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Cu nanowires with variable segment sizes
Accepted Manuscript Axially adjustable magnetic properties in arrays of multilayered Ni/Cu nanowires with variable segment sizes A. Shirazi Tehrani, M. Almasi Kashi, A. Ramazani, A.H. Montazer PII:
S0749-6036(16)30151-3
DOI:
10.1016/j.spmi.2016.04.009
Reference:
YSPMI 4286
To appear in:
Superlattices and Microstructures
Received Date: 28 November 2015 Revised Date:
2 April 2016
Accepted Date: 4 April 2016
Please cite this article as: A.S. Tehrani, M.A. Kashi, A. Ramazani, A.H. Montazer, Axially adjustable magnetic properties in arrays of multilayered Ni/Cu nanowires with variable segment sizes, Superlattices and Microstructures (2016), doi: 10.1016/j.spmi.2016.04.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Axially adjustable magnetic properties in arrays of multilayered Ni/Cu nanowires with variable segment sizes
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A. Shirazi Tehrania, M. Almasi Kashia,b*, A. Ramazania,b, A.H. Montazerb Department of Physics, University of Kashan, Kashan 87317-51167, Iran
b
Institute of Nanoscience and Nanotechnology, University of Kashan, Kashan 87317-51167, Iran
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*E-mail: [email protected]; Phone/Fax: +98 31 5591 2578
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a
Abstract
Arrays of multilayered Ni/Cu nanowires (NWs) with variable segment sizes were fabricated into anodic aluminum oxide templates using a pulsed electrodeposition method in a single bath for designated potential pulse times. Increasing the pules time between 0.125 and 2 s in the
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electrodeposition of Ni enabled the formation of segments with thicknesses ranging from 25 to 280 nm and 10 to 110 nm in 42 and 65 nm diameter NWs, respectively, leading to disk-shaped, rod-shaped and/or near wire-shaped geometries. Using hysteresis loop measurements at room
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temperature, the axial and perpendicular magnetic properties were investigated. Regardless of
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the segment geometry, the axial coercivity and squareness significantly increased with increasing Ni segment thickness, in agreement with a decrease in calculated demagnetizing factors along the NW length. On the contrary, the perpendicular magnetic properties were found to be independent of the segment sizes, indicating a competition between the intrawire interactions and the shape demagnetizing field.
Keywords: Ni/Cu multilayered nanowires; anodic aluminum oxide template; pulsed electrodeposition; single bath; coercivity; squareness.
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ACCEPTED MANUSCRIPT 1. Introduction Progress in magnetism, particularly at the nanoscale, requires new structures to be studied and understood. This ranges from studying their magnetization dynamics and magnetization reversal mechanisms to investigating magnetic properties, by which interesting effects and devices could be
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emerged and developed [1-4]. The key to fabricating functional magnetic nanostructured devices is to control materials and their properties effectively. In turn, this can provide a wide range of applications including high density magnetic recording media, spintronics, logic processors and magnetic sensing
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devices [5-8]. With regard to the sensing devices such as magnetic field sensors and magnetoresistive recording heads, the emerging effects such as the anisotropic magnetoresistance, magneto-thermopower
attentions from the researchers [9-13].
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and the giant magnetoresistance have received an enormous amount of experimental and theoretical
Theoretically, investigations on spin-dependent transport and effects of electron localization are interesting as the size of magnetic nanostructures becomes comparable with the mean free path of
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electrons [2, 14]. Experimentally, the giant magnetoresistance effect has so far been observed in a wide variety of nanostructured materials including thin films and nanowires (NWs), comprising alternating ferromagnetic (FM) and non-magnetic (NM) layers [13, 15-17]. In this direction, multilayered NW
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arrays with different segment sizes are expected to be perfect model systems and excellent candidates for the aspects mentioned.
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One of the most conventional methods for fabricating NWs is to use templates such as the polycarbonate and the anodic aluminum oxide (AAO) followed by electrochemical routes [5, 18-20]. Controlling the anodization parameters, AAO templates provide highly ordered nanopore arrays with different lengths and diameters. In this way, multilayered magnetic NWs such as Co/Cu, Ni/Cu, NiFe/Cu, CoFe/Cu, Co54Ni46/Co85Ni15 and CoNi/Cu have been fabricated by direct current (dc) and pulsed electrodeposition techniques in dual and single electrochemical bath [21-28].
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ACCEPTED MANUSCRIPT While the dual bath approach provides pure composition for the FM/NM segments by subsequent electrodeposition from different electrolytes [21], it is essential to choose elements with considerably different deposition potentials when using the single bath [24, 29], thereby restricting the fabrication of various multisegmented NWs. However, the use of single bath technique is prioritized over the dual
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bath. This is due to fact that latter approach suffers from some disadvantages including the oxidation of the layers whilst switching between the electrodes or electrolytes. Moreover, the presence of the residual electrolyte in the pores reduces the purity of the desired layers [30, 31].
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For the single bath method with the dc electrodeposition, one should control the current density or the cathode potential. Nevertheless, if the NM element has a lower deposition potential compared to that of
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the FM element, it would incorporate into the FM layer [29]. To decrease the rate of incorporation, one can make the concentration of the NM ions more dilute, leading to a diffusion-limited deposition of the NM layer over a wide range of potentials [32]. On the other hand, using the single bath with the pulsed electrodeposition enables us to adjust the composition of FM/NM layer by simultaneous change in the
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deposition potential. Additionally, the size of the segments could be readily changed by changing the pulse number and/or the pulse time thereby influencing the physical parameters involved, including the shape anisotropy contribution (through the aspect ratio) and the dipolar interactions between segments
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[29, 32, 33]. This may provide magnetic tunability in terms of magnetic properties including the coercivity and remanence ratio (i.e., squareness) [32].
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By modulating the potential in the fabrication of multisegmented Cu/Ni NWs with diameters ranging between 40 and 140 nm, Chen et al. [22] were able to tune the magnetic properties of the resulting NWs embedded in polycarbonate templates. In this case, the aspect ratio of the FM segment increased from 0.1 to 2.5 while also increasing both the coercivity and squareness values for a magnetic field applied parallel and perpendicular to the long axis of NWs [22]. With the small magnetocrystalline anisotropy energy constant of Ni (Kmc= –4.5 × 10-4 erg/cm3) [32] the magnetic behavior of the disk-shaped and rodshaped Ni segments is dominated by the shape anisotropy contribution with an easy axis located
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ACCEPTED MANUSCRIPT perpendicular and parallel to the NW axis, respectively. It is worth noting that the NM layer thickness could also play an important role in affecting the dipolar interaction contribution between the FM segments [22]. Regarding the microstructural properties, it is important to avoid the lattice mismatch between the FM/NM layers thus improving the FM segment crystal structure and the adhesion with the
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subsequent NM layer, providing a smooth morphology [19]. In turn, this may enhance the functionality and magnetic properties of the resulting NWs.
In this study, Ni/Cu multilayered NW arrays with different segment sizes were fabricated inside 42
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and 65 nm pore diameter AAO templates using a pulsed alternating current (ac) electrodeposition with a single bath technique. After optimizing the FM/NM layers in terms of purity by fabricating NiCu alloy
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NWs using a Cu electrolyte with a concentration of 0.005, Ni-rich segments with different sizes were obtained by increasing the potential pulse time from 0.125 to 2 s.
The morphological, structural and magnetic properties of the Ni/Cu NWs will be presented and compared. While the magnetic properties of the resulting NWs remain constant for the magnetic field
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applied perpendicular to NW axis, this study will give insights into the role of segment size in adjusting the axial magnetic properties of the multilayered NWs.
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2. Experimental details
Using a template-assisted technique with AAO, arrays of Ni/Cu multilayered NWs were fabricated.
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To obtain the AAO templates, Al disk samples with high purity (99.999% from Alfa Aesar; 0.3 mm in thickness with a diameter of 8 mm) were initially electropolished in a mixed solution of ethanol and perchloric acid (4:1 in volume) at 4 °C using a constant potential of 20 V during 3 min. To obtain ordered arrays of nanopores, the two-step anodization method was employed, as described elsewhere [34]. In brief, for the first step, the samples were anodized in 0.3 M oxalic acid at 17 °C using a constant potential of 40 V during 6 h. Afterward, the oxidized layer was etched by a mixed solution of 0.3 M chromic acid and 0.5 M phosphoric acid at 60 °C. For the second anodization step, the duration of the first step was only reduced to 2 h. 4
ACCEPTED MANUSCRIPT The resulting nanopore arrays will have an inter-pore distance of 100 nm with a diameter of approximately 30 nm [34]. To prepare nanopores with larger diameters, the AAO templates were subjected to an etching treatment in 0.3 M phosphoric acid at 30 °C for 10 and 30 min, leading to AAO templates with pore diameters of 42 and 65 nm, respectively [35]. The fabrication of NWs was
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performed using a pulsed ac electrodeposition technique before which the alumina barrier layer at the bottom of nanopores was sufficiently thinned in order to facilitate the electrodeposition process. In this respect, the AAO templates were subjected to an asymmetric anodization by step-wise reducing the
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potential from 40 to 12 V, creating dendritic nanopores at the pores’ bottom [36].
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The fabrication of Ni/Cu multilayered NWs was realized using a single bath method by serving the remaining aluminum of the template as the working electrode and a graphite roll as the counter electrode. However, to evaluate and reach the appropriate composition for the FM/NM segments, four NiCu alloy NW samples were initially prepared by electrodeposition into the 42 nm pore diameter AAO templates. In this regard, an electrochemical bath containing of 0.6 M NiSO4·7H2O with 45 g/l boric
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acid together with Cu electrolyte with a concentration of 0.005 M was employed at 30 °C. The acidity of the bath was set to 3. Using a pulsed ac power supply (SFG-830) with a
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programmable logger, asymmetric sine waveforms with constant reduction/oxidation potentials (Vred/Vox) of 18/12 V and 12/12 V together with pulse times of 5 ms and 200 ms were created in order to
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obtain Ni-rich and Cu-rich NiCu NWs, respectively. The amount of charge passed during the electrodeposition was 1 C. In should be noted that, to reduce the effect of the dendritic structures on the resulting properties, several pulses with Vred/Vox of 12/12 V were applied at the onset of the process, corresponding to the filling of the dendrites with the non-magnetic element Cu. X-ray fluorescence (XRF, equipped with a copper target) analysis was then carried out and the results are presented in Table 1.
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ACCEPTED MANUSCRIPT Table 1. The composition of NiCu alloy NWs using Ni concentration of 0.6 M, determined by XRF analysis.
Ni (at.%) Cu (at.%)
Cu concentration= 0.005 M Vred/Vox 18/12 V 82 18
Vred/Vox 12/12V 16 84
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Element
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As observed, using the selected concentration of Cu electrolyte, Ni-rich and Cu-rich NiCu NWs are fabricated by pulse times of 5 and 200 ms in conjunction with Vred/Vox of 18/12 V and 12/12 V,
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respectively. Subsequently, by alternating Vred/Vox between 18/12 and 12/12 V together with pulse times of 5 and 200 ms, multilayered arrays of Ni/Cu NWs were fabricated into the 42 and 65 nm pore diameter AAO templates. While the Cu-rich segment size was kept constant at each diameter for a potential pulse time of 40 s (i.e., a pulse number of 200), the Ni-rich segment size was varied using successive pulse
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times of 0.125, 0.25, 0.5, 1 and 2 s (corresponding to pulse numbers of 25, 50, 100, 200 and 400). Note that, at each diameter, the total length of NWs composed of the FM layer remained the same for all the selected conditions since the overall pulse number for electrodepositing the Ni segments was set to be
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8000. In Fig. 1(a), using Vred/Vox of 18/12 V and pulse time of 5 ms, the potential-time and current-time curves recorded during the electrodeposition of Ni segment are depicted. From Fig. 1(b), the charge
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passed during the electrodeposition process of multilayered NWs is shown as a function of deposition time. In this case, the potential pulse times for the deposition of Ni and Cu segments were set to be 2 and 40 s, so that 20 FM/NM segments are fabricated with the increase in the charge and deposition time up to 0.42 C and 840 s, respectively.
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Fig. 1. (a) Potential-time and current-time curves recorded during the electrodeposition of Ni segments
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at Vred/Vox of 18/12 V and pulse time of 5 ms. (b) Variation of charged passed during the electrodeposition of Ni/Cu NWs as a function of deposition time using successive potential pulse times of 2 s and 40 s for the deposition of Ni and Cu segments, respectively.
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The morphological properties of resulting multilayered NWs were obtained using field emission scanning electron microscopy (FESEM, MIRA3 TESCAN, using the backscatter mode operating at 15 kV) and transmission electron microscopy (TEM, Zeiss EM10C operating at 80 kV). For FESEM
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measurements, the NW samples were soaked in 0.3 M NaOH solution for 20 min so that the NWs were partially released from the AAO template. For TEM measurements, the AAO template was completely
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dissolved in 0.3 M NaOH during 45 min at room temperature and rinsed with double-distilled water. After removing the remaining Al at the back of the NW samples using a saturated solution of CuCl2, the crystalline characteristics were investigated by X-ray diffraction (XRD; Philips, model X’PertPro; Cu Kα radiation) with λ= 0.154 nm. On the other hand, the magnetic properties were studied using a vibrating sample magnetometer (VSM, Meghnatis Daghigh Kavir Co, Iran) at room temperature for a magnetic field applied parallel and perpendicular to the NW axis up to 3.5 kOe.
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ACCEPTED MANUSCRIPT 3. Results and discussion 3.1 Morphological properties Arrays of multilayered NWs were fabricated in 42 and 65 nm pore diameter AAO templates by
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modulating Vred/Vox between 18/12 and 12/12 V together with pulse times of 5 and 200 ms in order to obtain Ni-rich and Cu-rich Ni/Cu NWs, respectively. Additionally, with increasing potential pulse time between 0.125 and 4 s when electrodepositing Ni, different segment sizes are expected to form. To
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generally investigate the morphologies of multilayered Ni/Cu NWs, FESEM images were obtained. Fig. 1 shows the cross-sectional view of Ni/Cu NWs obtained with the potential pulse time of 1 s and 40 s for
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the deposition of Ni and Cu segments, respectively, partially released from the AAO template with pore diameters of 42 nm and 65 nm. The resulting NWs with a diameter of 42 nm show no clear segmented structure (Fig 1(a)). However, with the increase in diameter from 42 to 65 nm, the multilayered Ni/Cu NWs can be observed by FESEM (see Fig. 2(b)). The thickness of Ni segment is found to be
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approximately 65 nm using the pulse time of 1 s. On the other hand, to carefully investigate the segment sizes, TEM analysis was employed. Fig. 3 shows bright field TEM images of 65 nm diameter Ni/Cu NWs obtained with the pulse times of 0.5 and 2 s for the deposition of Ni segments. As observed, the
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resulting NWs have a smooth surface with uniform segments along the length. While the thickness of Cu segment is approximately 20 nm, increasing pulse time from 0.5 to 2 s increases the Ni segment
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thickness from 35 to 110 nm. In this way, Table 2 represents the thickness of Ni segments and the corresponding thickness/diameter ratio (T/D) for the samples in study using different pulse times. Basically, increasing the potential pulse time increases the thickness of Ni segment, thereby increasing the ratio of T/D. In turn, this causes the transformation of disk-shaped segments (0 < T/D < 1) to rodshaped ones (1 < T/D < 5) and/or to near wire-shaped segments (5 < T/D < 10) along the length of NWs.
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ACCEPTED MANUSCRIPT As a result, one could expect these changes in the morphology to be reflected in the corresponding crystalline characteristics and magnetic properties of multilayered Ni/Cu NWs due to the changes in the
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segment lengths and shape anisotropy contribution [32, 37], which will be discussed in the next sections.
Fig. 2. Cross sectional view FESEM images obtained from multilayered Ni/Cu NWs partially released
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from AAO templates with pore diameters of (a) 42 nm and (b) 65 nm. The potential pulse time for the deposition of Ni segment was 1 s.
Table 2. Ni segment thickness and corresponding thickness/diameter ratio (T/D) of Ni/Cu NWs
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obtained from different potential pulse times.
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Pulse time (s)
Diameter= 42 nm
Thickness (nm)
T/D
Diameter= 65 nm
Thickness (nm)
T/D
0.125
25
0.6
10
0.15
0.25
50
1.2
20
0.30
0.5
90
2.1
35
0.54
1
180
4.3
62
0.95
2
280
6.7
110
1.7
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Fig. 3. Bright field TEM images obtained from multilayered Ni/Cu NWs with a diameter of 65 nm using successive potential pulse times of (a) 0.5 s and (b) 2 s, for the deposition of Ni segment. The pulse time
rod-shaped segments.
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3.2 Crystalline characteristics
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for the deposition of Cu was constant to 40 s. The arrows indicate the semi-axes of the disk-shaped and
Initially, using XRD analysis, the crystalline characteristics of 42 nm diameter NiCu alloy NWs were
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investigated and the results are shown in Fig. 4. In the case Vred/Vox of 18/12 V with pulse time= 5 ms using Cu concentration of 0.005 M, Ni-rich NiCu (Ni85Cu15) NWs show only a face-centered cubic (fcc) structure with a preferential direction along [111], as can be seen in Fig. 4(a). For Cu-rich NiCu (Ni25Cu75) NWs (obtained using Vred/Vox and pulse time of 12/12 V; 200 ms), the XRD pattern in Fig. 4(b) indicates the formation of Cu (111)-fcc and Cu (220)-fcc peaks with enhanced crystallinity compared to that of the Ni-rich NiCu NWs. Consequently, the XRD patterns of Ni-rich and Cu-rich alloy NWs show reflections related only to the crystalline structure of Ni and Cu, respectively.
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Fig. 4. XRD patterns obtained from NiCu alloy NWs using Vred/Vox and pulse time of: (a) 18/12 V; 5
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ms; (b) 12/12 V; 200 ms.
Using Cu concentrations of 0.005 M, the XRD patterns obtained from 42 nm diameter Ni/Cu multilayered NWs with a Cu segment thickness of 50 nm (estimated from TEM images) and Ni segment thicknesses of 25, 90 and 280 nm are depicted in Fig. 5. For Ni25 nm/Cu50 nm NWs obtained by Cu
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concentration of 0.005 M, the corresponding XRD pattern in Fig. 5(a) shows the presence of both Ni-fcc and Cu-fcc so that no alloy peaks of Ni and Cu are observed, indicating the formation of multilayered structures with high purity FM/NM layers. Increasing Ni segment thickness up to 280 nm causes the
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reflections of Ni structure to dominate those of Cu while also increasing the crystallinity of Ni segments. Moreover, the growth of 42 nm diameter Ni and Cu segments with the same crystallographic textures
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may induce the formation of superlattice Ni/Cu NWs [37]. Increasing diameter up to 65 nm, the XRD patterns of Ni10 nm/Cu20 nm, Ni35 nm/Cu20 nm and Ni110 nm/Cu20 nm NWs are shown in Fig. 5b. The fcc structure of Ni and Cu segments are observed, indicating that the increased diameter has no considerable effect on the growth orientation and crystalline characteristics of Ni/Cu NWs. However, due to the small thickness of Ni segment (10 nm), the incorporation of Cu (with a segment thickness of 20 nm) into the fcc structure of Ni has induced a shift in the Ni (111) and (220) peak positions to lower angles compared to Ni (111) and (220) peaks of pure Ni NWs [38].
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Fig. 5. XRD patterns of Ni/Cu multilayered NWs with variable segment sizes for NW diameters of (a) 42 nm and (b) 65 nm.
The average grain sizes of the Ni/Cu NWs were also evaluated by the Scherrer equation. As a general
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trend, it was found that increasing the thickness of Ni segment decreases the corresponding average grain size along the [111] direction. For example, while the grain size of Ni25 nm/Cu50 nm NWs was estimated to be 14 nm, increasing the Ni segment thickness to 90 nm decreased the grain size to
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approximately 8 nm.
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3.3 Magnetic properties
As mentioned previously, with increasing pulse time when electrodepositing Ni at each NW diameter, magnetic properties of the resulting Ni/Cu NWs with disk-shaped, rod-shaped and/or near wire-shaped segments are expected to differ from each other [22, 32]. This arises due to the influence of the shape of the material on the demagnetizing field. This field, originated from the inside of the material, tends to demagnetize the magnetization, depending on the magnitude of the demagnetizing factors [32]. These factors along the semi-axes of a typical ellipsoid, namely Na, Nb and Nc (with c ≥ b ≥ a), satisfy the relation Na + Nb + Nc = 4π. 12
ACCEPTED MANUSCRIPT While an oblate spheroid could be approximated to the disk-shaped segments (where c = b > a; Fig. 3(a)), a prolate ellipsoid with a circular cross section could represent the rod-shaped and/or near wireshaped segments (where c > b = a; see Fig. 3(b)). For an oblate spheroid, the axial demagnetizing factor (Na) and perpendicular demagnetizing factors
)
1/ 2 2 A − 1 A 1 N a = 4π 2 × 1 − × arcsin 1/ 2 A A −1 A2 − 1 2
(
)
(
(1)
)
1/ 2 A2 − 1 A2 N b = N c = 4π × × arcsin − 1 2 1/ 2 A 2( A − 1) A2 − 1
1
)
(2)
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(
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(
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(Nb = Nc) are given by [32]:
where A is the aspect ratio (A= c/a).
Alternatively, for a prolate ellipsoid, the axial demagnetizing factor (Nc) and perpendicular
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demagnetizing factors (Na = Nb) are obtained by Eqs. (3) and (4) as follows [32]:
( (
) )
1/ 2 2 A A + A −1 N c = 4π 2 × × ln 1/ 2 A − 1 2 A2 − 1 1/ 2 A − A2 − 1
1
)
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(
− 1
( (
(3)
) )
1/2 2 1 A + A − 1 ×A− × ln Na = Nb = 4π 1/2 2 ( A2 − 1) 2( A2 − 1)1/2 A − A2 − 1
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A
(4)
Here, the demagnetizing factors of Ni segments (Na, Nb and Nc) were calculated from the above equations (depending on the segment geometry) and the results obtained are presented in Table 3.
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ACCEPTED MANUSCRIPT Table 3. The demagnetizing factors of Ni segments with diameters of 42 and 65 nm for different potential pulse times. Pulse time (s)
Diameter= 42 nm Nb
Nc
A= c/a
Na
Nb
Nc
0.125
1.68
6.05
3.25
3.25
6.50
9.98
1.25
1.25
0.25
1.19
4.45
4.45
3.63
3.25
8.32
2.16
2.16
0.5
2.14
5.68
5.68
2.10
1.86
6.45
3.06
3.06
1
4.28
5.93
5.93
0.65
1.05
1.19
3.90
3.90
2
6.67
6.06
6.06
0.46
1.69
4.97
4.97 2.56
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Na
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A= c/a
Diameter= 65 nm
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As expected, increasing A in the disk-shaped segments increases the demagnetizing factor along the NW axis (Na). In other words, the shape hard axis has a tendency to lie along the basal plane of diskshaped segments when increasing A, thereby inducing a demagnetizing field. On the other hand, in the case of rod-shaped and/or near wire-shaped segments, increasing A decreases Nc, thereby inducing a
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magnetizing field due to the shape easy axis along the NW axis. Accordingly, depending on the direction of the magnetic field applied (axial and/or perpendicular), the corresponding magnetic properties of the segments with variable sizes are expected to change due to the magnetizing and/or
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demagnetizing effects. In this respect, the magnetic properties of Ni/Cu multilayered NWs with different segment sizes were obtained at room temperature using axial and perpendicular hysteresis loop
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measurements and the results are shown in Figs. 6 and 7. Figures 6(a)–(c) show the hysteresis loops of 42 nm diameter Ni/Cu NWs with the disk-shaped to rod-shaped and/or near wire-shaped segments obtained using pulse times of 0.125, 0.5 and 2s, respectively. As inferred from the corresponding axial coercivity values presented in Fig. 7a, increasing pulse time from 0.125 to 2 s (thus increasing the Ni segment thickness from 25 to 280 nm) increases the coercivity from 250 to 820 Oe, demonstrating a significant enhancement in the axial coercivity by approximately
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Na= 6.05 to Nc= 0.46), as can be seen in Table 3.
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Fig. 6. Axial and perpendicular hysteresis loops of 42 nm and 65 nm diameter Ni/Cu NW arrays using pulse times of (a),(d) 0.125 s, (b),(e) 0.5 s and (c),(f) 2 s.
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In the case of 75 nm diameter Ni/Cu NWs, one can find the same variation behavior of coercivity with increasing Ni segment thickness from 10 to 110 nm. Note that, with the increase in diameter, the
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demagnetizing factors along the NW length increase, thereby decreasing the coercivities of 65 nm diameter Ni/Cu NWs compared to those of 42 nm diameter as a function of pulse time. For example, in the case of rod-shaped segments obtained using the pule time of 2 s, increasing Nc from 0.46 to 2.56 decreases the coercivity of 65 nm diameter Ni/Cu NWs to 650 Oe, starting from 820 Oe for 42 nm diameter NWs.
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ACCEPTED MANUSCRIPT On the other hand, increasing pulse time between 0.125 and 2 s, the corresponding axial squareness values for 42 and 75 nm diameter NWs increase from 0.5 to 0.8 and 0.3 to 0.55, respectively (see Figs. 7(a) and (b)). This is indicative of enhanced magnetizing magnetostatic interactions between the Ni segments (the intrawire interactions) when increasing the thickness of the FM layer, inducing a
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magnetizing field in the axial direction [39, 40].
Fig. 7. Variations of axial and perpendicular coercivity and squareness in (a),(c) 42 nm and (b),(d) 65 nm diameter Ni/Cu NW arrays as a function of pulse time. It is worthy to note that the intrawire interactions in multilayered NWs could be divided into intrawire interactions between the single segments and the demagnetizing fields of these segments [39].
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ACCEPTED MANUSCRIPT On the contrary, for the perpendicularly magnetized Ni/Cu NWs with diameters of 42 and 75 nm, increasing pulse time (thus increasing the perpendicular demagnetizing factors) has no considerable effect on the perpendicular coercivity and squareness values, according to the hysteresis loops in Fig. 6
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and the quantitative results in Fig. 7. Beron et al. [40] investigated the magnetic behavior of Ni/Cu multilayered NW arrays using the firstorder reversal curve (FORC) analysis for a magnetic field applied axial and perpendicular to the long axis of NWs. From their results, decreasing the Cu thickness to Ni thickness ratio (TCu/TNi) increased the
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reversible component of magnetization for perpendicularly magnetized NWs. This coincided with a
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significant increase in the intrawire demagnetizing interactions, leading to a nearly coherent rotation [40]. Here, increasing the pulse time between 0.125 and 2 s decreases TCu/TNi ratio from 2 to 0.18 for both 42 and 65 nm diameter Ni/Cu NWs, inducing an increase in the reversible component as well as in the intrawire interactions. Meanwhile, the shape demagnetizing field (in the perpendicular direction) decreases due to the increase in the Ni segment thickness. Altogether, the competition between the
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intrawire interactions and the shape demagnetizing field causes the magnetic properties in the perpendicular direction to remain almost the same. Particularly, this is realized by the fact that the magnetocrystalline anisotropy, surface anisotropy and exchange interactions are also negligible in the
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case of segmented Ni NWs [39]. Therefore, with increasing the Ni segment thickness, the enhanced demagnetizing factors do not contribute to the perpendicular magnetic behavior of Ni/Cu NWs with
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disk-shaped and/or rod-shaped segments. In addition to the demagnetizing factors, another possible aspect which should be taken into consideration is the effectiveness of segment size on the coercivity. In fact, regardless of the direction of applied magnetic field, increasing the segment thickness is expected to increase the coercivity in single domain state (see Fig. 11.2 in Ref. [41]). As predicted by Sun et al. [32], Ni segments with T/D ≤ 10 are single domain for diameters less than 600 nm. In the present case, the Ni segments are then considered to be single domain for different pulse times, according to Table 2. As a result, while increasing the
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ACCEPTED MANUSCRIPT pulse time is expected to decrease the coercivity in the perpendicular direction (due to the increased demagnetizing factors), the enhancement in the thickness of single domain segments (thus increasing coercivity) has nullified the effectiveness of demagnetizing factors, as can be seen in Figs. 7(a) and (b). Consequently, one can tune the magnetic properties of the multilayered NWs with different diameters in
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the axial direction with increasing only the Ni segment thickness. In particular, this anisotropic behavior of magnetic properties may find functionalities in high density perpendicular magnetic recording media
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and high quality magnetic field sensors [6, 17].
4. Conclusions
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In summary, using the pulsed ac electrodeposition in a single bath, arrays of Ni/Cu multilayered NWs with diameters of 42 and 75 nm were fabricated into AAO templates. Increasing potential pulse time for electrodepositing Ni between 0.125 and 2 s increased the Ni segment thickness from 25 to 280 nm and 10 to 110 nm in 42 nm and 65 nm diameter NWs, respectively. This enabled the fabrication of
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multilayered NWs with disk-shaped, rod shaped and/or near wire-shaped FM segments. Investigating the crystalline characteristics, XRD patterns indicated the formation of highly pure multilayers of Ni and Cu with fcc crystalline structure. Moreover, it was found that increasing diameter had no considerable
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influence on the growth orientation and crystalline characteristics of Ni/Cu NWs. Regarding the magnetic properties, the axial coercivity increased up to 820 and 650 Oe for 42 and 65 nm diameter
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NWs, starting from 250 and 180 Oe, respectively. This was attributed to the significant reduction in the demagnetizing factors along the NW axis when increasing Ni segment thickness, being consistent with the calculated demagnetization factors for oblate spheroid and prolate ellipsoid. Concurrently, due to the enhanced magnetizing magnetostatic interactions, the axial squareness increased up to 0.8 and 0.55. In contrast, increasing pulse-time did not affect the magnetic properties including the coercivity and squareness of the perpendicularly magnetized multilayered NWs.
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ACCEPTED MANUSCRIPT Acknowledgements The authors gratefully acknowledge the University of Kashan for providing the financial support of this work by Grant No. 159023/38.
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References
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Fabrication of multilayered Ni/Cu nanowires by a pulsed alternating current method. Using designated pulse times, segments with different geometries were created. Investigation of morphologies, crystalline characteristics and magnetic properties. Obtaining adjustable magnetic coercivity and squareness in the axial direction. Correlating the axial magnetic properties with calculated demagnetizing factors.
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• • • • •
S0749-6036(16)30151-3
DOI:
10.1016/j.spmi.2016.04.009
Reference:
YSPMI 4286
To appear in:
Superlattices and Microstructures
Received Date: 28 November 2015 Revised Date:
2 April 2016
Accepted Date: 4 April 2016
Please cite this article as: A.S. Tehrani, M.A. Kashi, A. Ramazani, A.H. Montazer, Axially adjustable magnetic properties in arrays of multilayered Ni/Cu nanowires with variable segment sizes, Superlattices and Microstructures (2016), doi: 10.1016/j.spmi.2016.04.009. This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
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Axially adjustable magnetic properties in arrays of multilayered Ni/Cu nanowires with variable segment sizes
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A. Shirazi Tehrania, M. Almasi Kashia,b*, A. Ramazania,b, A.H. Montazerb Department of Physics, University of Kashan, Kashan 87317-51167, Iran
b
Institute of Nanoscience and Nanotechnology, University of Kashan, Kashan 87317-51167, Iran
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*E-mail: [email protected]; Phone/Fax: +98 31 5591 2578
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a
Abstract
Arrays of multilayered Ni/Cu nanowires (NWs) with variable segment sizes were fabricated into anodic aluminum oxide templates using a pulsed electrodeposition method in a single bath for designated potential pulse times. Increasing the pules time between 0.125 and 2 s in the
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electrodeposition of Ni enabled the formation of segments with thicknesses ranging from 25 to 280 nm and 10 to 110 nm in 42 and 65 nm diameter NWs, respectively, leading to disk-shaped, rod-shaped and/or near wire-shaped geometries. Using hysteresis loop measurements at room
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temperature, the axial and perpendicular magnetic properties were investigated. Regardless of
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the segment geometry, the axial coercivity and squareness significantly increased with increasing Ni segment thickness, in agreement with a decrease in calculated demagnetizing factors along the NW length. On the contrary, the perpendicular magnetic properties were found to be independent of the segment sizes, indicating a competition between the intrawire interactions and the shape demagnetizing field.
Keywords: Ni/Cu multilayered nanowires; anodic aluminum oxide template; pulsed electrodeposition; single bath; coercivity; squareness.
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ACCEPTED MANUSCRIPT 1. Introduction Progress in magnetism, particularly at the nanoscale, requires new structures to be studied and understood. This ranges from studying their magnetization dynamics and magnetization reversal mechanisms to investigating magnetic properties, by which interesting effects and devices could be
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emerged and developed [1-4]. The key to fabricating functional magnetic nanostructured devices is to control materials and their properties effectively. In turn, this can provide a wide range of applications including high density magnetic recording media, spintronics, logic processors and magnetic sensing
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devices [5-8]. With regard to the sensing devices such as magnetic field sensors and magnetoresistive recording heads, the emerging effects such as the anisotropic magnetoresistance, magneto-thermopower
attentions from the researchers [9-13].
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and the giant magnetoresistance have received an enormous amount of experimental and theoretical
Theoretically, investigations on spin-dependent transport and effects of electron localization are interesting as the size of magnetic nanostructures becomes comparable with the mean free path of
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electrons [2, 14]. Experimentally, the giant magnetoresistance effect has so far been observed in a wide variety of nanostructured materials including thin films and nanowires (NWs), comprising alternating ferromagnetic (FM) and non-magnetic (NM) layers [13, 15-17]. In this direction, multilayered NW
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arrays with different segment sizes are expected to be perfect model systems and excellent candidates for the aspects mentioned.
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One of the most conventional methods for fabricating NWs is to use templates such as the polycarbonate and the anodic aluminum oxide (AAO) followed by electrochemical routes [5, 18-20]. Controlling the anodization parameters, AAO templates provide highly ordered nanopore arrays with different lengths and diameters. In this way, multilayered magnetic NWs such as Co/Cu, Ni/Cu, NiFe/Cu, CoFe/Cu, Co54Ni46/Co85Ni15 and CoNi/Cu have been fabricated by direct current (dc) and pulsed electrodeposition techniques in dual and single electrochemical bath [21-28].
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ACCEPTED MANUSCRIPT While the dual bath approach provides pure composition for the FM/NM segments by subsequent electrodeposition from different electrolytes [21], it is essential to choose elements with considerably different deposition potentials when using the single bath [24, 29], thereby restricting the fabrication of various multisegmented NWs. However, the use of single bath technique is prioritized over the dual
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bath. This is due to fact that latter approach suffers from some disadvantages including the oxidation of the layers whilst switching between the electrodes or electrolytes. Moreover, the presence of the residual electrolyte in the pores reduces the purity of the desired layers [30, 31].
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For the single bath method with the dc electrodeposition, one should control the current density or the cathode potential. Nevertheless, if the NM element has a lower deposition potential compared to that of
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the FM element, it would incorporate into the FM layer [29]. To decrease the rate of incorporation, one can make the concentration of the NM ions more dilute, leading to a diffusion-limited deposition of the NM layer over a wide range of potentials [32]. On the other hand, using the single bath with the pulsed electrodeposition enables us to adjust the composition of FM/NM layer by simultaneous change in the
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deposition potential. Additionally, the size of the segments could be readily changed by changing the pulse number and/or the pulse time thereby influencing the physical parameters involved, including the shape anisotropy contribution (through the aspect ratio) and the dipolar interactions between segments
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[29, 32, 33]. This may provide magnetic tunability in terms of magnetic properties including the coercivity and remanence ratio (i.e., squareness) [32].
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By modulating the potential in the fabrication of multisegmented Cu/Ni NWs with diameters ranging between 40 and 140 nm, Chen et al. [22] were able to tune the magnetic properties of the resulting NWs embedded in polycarbonate templates. In this case, the aspect ratio of the FM segment increased from 0.1 to 2.5 while also increasing both the coercivity and squareness values for a magnetic field applied parallel and perpendicular to the long axis of NWs [22]. With the small magnetocrystalline anisotropy energy constant of Ni (Kmc= –4.5 × 10-4 erg/cm3) [32] the magnetic behavior of the disk-shaped and rodshaped Ni segments is dominated by the shape anisotropy contribution with an easy axis located
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subsequent NM layer, providing a smooth morphology [19]. In turn, this may enhance the functionality and magnetic properties of the resulting NWs.
In this study, Ni/Cu multilayered NW arrays with different segment sizes were fabricated inside 42
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and 65 nm pore diameter AAO templates using a pulsed alternating current (ac) electrodeposition with a single bath technique. After optimizing the FM/NM layers in terms of purity by fabricating NiCu alloy
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NWs using a Cu electrolyte with a concentration of 0.005, Ni-rich segments with different sizes were obtained by increasing the potential pulse time from 0.125 to 2 s.
The morphological, structural and magnetic properties of the Ni/Cu NWs will be presented and compared. While the magnetic properties of the resulting NWs remain constant for the magnetic field
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applied perpendicular to NW axis, this study will give insights into the role of segment size in adjusting the axial magnetic properties of the multilayered NWs.
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2. Experimental details
Using a template-assisted technique with AAO, arrays of Ni/Cu multilayered NWs were fabricated.
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To obtain the AAO templates, Al disk samples with high purity (99.999% from Alfa Aesar; 0.3 mm in thickness with a diameter of 8 mm) were initially electropolished in a mixed solution of ethanol and perchloric acid (4:1 in volume) at 4 °C using a constant potential of 20 V during 3 min. To obtain ordered arrays of nanopores, the two-step anodization method was employed, as described elsewhere [34]. In brief, for the first step, the samples were anodized in 0.3 M oxalic acid at 17 °C using a constant potential of 40 V during 6 h. Afterward, the oxidized layer was etched by a mixed solution of 0.3 M chromic acid and 0.5 M phosphoric acid at 60 °C. For the second anodization step, the duration of the first step was only reduced to 2 h. 4
ACCEPTED MANUSCRIPT The resulting nanopore arrays will have an inter-pore distance of 100 nm with a diameter of approximately 30 nm [34]. To prepare nanopores with larger diameters, the AAO templates were subjected to an etching treatment in 0.3 M phosphoric acid at 30 °C for 10 and 30 min, leading to AAO templates with pore diameters of 42 and 65 nm, respectively [35]. The fabrication of NWs was
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performed using a pulsed ac electrodeposition technique before which the alumina barrier layer at the bottom of nanopores was sufficiently thinned in order to facilitate the electrodeposition process. In this respect, the AAO templates were subjected to an asymmetric anodization by step-wise reducing the
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potential from 40 to 12 V, creating dendritic nanopores at the pores’ bottom [36].
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The fabrication of Ni/Cu multilayered NWs was realized using a single bath method by serving the remaining aluminum of the template as the working electrode and a graphite roll as the counter electrode. However, to evaluate and reach the appropriate composition for the FM/NM segments, four NiCu alloy NW samples were initially prepared by electrodeposition into the 42 nm pore diameter AAO templates. In this regard, an electrochemical bath containing of 0.6 M NiSO4·7H2O with 45 g/l boric
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acid together with Cu electrolyte with a concentration of 0.005 M was employed at 30 °C. The acidity of the bath was set to 3. Using a pulsed ac power supply (SFG-830) with a
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programmable logger, asymmetric sine waveforms with constant reduction/oxidation potentials (Vred/Vox) of 18/12 V and 12/12 V together with pulse times of 5 ms and 200 ms were created in order to
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obtain Ni-rich and Cu-rich NiCu NWs, respectively. The amount of charge passed during the electrodeposition was 1 C. In should be noted that, to reduce the effect of the dendritic structures on the resulting properties, several pulses with Vred/Vox of 12/12 V were applied at the onset of the process, corresponding to the filling of the dendrites with the non-magnetic element Cu. X-ray fluorescence (XRF, equipped with a copper target) analysis was then carried out and the results are presented in Table 1.
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ACCEPTED MANUSCRIPT Table 1. The composition of NiCu alloy NWs using Ni concentration of 0.6 M, determined by XRF analysis.
Ni (at.%) Cu (at.%)
Cu concentration= 0.005 M Vred/Vox 18/12 V 82 18
Vred/Vox 12/12V 16 84
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Element
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As observed, using the selected concentration of Cu electrolyte, Ni-rich and Cu-rich NiCu NWs are fabricated by pulse times of 5 and 200 ms in conjunction with Vred/Vox of 18/12 V and 12/12 V,
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respectively. Subsequently, by alternating Vred/Vox between 18/12 and 12/12 V together with pulse times of 5 and 200 ms, multilayered arrays of Ni/Cu NWs were fabricated into the 42 and 65 nm pore diameter AAO templates. While the Cu-rich segment size was kept constant at each diameter for a potential pulse time of 40 s (i.e., a pulse number of 200), the Ni-rich segment size was varied using successive pulse
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times of 0.125, 0.25, 0.5, 1 and 2 s (corresponding to pulse numbers of 25, 50, 100, 200 and 400). Note that, at each diameter, the total length of NWs composed of the FM layer remained the same for all the selected conditions since the overall pulse number for electrodepositing the Ni segments was set to be
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8000. In Fig. 1(a), using Vred/Vox of 18/12 V and pulse time of 5 ms, the potential-time and current-time curves recorded during the electrodeposition of Ni segment are depicted. From Fig. 1(b), the charge
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passed during the electrodeposition process of multilayered NWs is shown as a function of deposition time. In this case, the potential pulse times for the deposition of Ni and Cu segments were set to be 2 and 40 s, so that 20 FM/NM segments are fabricated with the increase in the charge and deposition time up to 0.42 C and 840 s, respectively.
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Fig. 1. (a) Potential-time and current-time curves recorded during the electrodeposition of Ni segments
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at Vred/Vox of 18/12 V and pulse time of 5 ms. (b) Variation of charged passed during the electrodeposition of Ni/Cu NWs as a function of deposition time using successive potential pulse times of 2 s and 40 s for the deposition of Ni and Cu segments, respectively.
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The morphological properties of resulting multilayered NWs were obtained using field emission scanning electron microscopy (FESEM, MIRA3 TESCAN, using the backscatter mode operating at 15 kV) and transmission electron microscopy (TEM, Zeiss EM10C operating at 80 kV). For FESEM
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measurements, the NW samples were soaked in 0.3 M NaOH solution for 20 min so that the NWs were partially released from the AAO template. For TEM measurements, the AAO template was completely
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dissolved in 0.3 M NaOH during 45 min at room temperature and rinsed with double-distilled water. After removing the remaining Al at the back of the NW samples using a saturated solution of CuCl2, the crystalline characteristics were investigated by X-ray diffraction (XRD; Philips, model X’PertPro; Cu Kα radiation) with λ= 0.154 nm. On the other hand, the magnetic properties were studied using a vibrating sample magnetometer (VSM, Meghnatis Daghigh Kavir Co, Iran) at room temperature for a magnetic field applied parallel and perpendicular to the NW axis up to 3.5 kOe.
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ACCEPTED MANUSCRIPT 3. Results and discussion 3.1 Morphological properties Arrays of multilayered NWs were fabricated in 42 and 65 nm pore diameter AAO templates by
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modulating Vred/Vox between 18/12 and 12/12 V together with pulse times of 5 and 200 ms in order to obtain Ni-rich and Cu-rich Ni/Cu NWs, respectively. Additionally, with increasing potential pulse time between 0.125 and 4 s when electrodepositing Ni, different segment sizes are expected to form. To
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generally investigate the morphologies of multilayered Ni/Cu NWs, FESEM images were obtained. Fig. 1 shows the cross-sectional view of Ni/Cu NWs obtained with the potential pulse time of 1 s and 40 s for
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the deposition of Ni and Cu segments, respectively, partially released from the AAO template with pore diameters of 42 nm and 65 nm. The resulting NWs with a diameter of 42 nm show no clear segmented structure (Fig 1(a)). However, with the increase in diameter from 42 to 65 nm, the multilayered Ni/Cu NWs can be observed by FESEM (see Fig. 2(b)). The thickness of Ni segment is found to be
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approximately 65 nm using the pulse time of 1 s. On the other hand, to carefully investigate the segment sizes, TEM analysis was employed. Fig. 3 shows bright field TEM images of 65 nm diameter Ni/Cu NWs obtained with the pulse times of 0.5 and 2 s for the deposition of Ni segments. As observed, the
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resulting NWs have a smooth surface with uniform segments along the length. While the thickness of Cu segment is approximately 20 nm, increasing pulse time from 0.5 to 2 s increases the Ni segment
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thickness from 35 to 110 nm. In this way, Table 2 represents the thickness of Ni segments and the corresponding thickness/diameter ratio (T/D) for the samples in study using different pulse times. Basically, increasing the potential pulse time increases the thickness of Ni segment, thereby increasing the ratio of T/D. In turn, this causes the transformation of disk-shaped segments (0 < T/D < 1) to rodshaped ones (1 < T/D < 5) and/or to near wire-shaped segments (5 < T/D < 10) along the length of NWs.
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ACCEPTED MANUSCRIPT As a result, one could expect these changes in the morphology to be reflected in the corresponding crystalline characteristics and magnetic properties of multilayered Ni/Cu NWs due to the changes in the
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segment lengths and shape anisotropy contribution [32, 37], which will be discussed in the next sections.
Fig. 2. Cross sectional view FESEM images obtained from multilayered Ni/Cu NWs partially released
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from AAO templates with pore diameters of (a) 42 nm and (b) 65 nm. The potential pulse time for the deposition of Ni segment was 1 s.
Table 2. Ni segment thickness and corresponding thickness/diameter ratio (T/D) of Ni/Cu NWs
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obtained from different potential pulse times.
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Pulse time (s)
Diameter= 42 nm
Thickness (nm)
T/D
Diameter= 65 nm
Thickness (nm)
T/D
0.125
25
0.6
10
0.15
0.25
50
1.2
20
0.30
0.5
90
2.1
35
0.54
1
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4.3
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0.95
2
280
6.7
110
1.7
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Fig. 3. Bright field TEM images obtained from multilayered Ni/Cu NWs with a diameter of 65 nm using successive potential pulse times of (a) 0.5 s and (b) 2 s, for the deposition of Ni segment. The pulse time
rod-shaped segments.
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3.2 Crystalline characteristics
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for the deposition of Cu was constant to 40 s. The arrows indicate the semi-axes of the disk-shaped and
Initially, using XRD analysis, the crystalline characteristics of 42 nm diameter NiCu alloy NWs were
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investigated and the results are shown in Fig. 4. In the case Vred/Vox of 18/12 V with pulse time= 5 ms using Cu concentration of 0.005 M, Ni-rich NiCu (Ni85Cu15) NWs show only a face-centered cubic (fcc) structure with a preferential direction along [111], as can be seen in Fig. 4(a). For Cu-rich NiCu (Ni25Cu75) NWs (obtained using Vred/Vox and pulse time of 12/12 V; 200 ms), the XRD pattern in Fig. 4(b) indicates the formation of Cu (111)-fcc and Cu (220)-fcc peaks with enhanced crystallinity compared to that of the Ni-rich NiCu NWs. Consequently, the XRD patterns of Ni-rich and Cu-rich alloy NWs show reflections related only to the crystalline structure of Ni and Cu, respectively.
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Fig. 4. XRD patterns obtained from NiCu alloy NWs using Vred/Vox and pulse time of: (a) 18/12 V; 5
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ms; (b) 12/12 V; 200 ms.
Using Cu concentrations of 0.005 M, the XRD patterns obtained from 42 nm diameter Ni/Cu multilayered NWs with a Cu segment thickness of 50 nm (estimated from TEM images) and Ni segment thicknesses of 25, 90 and 280 nm are depicted in Fig. 5. For Ni25 nm/Cu50 nm NWs obtained by Cu
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concentration of 0.005 M, the corresponding XRD pattern in Fig. 5(a) shows the presence of both Ni-fcc and Cu-fcc so that no alloy peaks of Ni and Cu are observed, indicating the formation of multilayered structures with high purity FM/NM layers. Increasing Ni segment thickness up to 280 nm causes the
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reflections of Ni structure to dominate those of Cu while also increasing the crystallinity of Ni segments. Moreover, the growth of 42 nm diameter Ni and Cu segments with the same crystallographic textures
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may induce the formation of superlattice Ni/Cu NWs [37]. Increasing diameter up to 65 nm, the XRD patterns of Ni10 nm/Cu20 nm, Ni35 nm/Cu20 nm and Ni110 nm/Cu20 nm NWs are shown in Fig. 5b. The fcc structure of Ni and Cu segments are observed, indicating that the increased diameter has no considerable effect on the growth orientation and crystalline characteristics of Ni/Cu NWs. However, due to the small thickness of Ni segment (10 nm), the incorporation of Cu (with a segment thickness of 20 nm) into the fcc structure of Ni has induced a shift in the Ni (111) and (220) peak positions to lower angles compared to Ni (111) and (220) peaks of pure Ni NWs [38].
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Fig. 5. XRD patterns of Ni/Cu multilayered NWs with variable segment sizes for NW diameters of (a) 42 nm and (b) 65 nm.
The average grain sizes of the Ni/Cu NWs were also evaluated by the Scherrer equation. As a general
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trend, it was found that increasing the thickness of Ni segment decreases the corresponding average grain size along the [111] direction. For example, while the grain size of Ni25 nm/Cu50 nm NWs was estimated to be 14 nm, increasing the Ni segment thickness to 90 nm decreased the grain size to
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approximately 8 nm.
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3.3 Magnetic properties
As mentioned previously, with increasing pulse time when electrodepositing Ni at each NW diameter, magnetic properties of the resulting Ni/Cu NWs with disk-shaped, rod-shaped and/or near wire-shaped segments are expected to differ from each other [22, 32]. This arises due to the influence of the shape of the material on the demagnetizing field. This field, originated from the inside of the material, tends to demagnetize the magnetization, depending on the magnitude of the demagnetizing factors [32]. These factors along the semi-axes of a typical ellipsoid, namely Na, Nb and Nc (with c ≥ b ≥ a), satisfy the relation Na + Nb + Nc = 4π. 12
ACCEPTED MANUSCRIPT While an oblate spheroid could be approximated to the disk-shaped segments (where c = b > a; Fig. 3(a)), a prolate ellipsoid with a circular cross section could represent the rod-shaped and/or near wireshaped segments (where c > b = a; see Fig. 3(b)). For an oblate spheroid, the axial demagnetizing factor (Na) and perpendicular demagnetizing factors
)
1/ 2 2 A − 1 A 1 N a = 4π 2 × 1 − × arcsin 1/ 2 A A −1 A2 − 1 2
(
)
(
(1)
)
1/ 2 A2 − 1 A2 N b = N c = 4π × × arcsin − 1 2 1/ 2 A 2( A − 1) A2 − 1
1
)
(2)
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(
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(
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(Nb = Nc) are given by [32]:
where A is the aspect ratio (A= c/a).
Alternatively, for a prolate ellipsoid, the axial demagnetizing factor (Nc) and perpendicular
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demagnetizing factors (Na = Nb) are obtained by Eqs. (3) and (4) as follows [32]:
( (
) )
1/ 2 2 A A + A −1 N c = 4π 2 × × ln 1/ 2 A − 1 2 A2 − 1 1/ 2 A − A2 − 1
1
)
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(
− 1
( (
(3)
) )
1/2 2 1 A + A − 1 ×A− × ln Na = Nb = 4π 1/2 2 ( A2 − 1) 2( A2 − 1)1/2 A − A2 − 1
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A
(4)
Here, the demagnetizing factors of Ni segments (Na, Nb and Nc) were calculated from the above equations (depending on the segment geometry) and the results obtained are presented in Table 3.
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ACCEPTED MANUSCRIPT Table 3. The demagnetizing factors of Ni segments with diameters of 42 and 65 nm for different potential pulse times. Pulse time (s)
Diameter= 42 nm Nb
Nc
A= c/a
Na
Nb
Nc
0.125
1.68
6.05
3.25
3.25
6.50
9.98
1.25
1.25
0.25
1.19
4.45
4.45
3.63
3.25
8.32
2.16
2.16
0.5
2.14
5.68
5.68
2.10
1.86
6.45
3.06
3.06
1
4.28
5.93
5.93
0.65
1.05
1.19
3.90
3.90
2
6.67
6.06
6.06
0.46
1.69
4.97
4.97 2.56
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Na
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A= c/a
Diameter= 65 nm
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As expected, increasing A in the disk-shaped segments increases the demagnetizing factor along the NW axis (Na). In other words, the shape hard axis has a tendency to lie along the basal plane of diskshaped segments when increasing A, thereby inducing a demagnetizing field. On the other hand, in the case of rod-shaped and/or near wire-shaped segments, increasing A decreases Nc, thereby inducing a
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magnetizing field due to the shape easy axis along the NW axis. Accordingly, depending on the direction of the magnetic field applied (axial and/or perpendicular), the corresponding magnetic properties of the segments with variable sizes are expected to change due to the magnetizing and/or
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demagnetizing effects. In this respect, the magnetic properties of Ni/Cu multilayered NWs with different segment sizes were obtained at room temperature using axial and perpendicular hysteresis loop
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measurements and the results are shown in Figs. 6 and 7. Figures 6(a)–(c) show the hysteresis loops of 42 nm diameter Ni/Cu NWs with the disk-shaped to rod-shaped and/or near wire-shaped segments obtained using pulse times of 0.125, 0.5 and 2s, respectively. As inferred from the corresponding axial coercivity values presented in Fig. 7a, increasing pulse time from 0.125 to 2 s (thus increasing the Ni segment thickness from 25 to 280 nm) increases the coercivity from 250 to 820 Oe, demonstrating a significant enhancement in the axial coercivity by approximately
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Na= 6.05 to Nc= 0.46), as can be seen in Table 3.
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Fig. 6. Axial and perpendicular hysteresis loops of 42 nm and 65 nm diameter Ni/Cu NW arrays using pulse times of (a),(d) 0.125 s, (b),(e) 0.5 s and (c),(f) 2 s.
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In the case of 75 nm diameter Ni/Cu NWs, one can find the same variation behavior of coercivity with increasing Ni segment thickness from 10 to 110 nm. Note that, with the increase in diameter, the
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demagnetizing factors along the NW length increase, thereby decreasing the coercivities of 65 nm diameter Ni/Cu NWs compared to those of 42 nm diameter as a function of pulse time. For example, in the case of rod-shaped segments obtained using the pule time of 2 s, increasing Nc from 0.46 to 2.56 decreases the coercivity of 65 nm diameter Ni/Cu NWs to 650 Oe, starting from 820 Oe for 42 nm diameter NWs.
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ACCEPTED MANUSCRIPT On the other hand, increasing pulse time between 0.125 and 2 s, the corresponding axial squareness values for 42 and 75 nm diameter NWs increase from 0.5 to 0.8 and 0.3 to 0.55, respectively (see Figs. 7(a) and (b)). This is indicative of enhanced magnetizing magnetostatic interactions between the Ni segments (the intrawire interactions) when increasing the thickness of the FM layer, inducing a
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magnetizing field in the axial direction [39, 40].
Fig. 7. Variations of axial and perpendicular coercivity and squareness in (a),(c) 42 nm and (b),(d) 65 nm diameter Ni/Cu NW arrays as a function of pulse time. It is worthy to note that the intrawire interactions in multilayered NWs could be divided into intrawire interactions between the single segments and the demagnetizing fields of these segments [39].
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ACCEPTED MANUSCRIPT On the contrary, for the perpendicularly magnetized Ni/Cu NWs with diameters of 42 and 75 nm, increasing pulse time (thus increasing the perpendicular demagnetizing factors) has no considerable effect on the perpendicular coercivity and squareness values, according to the hysteresis loops in Fig. 6
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and the quantitative results in Fig. 7. Beron et al. [40] investigated the magnetic behavior of Ni/Cu multilayered NW arrays using the firstorder reversal curve (FORC) analysis for a magnetic field applied axial and perpendicular to the long axis of NWs. From their results, decreasing the Cu thickness to Ni thickness ratio (TCu/TNi) increased the
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reversible component of magnetization for perpendicularly magnetized NWs. This coincided with a
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significant increase in the intrawire demagnetizing interactions, leading to a nearly coherent rotation [40]. Here, increasing the pulse time between 0.125 and 2 s decreases TCu/TNi ratio from 2 to 0.18 for both 42 and 65 nm diameter Ni/Cu NWs, inducing an increase in the reversible component as well as in the intrawire interactions. Meanwhile, the shape demagnetizing field (in the perpendicular direction) decreases due to the increase in the Ni segment thickness. Altogether, the competition between the
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intrawire interactions and the shape demagnetizing field causes the magnetic properties in the perpendicular direction to remain almost the same. Particularly, this is realized by the fact that the magnetocrystalline anisotropy, surface anisotropy and exchange interactions are also negligible in the
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case of segmented Ni NWs [39]. Therefore, with increasing the Ni segment thickness, the enhanced demagnetizing factors do not contribute to the perpendicular magnetic behavior of Ni/Cu NWs with
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disk-shaped and/or rod-shaped segments. In addition to the demagnetizing factors, another possible aspect which should be taken into consideration is the effectiveness of segment size on the coercivity. In fact, regardless of the direction of applied magnetic field, increasing the segment thickness is expected to increase the coercivity in single domain state (see Fig. 11.2 in Ref. [41]). As predicted by Sun et al. [32], Ni segments with T/D ≤ 10 are single domain for diameters less than 600 nm. In the present case, the Ni segments are then considered to be single domain for different pulse times, according to Table 2. As a result, while increasing the
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ACCEPTED MANUSCRIPT pulse time is expected to decrease the coercivity in the perpendicular direction (due to the increased demagnetizing factors), the enhancement in the thickness of single domain segments (thus increasing coercivity) has nullified the effectiveness of demagnetizing factors, as can be seen in Figs. 7(a) and (b). Consequently, one can tune the magnetic properties of the multilayered NWs with different diameters in
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the axial direction with increasing only the Ni segment thickness. In particular, this anisotropic behavior of magnetic properties may find functionalities in high density perpendicular magnetic recording media
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and high quality magnetic field sensors [6, 17].
4. Conclusions
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In summary, using the pulsed ac electrodeposition in a single bath, arrays of Ni/Cu multilayered NWs with diameters of 42 and 75 nm were fabricated into AAO templates. Increasing potential pulse time for electrodepositing Ni between 0.125 and 2 s increased the Ni segment thickness from 25 to 280 nm and 10 to 110 nm in 42 nm and 65 nm diameter NWs, respectively. This enabled the fabrication of
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multilayered NWs with disk-shaped, rod shaped and/or near wire-shaped FM segments. Investigating the crystalline characteristics, XRD patterns indicated the formation of highly pure multilayers of Ni and Cu with fcc crystalline structure. Moreover, it was found that increasing diameter had no considerable
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influence on the growth orientation and crystalline characteristics of Ni/Cu NWs. Regarding the magnetic properties, the axial coercivity increased up to 820 and 650 Oe for 42 and 65 nm diameter
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NWs, starting from 250 and 180 Oe, respectively. This was attributed to the significant reduction in the demagnetizing factors along the NW axis when increasing Ni segment thickness, being consistent with the calculated demagnetization factors for oblate spheroid and prolate ellipsoid. Concurrently, due to the enhanced magnetizing magnetostatic interactions, the axial squareness increased up to 0.8 and 0.55. In contrast, increasing pulse-time did not affect the magnetic properties including the coercivity and squareness of the perpendicularly magnetized multilayered NWs.
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ACCEPTED MANUSCRIPT Acknowledgements The authors gratefully acknowledge the University of Kashan for providing the financial support of this work by Grant No. 159023/38.
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Fabrication of multilayered Ni/Cu nanowires by a pulsed alternating current method. Using designated pulse times, segments with different geometries were created. Investigation of morphologies, crystalline characteristics and magnetic properties. Obtaining adjustable magnetic coercivity and squareness in the axial direction. Correlating the axial magnetic properties with calculated demagnetizing factors.
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• • • • •