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The pH and current density dependence of DC electrodeposited CoCu thin films
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 304 (2006) e100–e102 www.elsevier.com/locate/jmmm
The pH and current density dependence of DC electrodeposited CoCu thin films Ji Hyun Mina, Jun-Hua Wub, Ji Ung Choa, Qun Xian Liub, Ju Hun Leea, Young-Dong Kob, Jin-Seok Chungc, Jae-Ho Leed, Young Keun Kima, a
Department of Materials Science and Engineering, Korea University, Seoul 136-713, Korea Research Institute of Engineering and Technology, Korea University, Seoul 136-713, Korea c AMCRCD and Department of Physics, Soongsil University, Seoul 156-743, Korea d Department of Materials Science and Engineering, Hongik University, Seoul 121-791, Korea b
Available online 3 March 2006
Abstract CoCu thin films were fabricated on a nanocrystalline substrate by DC electrodeposition. It is found that the composition, structure and magnetic properties of the thin films exhibit a strong dependence on the current density and pH values of the bath electrolyte. The effect of annealing as well as the structure–property relation was investigated. r 2006 Elsevier B.V. All rights reserved. PACS: 81.15.Pq; 82.65.+r; 75.70.Rf; 68.55.a Keywords: CoCu alloy; Thin films; Electrodeposition; Structure; Magnetic property; Annealing
The fabrication of thin film multilayers or granular films using electrodeposition has been of great interest for magnetic and magneto-transport studies [1–4]. During the electrodeposition process, pH and current density are two key processing variables that determine physical properties of the deposited films. The Co/Cu multilayer system has been extensively investigated for the effect of giant magnetoresistance (GMR) [5] and a CoCu alloy film fabricated by electrodeposition provides an economic alternative for potential GMR applications such as magnetic sensors [1–4]. In a previous research, we studied electrodeposited CoCu alloy films and reported their surface morphology dependent on pH values of the bath and their composition on both pH values and current densities [6]. In this paper, we focus on the dependence of the composition, structure and magnetic properties of the DC electrodeposited CoCu thin films on both bath pH and current density, and investigate their structure-property relation. In addition, the effect of annealing is studied. Corresponding author. Tel.: +82 2 3290 3281; 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.2006.01.191
The CoCu thin films were fabricated on the nanocrystalline FinemetTM substrates. The deposition bath comprised CoSO4 (1 M) and CuSO4 (0.025 M), without any other buffering agent or additives. The electrolyte pH was adjusted from 2.5 to 5.0 by adding sulfuric acid. A Cu sheet was used as the counterelectrode. The electrodeposition process was conducted at ambient temperature without stirring. The composition of the thin films was investigated in terms of deposition parameters, i.e., current density and electrolyte pH. The deposited films were analyzed by scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS) and inductively coupled plasma atomic emission spectroscopy (ICP-AE). The microstructure was characterized by X-ray diffraction (XRD), and magnetic properties were measured using a vibrating sample magnetometer (VSM), respectively. The annealing of films was performed under a condition of 450 1C for 20 min in a vacuum of 106 Torr. The morphology of the resultant films studied with SEM presents a particulate surface, similar to our previous observation [6]. The crystal structure was investigated by XRD, as shown in Fig. 1 for the films fabricated at pH ¼ 3
ARTICLE IN PRESS J.H. Min et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e100–e102
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Table 1 Parameters derived from the XRD patterns Sample
Peak type
Peak position (1)
FWHM (1)
Grain size (nm)
Pure Cu
Cu (1 1 1)
43.36
0.11
75
Cu76Co24
CuCo alloy Co fcc(1 1 1)
43.44 44.34
0.14 0.27
62 32
Cu54Co46
CuCo alloy Co fcc(1 1 1)
43.47 44.22
0.20 0.37
43 23
Fig. 1. XRD patterns at pH ¼ 3 for three current densities of (A) 5 mA/cm2, (B) 2 mA/cm2, and (C) 1 mA/cm2.
with 3 current densities, 1, 2 and 5 mA/cm2. At the current density of 1 mA/cm2, only (1 1 1) and (2 0 0) of cubic Cu is observed, indicating the formation of pure Cu film under the deposition condition. As the current density increases to 2 and 5 mA/cm2, the second set of peaks representing FCC phase of Co appears. The analysis shows the peak positions shift with the current densities. As shown in the inset, Cu (1 1 1) peaks move to high angles, whereas Co (0 0 2) peaks move to low angle as the current density increases. The phenomenon could be attributed to the formation of CoCu alloys and/or the effect of stress created during the deposition. As discussed below, the observation that annealing does not lead to any change of peak positions implies the shifting probably arise from the alloy formation, rather than stress. In this case, the alloy composition can be derived according to the Vergard’s rule [7]. The grain size was analyzed by Scherrer’s formula [7] (see Table 1). It reduces with increasing current density. The composition of the thin films was obtained by means of EDS and ICP-AE. Fig. 2 shows the dependence of the Co concentration in the thin films both on the electrodeposition current density and the pH values of the bath electrolyte. For a given pH in the range of 2.5–5.0, the Co concentration increases rapidly first and then level off with increasing current density. This fact implies that at high current density Co reduction is a dominant reaction, whereas Cu competes against Co for reduction resulting in rich-Cu composition at lower density, owing to limited cathodic polarization [6]. In our work, the highest Co concentration of about 90 at% was obtained at a condition of pH ¼ 3 and current density of 40 mA/cm2. Furthermore, it is observed from the figure that for a constant current density, the Co concentration increases as pH decreases. The result is different from previous research where the Co concentration decreases as pH decreases, probably due to the different ranges of pH used. In our case, pH ranges
Fig. 2. Co concentration in thin films as a function of pH and electrodeposition current density.
from 2.5 to 5.0, compared the literature data of 4.7 to 6.5 [8]. The magnetic property of the electrodeposited films was investigated by VSM for pH ¼ 3 at four current densities of 1, 1.5, 2 and 10 mA/cm2, respectively. For the case of 1 mA/cm2, the film shows a weak paramagnetic behavior, as evidenced by the XRD observation above. Fig. 3 is the hysteresis curves for the in-plane measurements on the films fabricated at the current densities of 1.5, 2 and 10 mA/cm2. Clearly, the films are ferromagnetic, with the corresponding coercivity (Hc) of 120, 100, and 80 Oe. The fact that the Hc decreases as the current density increases could arise from the reduced grain size with the current density (see Table 1). The effect of annealing on the crystalline structure and magnetic properties was conducted under the annealing condition of 450 1C for 20 min for the films electrodeposited at pH ¼ 3.0 and current density of 3 mA/cm2. As shown in Fig. 4, the Hc increases from 100 Oe as deposited to 120 Oe after annealing. The result is interpreted by the increase of grain size resultant from annealing, as supported by the observation that the FWHM of the peaks almost reduces to half after the film being annealed and the intensity of all peaks become stronger. It indicates
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Fig. 3. Hysteresis loops of the electrodeposited films at pH ¼ 3 for three current densities of 1.5, 2 and 10 mA/cm2, respectively.
that crystallization improves after being annealed and the grain size becomes larger. Moreover, the peak positions remain the same before and after annealing. This fact supports the formation of CoCu alloys by the electrodeposition, as the peak positions would change after stress release during annealing if the shifting in the peak positions in our experiments were due to stress in the films. In summary, we have fabricated the CoCu thin films on the amorphous substrate by DC electrodeposition. The Co concentration of the thin films is a strong function of current density and pH values of the bath electrolyte. At a given pH between 2.5 and 5.0, the Co concentration increases with increasing current density. In the case of a constant current density, the concentration decrease as the pH value increase. The XRD analysis shows that the grain size reduces with increasing current density, explaining the decrease in the coercivity. The result of annealing supports the formation of CoCu alloys in our films. The structure and magnetic property of CoCu thin films could be finetuned by adjusting the electrodeposition processing parameters. This work was supported by the Korea Research Foundation grant KRF-2005-210-D00023 and KRF2004-005-D00057, by the Grant A050750 of the Korea Health 21 R&D Project, Ministry of Health & Welfare, and by Grant M10500000105-05J0000-10510 from the National Research Laboratory Program of the Korea Science and Engineering Foundation. References
Fig. 4. (a) VSM data for the film as-deposited and after annealing at 450 1C for 20 min in vacuum, and (b) the XRD data of as-deposited state and after annealing.
[1] W. Schwarzacher, D.S. Lashmore, IEEE. Trans. Magn. 32 (1996) 3133. [2] E. Chassaing, A. Morrone, J.E. Schmidt, J. Electrochem. Soc. 146 (1999) 1794. [3] S. Kainuma, K. Takayanagi, K. Hisatake, T. Watanabe, J. Magn. Magn. Mater. 246 (2002) 207. [4] Q. Liu, L. Pe´ter, J. Pa´da´r, I. Bakonyi, J. Electrochem. Soc. 152 (2005) C316. [5] M.N. Baibich, J.M. Broto, A. Fert, F.N.V. Dau, F. Petroff, P. Eitenne, G. Creuset, A. Friederich, J. Chazelas, Phys. Rev. Lett. 61 (1988) 2472; A. Barthe´le´my, A. Fert, F. Petroff, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol.12, Elsevier, Amsterdam, 1999, pp. 1–96. [6] Q. Liu, J.H. Min, J.U. Cho, Y.K. Kim, IEEE. Trans. Magn. 41 (2005) 930. [7] B.D. Cullity, S.R. Stock, Elements of X-ray Diffraction, 3rd ed, Prentice Hall, New Jersey, 2001, pp. 170. [8] G.R. Pattanaik, D.K. Randya, S.C. Kashyap, J. Electrochem. Soc. 149 (2002) C363.
Journal of Magnetism and Magnetic Materials 304 (2006) e100–e102 www.elsevier.com/locate/jmmm
The pH and current density dependence of DC electrodeposited CoCu thin films Ji Hyun Mina, Jun-Hua Wub, Ji Ung Choa, Qun Xian Liub, Ju Hun Leea, Young-Dong Kob, Jin-Seok Chungc, Jae-Ho Leed, Young Keun Kima, a
Department of Materials Science and Engineering, Korea University, Seoul 136-713, Korea Research Institute of Engineering and Technology, Korea University, Seoul 136-713, Korea c AMCRCD and Department of Physics, Soongsil University, Seoul 156-743, Korea d Department of Materials Science and Engineering, Hongik University, Seoul 121-791, Korea b
Available online 3 March 2006
Abstract CoCu thin films were fabricated on a nanocrystalline substrate by DC electrodeposition. It is found that the composition, structure and magnetic properties of the thin films exhibit a strong dependence on the current density and pH values of the bath electrolyte. The effect of annealing as well as the structure–property relation was investigated. r 2006 Elsevier B.V. All rights reserved. PACS: 81.15.Pq; 82.65.+r; 75.70.Rf; 68.55.a Keywords: CoCu alloy; Thin films; Electrodeposition; Structure; Magnetic property; Annealing
The fabrication of thin film multilayers or granular films using electrodeposition has been of great interest for magnetic and magneto-transport studies [1–4]. During the electrodeposition process, pH and current density are two key processing variables that determine physical properties of the deposited films. The Co/Cu multilayer system has been extensively investigated for the effect of giant magnetoresistance (GMR) [5] and a CoCu alloy film fabricated by electrodeposition provides an economic alternative for potential GMR applications such as magnetic sensors [1–4]. In a previous research, we studied electrodeposited CoCu alloy films and reported their surface morphology dependent on pH values of the bath and their composition on both pH values and current densities [6]. In this paper, we focus on the dependence of the composition, structure and magnetic properties of the DC electrodeposited CoCu thin films on both bath pH and current density, and investigate their structure-property relation. In addition, the effect of annealing is studied. Corresponding author. Tel.: +82 2 3290 3281; 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.2006.01.191
The CoCu thin films were fabricated on the nanocrystalline FinemetTM substrates. The deposition bath comprised CoSO4 (1 M) and CuSO4 (0.025 M), without any other buffering agent or additives. The electrolyte pH was adjusted from 2.5 to 5.0 by adding sulfuric acid. A Cu sheet was used as the counterelectrode. The electrodeposition process was conducted at ambient temperature without stirring. The composition of the thin films was investigated in terms of deposition parameters, i.e., current density and electrolyte pH. The deposited films were analyzed by scanning electron microscope (SEM) with energy dispersive spectroscopy (EDS) and inductively coupled plasma atomic emission spectroscopy (ICP-AE). The microstructure was characterized by X-ray diffraction (XRD), and magnetic properties were measured using a vibrating sample magnetometer (VSM), respectively. The annealing of films was performed under a condition of 450 1C for 20 min in a vacuum of 106 Torr. The morphology of the resultant films studied with SEM presents a particulate surface, similar to our previous observation [6]. The crystal structure was investigated by XRD, as shown in Fig. 1 for the films fabricated at pH ¼ 3
ARTICLE IN PRESS J.H. Min et al. / Journal of Magnetism and Magnetic Materials 304 (2006) e100–e102
e101
Table 1 Parameters derived from the XRD patterns Sample
Peak type
Peak position (1)
FWHM (1)
Grain size (nm)
Pure Cu
Cu (1 1 1)
43.36
0.11
75
Cu76Co24
CuCo alloy Co fcc(1 1 1)
43.44 44.34
0.14 0.27
62 32
Cu54Co46
CuCo alloy Co fcc(1 1 1)
43.47 44.22
0.20 0.37
43 23
Fig. 1. XRD patterns at pH ¼ 3 for three current densities of (A) 5 mA/cm2, (B) 2 mA/cm2, and (C) 1 mA/cm2.
with 3 current densities, 1, 2 and 5 mA/cm2. At the current density of 1 mA/cm2, only (1 1 1) and (2 0 0) of cubic Cu is observed, indicating the formation of pure Cu film under the deposition condition. As the current density increases to 2 and 5 mA/cm2, the second set of peaks representing FCC phase of Co appears. The analysis shows the peak positions shift with the current densities. As shown in the inset, Cu (1 1 1) peaks move to high angles, whereas Co (0 0 2) peaks move to low angle as the current density increases. The phenomenon could be attributed to the formation of CoCu alloys and/or the effect of stress created during the deposition. As discussed below, the observation that annealing does not lead to any change of peak positions implies the shifting probably arise from the alloy formation, rather than stress. In this case, the alloy composition can be derived according to the Vergard’s rule [7]. The grain size was analyzed by Scherrer’s formula [7] (see Table 1). It reduces with increasing current density. The composition of the thin films was obtained by means of EDS and ICP-AE. Fig. 2 shows the dependence of the Co concentration in the thin films both on the electrodeposition current density and the pH values of the bath electrolyte. For a given pH in the range of 2.5–5.0, the Co concentration increases rapidly first and then level off with increasing current density. This fact implies that at high current density Co reduction is a dominant reaction, whereas Cu competes against Co for reduction resulting in rich-Cu composition at lower density, owing to limited cathodic polarization [6]. In our work, the highest Co concentration of about 90 at% was obtained at a condition of pH ¼ 3 and current density of 40 mA/cm2. Furthermore, it is observed from the figure that for a constant current density, the Co concentration increases as pH decreases. The result is different from previous research where the Co concentration decreases as pH decreases, probably due to the different ranges of pH used. In our case, pH ranges
Fig. 2. Co concentration in thin films as a function of pH and electrodeposition current density.
from 2.5 to 5.0, compared the literature data of 4.7 to 6.5 [8]. The magnetic property of the electrodeposited films was investigated by VSM for pH ¼ 3 at four current densities of 1, 1.5, 2 and 10 mA/cm2, respectively. For the case of 1 mA/cm2, the film shows a weak paramagnetic behavior, as evidenced by the XRD observation above. Fig. 3 is the hysteresis curves for the in-plane measurements on the films fabricated at the current densities of 1.5, 2 and 10 mA/cm2. Clearly, the films are ferromagnetic, with the corresponding coercivity (Hc) of 120, 100, and 80 Oe. The fact that the Hc decreases as the current density increases could arise from the reduced grain size with the current density (see Table 1). The effect of annealing on the crystalline structure and magnetic properties was conducted under the annealing condition of 450 1C for 20 min for the films electrodeposited at pH ¼ 3.0 and current density of 3 mA/cm2. As shown in Fig. 4, the Hc increases from 100 Oe as deposited to 120 Oe after annealing. The result is interpreted by the increase of grain size resultant from annealing, as supported by the observation that the FWHM of the peaks almost reduces to half after the film being annealed and the intensity of all peaks become stronger. It indicates
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Fig. 3. Hysteresis loops of the electrodeposited films at pH ¼ 3 for three current densities of 1.5, 2 and 10 mA/cm2, respectively.
that crystallization improves after being annealed and the grain size becomes larger. Moreover, the peak positions remain the same before and after annealing. This fact supports the formation of CoCu alloys by the electrodeposition, as the peak positions would change after stress release during annealing if the shifting in the peak positions in our experiments were due to stress in the films. In summary, we have fabricated the CoCu thin films on the amorphous substrate by DC electrodeposition. The Co concentration of the thin films is a strong function of current density and pH values of the bath electrolyte. At a given pH between 2.5 and 5.0, the Co concentration increases with increasing current density. In the case of a constant current density, the concentration decrease as the pH value increase. The XRD analysis shows that the grain size reduces with increasing current density, explaining the decrease in the coercivity. The result of annealing supports the formation of CoCu alloys in our films. The structure and magnetic property of CoCu thin films could be finetuned by adjusting the electrodeposition processing parameters. This work was supported by the Korea Research Foundation grant KRF-2005-210-D00023 and KRF2004-005-D00057, by the Grant A050750 of the Korea Health 21 R&D Project, Ministry of Health & Welfare, and by Grant M10500000105-05J0000-10510 from the National Research Laboratory Program of the Korea Science and Engineering Foundation. References
Fig. 4. (a) VSM data for the film as-deposited and after annealing at 450 1C for 20 min in vacuum, and (b) the XRD data of as-deposited state and after annealing.
[1] W. Schwarzacher, D.S. Lashmore, IEEE. Trans. Magn. 32 (1996) 3133. [2] E. Chassaing, A. Morrone, J.E. Schmidt, J. Electrochem. Soc. 146 (1999) 1794. [3] S. Kainuma, K. Takayanagi, K. Hisatake, T. Watanabe, J. Magn. Magn. Mater. 246 (2002) 207. [4] Q. Liu, L. Pe´ter, J. Pa´da´r, I. Bakonyi, J. Electrochem. Soc. 152 (2005) C316. [5] M.N. Baibich, J.M. Broto, A. Fert, F.N.V. Dau, F. Petroff, P. Eitenne, G. Creuset, A. Friederich, J. Chazelas, Phys. Rev. Lett. 61 (1988) 2472; A. Barthe´le´my, A. Fert, F. Petroff, in: K.H.J. Buschow (Ed.), Handbook of Magnetic Materials, Vol.12, Elsevier, Amsterdam, 1999, pp. 1–96. [6] Q. Liu, J.H. Min, J.U. Cho, Y.K. Kim, IEEE. Trans. Magn. 41 (2005) 930. [7] B.D. Cullity, S.R. Stock, Elements of X-ray Diffraction, 3rd ed, Prentice Hall, New Jersey, 2001, pp. 170. [8] G.R. Pattanaik, D.K. Randya, S.C. Kashyap, J. Electrochem. Soc. 149 (2002) C363.