant changes have occurred for higher than this percentage. The results show that the highest HC and Sq were obtained for samples with 2 and 3% Tungsten as impurity, which are higher than obtained value for pure Co nanowires. This confirms the success of the addition of W impurity to Cobalt nanowire in order to improve its magnetic properties. As noted above, due to the identically of shape anisotropy and magnetostatic interaction for all samples, the reason for the increase of the HC and Sq for the prepared samples can be attributed to

Fig. 8. The X-ray patterns for Co-W nanowires electrodeposited at different annealing temperatures and W concentration. 4

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high intensity in XRD patterns come from Al substrate and AAO template. In XRD patterns of Co97W3 nanowire at 15 °C and 600 °C, two Bragg diffraction peaks are appeared at about 41.75° and 44.86° which corresponds to crystal planes with Miller indexes (1 0 0) and (0 0 2) for Co with hcp phase respectively. Also, these peaks can be related to (1 1 1) crystal plane of Co with fcc structure phase and (2 0 0) crystal plane of Al, which are overlapped on each other. There is no other any peak related to Co-W alloys in XRD patterns, which means that the W has placed in Co structure as an impurity. Comparison of XRD patterns for as-deposited with annealed Co97W3 nanowires shows that the intensity of peak related to Co-hcp (1 0 0) has slightly decreased and the intensity of peak related to Co-hcp (0 0 2) or Co-fcc (1 1 1) has significantly increased. Therefore, the increase in peak intensity indicates an increase in crystallinity amount of the nanowires in this direction and consequently, an increase in magnetocrystelline anisotropy, which can be a reason for increase the coercivity of nanowires. The order of magnitude for magnetocrystalline anisotropy energy density (Km = 5.1 × 106 erg.cm−3) and shape anisotropy energy density (Ks = 6.0 × 106 erg.cm−3) is the same for Cobalt with hcp structure [47]. Thus, a significant role can be considered for magnetocrystalline anisotropy in the magnetic properties of the Cobalt nanowires with the hcp crystal phase. As it is known, the direction of easy magnetization of magnetocrystalline anisotropy for Cobalt with hcp structure is [0 0 2] direction, so the preferred [0 0 2] texture of Co nanowitres is parallel to the nanowires axis, the effect of magnetocrystalline anisotropy will be aligning the magnetic moment with the axis of the nanowire. In this case, the result of the combination of magnetocrystalline and shape anisotropies will be the effective anisotropy of the nanowire. This conclusion matches with results obtained from AGFM that HC increase from 853 to 1340 Oe and Sq increase from 0.71 to 0.84 for as-deposited and annealed Co97W3 nanowires respectively. On the other hand, for Co with fcc structure phase, the comparison of the magnetocrystalline anisotropy energy density (6.3 × 105 erg.cm−3) and the shape anisotropy energy density (6 × 106 erg.cm−3) show that the magnetocrystalline anisotropy energy can be relinquished [47]. As is known, for Co with hcp structure, the magnetocrystal easy axis is along the C axis of crystal, thus for examine the preferred orientation degree, relative intensity of peaks (I(0 0 2)/I(1 0 0)) for the as-deposited and annealed Co97W3 nanowires were calculated that were about 2.6 and 12 respectively. Comparison of peak intensity ratio (I(0 0 2)/I(1 0 0)) for asdeposited and annealed samples show that the c axis, preferred orientation, along the nanowire axis is increased for annealed sample. The XRD results show that with increasing content of W in the Co nanowires from 3 to 5, the nanowires structure, changes from mixed fcc-hcp to hcp phase. So that, the intensity of peak related to (1 0 0) plane for hcp phase decreases slightly, and peak related to fcc phase of Co ((1 1 1) plane) disappears that thereby the coercivity decreases from 1340 Oe to 1100 Oe. The peak (1 0 0) in XRD pattern with low intensity for annealed Co95W5 nanowires correspond to hcp structure, which indicates the long axis of nanowires is perpendicular to the C axis of hcp unit cell, and tend to amorphous structure [23]. In these cases, the magnetic moment tends to align perpendicular to the nanowires axis because of magnetocrystalline, but shape anisotropy tends to align magnetic moment along the nanowires axis. Therefore, the lowest HC value for nanowire with W concentration of 5% is the result of shape anisotropy and magnetocrystalline anisotropy competition, which indicate a weak effective anisotropy along the nanowire axis. Eliminating the stress in the crystalline structure by annealing can be another reason for improving the HC and Sq after annealing. Because the defects and stresses in the structure of as-deposited nanowires arising from the rapid accumulation of ions into the AAO pores are removed from the crystal structure by annealing.

Table 1 Co and W percentage in the Co97W3 nanowires fabricated at different pH values. Sample




Co97W3 Co97W3 Co97W3

2 3 3.5

98.9 98.6 97.0

1.0 1.3 2.9

fabricated Co97W3 nanowires at pH = 2, 3 and 3.5 were examined by the energy dispersive X-ray spectroscopy (EDX). The results obtained from EDX are listed in Table 1. Fig. 9 shows the variation of obtained W content from EDX versus different electrolyte solutions pH. As can be seen, the W content in the nanowires has increased with pH, which indicates the effect of pH on the amount of W in nanowires. Effects of the electrolytic solution pH on the magnetic properties of the Co-W nanowires are investigated by measuring the magnetic hysteresis loop of samples for applying a magnetic field along the axis of the nanowires that is illustrated in Fig. 10. It is found that the loops were narrowed with increasing pH, as reports [47]. The HC and Sq were determined using magnetic hysteresis loops, as shown in Fig. 11. It can be seen that increasing in pH up to a critical point (pH = 3.5) causes a rapid decrease in HC and Sq then remains almost constant beyond this pH. As the EDX analysis showed, Tungsten content as a non-magnetic impurity in the nanowires increases with increasing pH, which can be the cause of the decrease in magnetic properties of the nanowires. Also, the pH can be effected magnetic properties of nanowires through their structure. For this purpose, we study the crystalline structure of the prepared samples at different electrolyte pH. The X-ray diffraction patterns of the as-deposited nanowires at pH = 3 and 3.5 are shown in Fig. 12. As can be seen in XRD pattern, for the as-deposited nanowires at pH = 3 (with HC = 940 Oe) only a peak (1 1 1) related to the Cobalt with fcc structure is appeared. But, for the fabricated nanowires at pH = 3.5 (with HC = 850 Oe) two peaks corresponding to the Co-hcp (1 0 0) and Co-hcp (0 0 2) or Co-fcc (1 1 1) are observed, which confirms the existence of both crystalline phases (hcp and fcc structure) in the structure. These results are accordant with the Li's results that the crystal structure of Co nanowires is fcc at pH value lower than 3.5 and a mixture of hcp and fcc structures at pH = 3.5 [47]. The reason for decreasing magnetic properties of prepared nanowires at pH = 3.5 can be attributed to the appearance of (1 0 0) peak related to Co-hcp structure in XRD pattern, which means [1 0 0] direction of some crystalline grains are parallel to the nanowires axis. This is while the [1 0 0] is the hard axis of the magnetocrystalline anisotropy directions of hcp structure and the [0 0 2] direction is the easy axis. Therefore, the lower value of HC for prepared nanowires at pH = 3.5 is due to weak effective anisotropy along the nanowires axis, which is due to competition between shape and magnetocrystalline anisotropy. These results are accordance with HC curves in Fig. 11 that coercivity decrease from 940 Oe to 850 Oe when pH increase from 3 to 3.5 respectively.

3.3.1. Effect annealing on Co97W3nanowires fabricated at different pH In order to investigate the effect of annealing temperature on Hc and Sq of the Co97W3 nanowires prepared at different solution pH (2, 2.5, 3, 3.5 and 4), the samples were annealed in the high purity Ar atmosphere at different temperature of 300, 400, 500, 550, and 600 °C for 20 min. Figs. 13 and 14 show the HC and Sq curves as a function of pH for asdeposited and annealed Co97W3 nanowires. It is observed, the HC and Sq increased after annealing. So that, the Sq and HC of the samples increases with increasing temperature of annealing. The obtained results show, the nanowires electrodeposited at pH = 2 and 3 have the maximum coercivity (1500 Oe and 1445 Oe) and the maximum Sq (0.87 and 0.89) at 600 °C. The reason for this increase in HC and Sq is the change

3.3. Effect of the electrolyte solution pH on Co97W3nanowires properties To investigate elemental composition of the prepared samples, the 5

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Fig. 9. The variation of W content (obtain by EDX) in the Co97W3 nanowires deposited at different pH values.

Fig. 11. The HC and Sq of the Co97W3nanowires as a function of pH. Fig. 10. The hysteresis loops of the Co97W3 nanowires electrodeposited at different pH values.

structure changes from fcc phase to hcp phase by annealing and the peak is related to (2 0 0) crystal plane of hcp structure (It is more probable). Make crystal free of stress by annealing at high temperature can be another possibility. Fig. 16 illustrates XRD patterns of prepared Co97W3 nanowires at different electrolyte solution pH (2, 3 and 3.5) and annealed at 600 °C. The XRD pattern shows that the structure of the nanowires fabricated at pH = 2 after annealing is amorphous, since no obvious diffraction peak was observed. But the structure of the prepared Co97W3 nanowires at pH = 3 is fcc with the direction of [1 1 1] or is the hcp structure with (0 0 2) texture. As can be seen, with increasing the pH value of electrolyte to 3.5 the peak of crystal plane with (1 0 0) Miller index for hcp structure is appeared in addition to the crystal plane with [1 1 1] direction for fcc structure and crystal plane with [0 0 2] direction for hcp structure, which confirms that the cobalt nanowires in this case have a mixed crystalline structure of hcp and fcc. Given that magnetocrystalline anisotropy for nanowires with an amorphous structure is close to

in magnetocrystalline anisotropy (As described in the previous section). So, to study magnetocrystalline anisotropy changes, the prepared nanowires at different pH after annealing were characterized by X-ray diffractometer. XRD patterns of the Co97W3 nanowires electrodeposited at room temperature and annealed at 600 °C are illustrated in Fig. 15. The XRD pattern show that crystal structure of the Co97W3 nanowires prepared at pH = 3 are, with just (1 1 1) orientation. The XRD patterns show that the annealing process has no significant affect on reflections from others crystal planes of the Co97W3nanowires, only increases the intensity of peak. However, it was observed that HC and Sq of the nanowires increased. Therefore, the improvement in value of HC and Sq can be related to increasing in (1 1 1) crystal plane size, which means 〈1 1 1〉 is easy direction for Cobalt in fcc structure or the Co nanowire 6

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Fig. 12. X-ray diffraction patterns of the fabricated Co97W3 nanowires at pH = 3 and 3.5.

Fig. 15. XRD patterns of the Co97W3 nanowires at room temperature and 600 °C.

Fig. 13. Coercivity vs. pH for as-deposited and annealed Co97W3 nanowires.

Fig. 16. XRD patterns of annealed the prepared Co97W3 nanowires at different pH.

zero, therefore for the fabricated nanowires at pH = 2, the easy magnetization direction of the shape anisotropy that is along nanowire's axis, leading to an increasing in the HC and Sq of Co97W3 nanowire along its axis. Thus existence amorphous structure in this case, causes high magnetic properties (high HC and Sq) for Co97W3nanowires with pH = 2. 4. Conclusion Highly ordered nanowire arrays of Co100-XWX (0 ≤ X ≤ 8) with diameter of 33 nm and the high length-to-diameter ratio (about 100) were fabricated by AC electrodeposition into AAO templates. The effect of various W contents, pH and annealing temperature on magnetic properties and structure of fabricated nanowires were investigated by XRD, SEM, EDX, and AGFM. The obtained results showed that for aselectrodeposited nanowires HC and Sq were decreased with increasing in W content. Magnetic properties of the fabricated nanowires improved with increasing annealing temperature for W content more than one W% so that Co97W3 sample showed the most variation magnetic properties (ΔHC = 490 Oe). Also, for as-electrodeposited nanowires HC and Sq were decreased with increasing pH (from 2 to 4). But after annealing, magnetic properties of the nanowires were improved, and HC

Fig. 14. squareness vs. pH for as-deposited and annealed Co97W3 nanowires.


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and Sq both increased for all range of pH and various annealing temperatures which were due to crystal anisotropy changes. X-ray measurement showed that increasing the W content in nanowires led to change crystal structure from fcc and hcp mixture structure to hcp structure. Also, pH variation led to crystallized the nanowires at different crystal structure, so that amorphous phase, fcc structure and a mixture of hcp and fcc structure formed at pH = 2, 3 and 3.5, respectively. We founded that by adding the appropriate amount of Tungsten and changing the annealing temperature, the magnetic properties (HC and Sq) of the fabricated Cobalt nanowires by AC electrodeposition can be improved in comparison to the pure state. These results could be very helpful in making magnetic storage.

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