- Email: [email protected]
Cu nanowire array
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 304 (2006) e213–e215 www.elsevier.com/locate/jmmm
Synthesis and magnetic anisotropy of multilayered Co/Cu nanowire array Ji Ung Cho, Qun Xian Liu, Ji Hyun Min, Seung Pil Ko, Young Keun Kim Department of Materials Science and Engineering, Korea University, Seoul 136-713, Republic of Korea Available online 6 March 2006
Abstract The Co and multilayered Co/Cu nanowire arrays were fabricated by pulsed DC electrodeposition using anodized aluminum oxide (AAO) templates. Each nanowire has the length of 20 mm and the diameter of 200 nm. The layer thickness of Co layer was slightly decreased even if the deposition time was fixed as Cu deposition time increased due to the metal exchange. Unlike the Co nanowire array case, multilayered Co/Cu nanowire array exhibited an easy magnetization direction perpendicular to the nanowire axis due to the separation of Co layers. r 2006 Elsevier B.V. All rights reserved. PACS: 81.07.Bc; 81.40.Rs Keywords: Co/Cu multilayered nanowire; Electrodeposition; Magnetization direction
Metallic nanowires show interesting magnetic, superconducting and magnetotransport properties [1]. A template-based electrodeposition technique appears convenient and cost-effective to obtain nanowires. When the pores of nanoporous membranes are filled by alternating ferromagnetic metal and nonmagnetic metal layers, an array of multilayered nanowires with various lengths can be achieved. The layer interfaces in the nanowires are perpendicular to their long axis, making them suited for the current-perpendicular-to-the-plane magnetotransport measurement [2]. In the present study, we describe a procedure for multilayered Co/Cu nanowire preperation by the DC electrodeposition and evaluation of their magnetic characteristics such as anisotropy. The 60-mm-thick anodized aluminum oxide (AAO) templates possessing pore diameters of 200 nm were used. Before electrodeposition, one side of the AAO was evaporated with a gold layer that served as a working electrode. To fabricate Co/Cu multilayered nanowires, the bath consisted of CoSO4d7H2O and CuSO4d5H2O at the concentration of 1 and 0.025 M, respectively, was used [3]. Corresponding author. Tel.: +82 2 3290 3899; fax: +82 2 928 3584.
E-mail address: [email protected] (Y.K. Kim). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.02.034
The above-mentioned chemicals were first dissolved by deionized water, where the solution pH was adjusted to maintain at 3.0 by adding dilute H2SO4. A piece of 0.2mm-thick Cu sheet was used as a counter electrode. The electrodeposition was carried out under cathodic pulses of 40 and 0.5 mA/cm2 for a variety of durations by a Keithly 2400 power station. All the deposition processes were carried out in an ambient condition and without stirring. The microstructure and magnetic properties of nanowires embedded into the AAO template, i.e. an array state, were characterized by X-ray diffraction (XRD) and vibrating sample magnetometry (VSM), respectively. For compositional analysis, both line-scanning and element mapping methods by transmission electron microscopy (TEM) were employed. In this case, nanowires were dispersed on a gold mesh grid after removing the template selectively by immersing the nanowire-containing AAO template into a 1 M NaOH solution for 30 min at 300 K. As shown in Fig. 1, we were able to observe clear separations between Co and Cu layers in a single wire state using TEM linescan and mapping. In addition, a real thickness of each Co/Cu layer was detected. The current density and time for Co deposition were 40 mA/cm2 and
ARTICLE IN PRESS e214
J.U. Cho et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e213–e215
Cu
180
layer thickness (nm)
Co 150 Co : 40 mA / cm2 (fixed to 4 sec)
120
Cu : 0.5 mA / cm2 (variable) 90
60
30
4 s, and 0.5 mA/cm2 and 60 s for Cu, respectively. When the low current was applied only Cu was deposited, while in high current case Co with some Cu inclusion was inevitable as manifested by the inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurement: the concentration of Co nanowires was more than 90 wt% Co. The thickness of Co layer was decreased as Cu deposition time increased though a fixed Co deposition condition as shown in Fig. 2. A longer Cu pulse means more Co would be sacrificed by the metal exchange reaction [4] because Cu is a more noble metal than Co. It means that longer Cu deposition causes higher Co dissolution into an electrolyte during the Cu deposition time. Note that the Co and Cu layer thicknesses did not linearly change with the deposition current. The metal exchange between Cu and Co in nanowire appears different from that in multilayered films due to the nanopore confinement. We verified the condition of Co and Cu reduction, and confirmed corresponding charge consumption and thickness of reduced metals. Therefore, we were able to tailor-design Co/Cu nanowires with desired thicknesses in a reproducible manner by controlling deposition time and current density. The XRD analysis on the multilayered Co/Cu array showed that this material possessed polycrystalline structure without a preferred crystal orientation as displayed in Fig. 3. The microstructure of the Co layer was randomly oriented face-centered-cubic (FCC) phase. It means that the magnetic anisotropy of these Co layers will be affected primarily by shape anisotropy. Because the aspect ratio of each Co layer in the multilayered nanowire array is less than one, the easy magnetization direction is expected to be in-plane (i.e. perpendicular to wire axis). Magnetic properties of multilayered Co/Cu nanowire array were analyzed by VSM with magnetic fields applied
40 45 50 Cu Pulse time (sec)
55
60
Fig. 2. Variation of layer thicknesses of Co and Cu with respect to the Cu pulse time while Co deposition condition was fixed.
200 Intensity (Count per sec)
Fig. 1. Electrodeposited Co–Cu/Cu multilayers deposited in the AAO membrane: (a) TEM linescan image, and (b) and (c) elemental mappings of Co and Cu, respectively.
35
30
Cu(111) Co_fcc(111)
180
Co_fcc(200)
160
Co_fcc(311) Cu(311)
Cu(200)
Cu(200)
140
Co_fcc(200) 120 100 80 40
50
60
70 2θ
80
90
100
Fig. 3. XRD analysis of multilayered Co/Cu nanowire array where Co and Cu layer thicknesses were 145 and 80 nm, respectively.
both perpendicular and parallel directions to the nanowire axis. As shown in Fig. 4, multilayered nanowire arrays exhibited an easy magnetization direction perpendicular to the nanowire axis (layer in-plane) as expected. Meanwhile the pure Co nanowire array had an easy direction parallel to the wire axis (layer out-of-plane). In the case of a Co nanowire array, the easy direction was developed axially due to the high aspect ratio. This was not apparent for the multilayered Co/Cu nanowire array due to the fact that ferromagnetic Co layers were separated by nonmagnetic Cu layers. As a conclusion, pure Co and Co/Cu nanowire arrays have been fabricated by employing nanoporous templates and electrodeposition. Desired layer thicknesses were achieved by the precise control of current density and deposition time. Pure Co and multilayered Co/Cu nanowire
ARTICLE IN PRESS J.U. Cho et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e213–e215
arrays possessed layer out-of-plane and in-plane magnetic anisotropies, respectively.
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M (emu)
0.4
e215
In - plane Out - of - plane
This work was supported by the Korea Research Foundation Grant KRF-2004-005-D00057, by the Brain Pool Program 032-2-1 of the Korean Federation of Science and Technology Societies, by the Grant A050750 of the Korea Health 21 R and D Project, Ministry of Health and Welfare, and by Grant M10500000105-05J0000-10510 from the National Research Laboratory Program of the Korea Science and Engineering Foundation.
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[1] W. Wernsdorfer, B. Doudin, D. Mailly, K. Hasselbach, A. Benoit, J. Meier, J.-Ph. Ansermet, B. Barbara, Phys. Rev. Lett. 77 (1996) 1873. [2] S. Dubois, C. Marchal, J.M. Beuken, L. Piraux, Appl. Phys. Lett. 70 (1997) 396. [3] L. Piraux, J.M. George, J.F. Despres, C. Leroy, E. Ferain, R. Legras, Appl. Phys. Lett. 65 (1994) 2484. [4] Q.X. Liu, L. Pe´ter, J. To´th, L.F. Kiss, A´. Czira´ki, I. Bakonyi, J. Magn. Magn. Mater. 280 (2004) 60.
0.8
M (emu)
0.4
In - plane Out - of - plane
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-0.4
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(b)
-5000
0 H (Oe)
References
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Fig. 4. Hysteresis loops of (a) Co and (b) multilayered Co/Cu nanowire arrays. The diameter and length were 200 nm and 20 mm, respectively.
Journal of Magnetism and Magnetic Materials 304 (2006) e213–e215 www.elsevier.com/locate/jmmm
Synthesis and magnetic anisotropy of multilayered Co/Cu nanowire array Ji Ung Cho, Qun Xian Liu, Ji Hyun Min, Seung Pil Ko, Young Keun Kim Department of Materials Science and Engineering, Korea University, Seoul 136-713, Republic of Korea Available online 6 March 2006
Abstract The Co and multilayered Co/Cu nanowire arrays were fabricated by pulsed DC electrodeposition using anodized aluminum oxide (AAO) templates. Each nanowire has the length of 20 mm and the diameter of 200 nm. The layer thickness of Co layer was slightly decreased even if the deposition time was fixed as Cu deposition time increased due to the metal exchange. Unlike the Co nanowire array case, multilayered Co/Cu nanowire array exhibited an easy magnetization direction perpendicular to the nanowire axis due to the separation of Co layers. r 2006 Elsevier B.V. All rights reserved. PACS: 81.07.Bc; 81.40.Rs Keywords: Co/Cu multilayered nanowire; Electrodeposition; Magnetization direction
Metallic nanowires show interesting magnetic, superconducting and magnetotransport properties [1]. A template-based electrodeposition technique appears convenient and cost-effective to obtain nanowires. When the pores of nanoporous membranes are filled by alternating ferromagnetic metal and nonmagnetic metal layers, an array of multilayered nanowires with various lengths can be achieved. The layer interfaces in the nanowires are perpendicular to their long axis, making them suited for the current-perpendicular-to-the-plane magnetotransport measurement [2]. In the present study, we describe a procedure for multilayered Co/Cu nanowire preperation by the DC electrodeposition and evaluation of their magnetic characteristics such as anisotropy. The 60-mm-thick anodized aluminum oxide (AAO) templates possessing pore diameters of 200 nm were used. Before electrodeposition, one side of the AAO was evaporated with a gold layer that served as a working electrode. To fabricate Co/Cu multilayered nanowires, the bath consisted of CoSO4d7H2O and CuSO4d5H2O at the concentration of 1 and 0.025 M, respectively, was used [3]. Corresponding author. Tel.: +82 2 3290 3899; fax: +82 2 928 3584.
E-mail address: [email protected] (Y.K. Kim). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.02.034
The above-mentioned chemicals were first dissolved by deionized water, where the solution pH was adjusted to maintain at 3.0 by adding dilute H2SO4. A piece of 0.2mm-thick Cu sheet was used as a counter electrode. The electrodeposition was carried out under cathodic pulses of 40 and 0.5 mA/cm2 for a variety of durations by a Keithly 2400 power station. All the deposition processes were carried out in an ambient condition and without stirring. The microstructure and magnetic properties of nanowires embedded into the AAO template, i.e. an array state, were characterized by X-ray diffraction (XRD) and vibrating sample magnetometry (VSM), respectively. For compositional analysis, both line-scanning and element mapping methods by transmission electron microscopy (TEM) were employed. In this case, nanowires were dispersed on a gold mesh grid after removing the template selectively by immersing the nanowire-containing AAO template into a 1 M NaOH solution for 30 min at 300 K. As shown in Fig. 1, we were able to observe clear separations between Co and Cu layers in a single wire state using TEM linescan and mapping. In addition, a real thickness of each Co/Cu layer was detected. The current density and time for Co deposition were 40 mA/cm2 and
ARTICLE IN PRESS e214
J.U. Cho et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e213–e215
Cu
180
layer thickness (nm)
Co 150 Co : 40 mA / cm2 (fixed to 4 sec)
120
Cu : 0.5 mA / cm2 (variable) 90
60
30
4 s, and 0.5 mA/cm2 and 60 s for Cu, respectively. When the low current was applied only Cu was deposited, while in high current case Co with some Cu inclusion was inevitable as manifested by the inductively coupled plasma atomic emission spectroscopy (ICP-AES) measurement: the concentration of Co nanowires was more than 90 wt% Co. The thickness of Co layer was decreased as Cu deposition time increased though a fixed Co deposition condition as shown in Fig. 2. A longer Cu pulse means more Co would be sacrificed by the metal exchange reaction [4] because Cu is a more noble metal than Co. It means that longer Cu deposition causes higher Co dissolution into an electrolyte during the Cu deposition time. Note that the Co and Cu layer thicknesses did not linearly change with the deposition current. The metal exchange between Cu and Co in nanowire appears different from that in multilayered films due to the nanopore confinement. We verified the condition of Co and Cu reduction, and confirmed corresponding charge consumption and thickness of reduced metals. Therefore, we were able to tailor-design Co/Cu nanowires with desired thicknesses in a reproducible manner by controlling deposition time and current density. The XRD analysis on the multilayered Co/Cu array showed that this material possessed polycrystalline structure without a preferred crystal orientation as displayed in Fig. 3. The microstructure of the Co layer was randomly oriented face-centered-cubic (FCC) phase. It means that the magnetic anisotropy of these Co layers will be affected primarily by shape anisotropy. Because the aspect ratio of each Co layer in the multilayered nanowire array is less than one, the easy magnetization direction is expected to be in-plane (i.e. perpendicular to wire axis). Magnetic properties of multilayered Co/Cu nanowire array were analyzed by VSM with magnetic fields applied
40 45 50 Cu Pulse time (sec)
55
60
Fig. 2. Variation of layer thicknesses of Co and Cu with respect to the Cu pulse time while Co deposition condition was fixed.
200 Intensity (Count per sec)
Fig. 1. Electrodeposited Co–Cu/Cu multilayers deposited in the AAO membrane: (a) TEM linescan image, and (b) and (c) elemental mappings of Co and Cu, respectively.
35
30
Cu(111) Co_fcc(111)
180
Co_fcc(200)
160
Co_fcc(311) Cu(311)
Cu(200)
Cu(200)
140
Co_fcc(200) 120 100 80 40
50
60
70 2θ
80
90
100
Fig. 3. XRD analysis of multilayered Co/Cu nanowire array where Co and Cu layer thicknesses were 145 and 80 nm, respectively.
both perpendicular and parallel directions to the nanowire axis. As shown in Fig. 4, multilayered nanowire arrays exhibited an easy magnetization direction perpendicular to the nanowire axis (layer in-plane) as expected. Meanwhile the pure Co nanowire array had an easy direction parallel to the wire axis (layer out-of-plane). In the case of a Co nanowire array, the easy direction was developed axially due to the high aspect ratio. This was not apparent for the multilayered Co/Cu nanowire array due to the fact that ferromagnetic Co layers were separated by nonmagnetic Cu layers. As a conclusion, pure Co and Co/Cu nanowire arrays have been fabricated by employing nanoporous templates and electrodeposition. Desired layer thicknesses were achieved by the precise control of current density and deposition time. Pure Co and multilayered Co/Cu nanowire
ARTICLE IN PRESS J.U. Cho et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e213–e215
arrays possessed layer out-of-plane and in-plane magnetic anisotropies, respectively.
0.8
M (emu)
0.4
e215
In - plane Out - of - plane
This work was supported by the Korea Research Foundation Grant KRF-2004-005-D00057, by the Brain Pool Program 032-2-1 of the Korean Federation of Science and Technology Societies, by the Grant A050750 of the Korea Health 21 R and D Project, Ministry of Health and Welfare, and by Grant M10500000105-05J0000-10510 from the National Research Laboratory Program of the Korea Science and Engineering Foundation.
0.0
-0.4
-0.8 -6000
-4000
-2000
(a)
0 H (Oe)
2000
4000
6000
[1] W. Wernsdorfer, B. Doudin, D. Mailly, K. Hasselbach, A. Benoit, J. Meier, J.-Ph. Ansermet, B. Barbara, Phys. Rev. Lett. 77 (1996) 1873. [2] S. Dubois, C. Marchal, J.M. Beuken, L. Piraux, Appl. Phys. Lett. 70 (1997) 396. [3] L. Piraux, J.M. George, J.F. Despres, C. Leroy, E. Ferain, R. Legras, Appl. Phys. Lett. 65 (1994) 2484. [4] Q.X. Liu, L. Pe´ter, J. To´th, L.F. Kiss, A´. Czira´ki, I. Bakonyi, J. Magn. Magn. Mater. 280 (2004) 60.
0.8
M (emu)
0.4
In - plane Out - of - plane
0.0
-0.4
-0.8 -10000
(b)
-5000
0 H (Oe)
References
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-10000
Fig. 4. Hysteresis loops of (a) Co and (b) multilayered Co/Cu nanowire arrays. The diameter and length were 200 nm and 20 mm, respectively.