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Magnetic properties of CoP alloys electrodeposited at room temperature
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 290–291 (2005) 1513–1516 www.elsevier.com/locate/jmmm
Magnetic properties of CoP alloys electrodeposited at room temperature I. Lucasa, L. Pereza,, C. Arocab, P. Sa´nchezb, E. Lo´peza, M.C. Sa´ncheza a
Dpt. Fı´sica de Materiales, Facultad de C.C. Fı´sicas, Universidad Complutenese de Madrid, Ciudad Universitaria s/n. 28040 Madrid, Spain b ISOM & Dpt. Fı´sica Aplicada, E.T.S.I. Telecomunicacio´n, U.P.M. 28040 Madrid, Spain Available online 21 December 2004
Abstract CoP alloys have been electrodeposited at room temperature from electrolytes with different pH values and their magnetic properties have been studied. Cracks and fractures appear when using stiff substrates, showing that high internal stresses, due to hydrogen evolution, are involved in the electrodeposition process. Samples electrodeposited onto flexible substrates do not show cracks on the surface. We also report an increment in the coercivity of the alloys when the pH of the electrolyte decreases, and therefore, the hydrogen evolution and the internal stresses increase. r 2004 Elsevier B.V. All rights reserved. PACS: 75.50.Kj; 81.15.Pq Keywords: Amorphous systems—metallic glasses; Electrodeposition; Stress-internal
1. Introduction In the last few years there has been increasing interest in CoP electrodeposited alloys due to their excellent properties as soft magnetic materials (low coercivity and high permeability) [1] in addition to the GMI that they exhibit [2]. These alloys have been recently used in integrated sensors and inductors [3,4]. These alloys are normally electroplated from electrolytes at 60–80 1C [5], which is an important drawback for the integration of CoP alloys in small systems because high temperature degrades the photoresist used in some lithographic processes. In 1998, Djokic´ reported the possibility of plating CoP alloys at room temperature [6]. This work opened Corresponding author. Tel.: +34 91 394 4747; +34 91 394 4547. E-mail address: lucas.perez@fis.ucm.es (L. Perez).
fax:
new possibilities for the fabrication of these materials. Nevertheless, in the cited paper there are no magnetic measurements of the electrodeposited alloys. In this paper, we report the magnetic properties of CoP alloys electrodeposited at room temperature. We show the influence of the stiffness of the substrate in the adhesion of the thin films to them and also the dependence of the coercivity on the hydrogen evolution during the plating process.
2. Electrodeposition of samples Samples were prepared by galvanostatic electrodeposition from an electrolyte with the composition proposed by Djokic´ [6] (see Table 1). The pH of the electrolyte was adjusted to different values (between 0.4 and 1.4) by adding sodium hydroxide or sulphuric acid. The
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.11.563
ARTICLE IN PRESS 1514
I. Lucas et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 1513–1516
current density used for plating all the samples was 100 mA cm2 and the plating time was 10 min. A cobalt foil was used as anode. As cathode, two different kinds of substrates were used: electropolished copper foils (flexible substrates similar to the ones used by Djokic´) and sputtered Si/Ta/Cu wafers (stiff sub-
Table 1 Composition of the electrolyte used for the electrodeposition of the CoP alloys, based on a previous work by Djokic´ [6] Chemical
Concentration (g/l)
CoSO4 6H2 O CoCl2 6H2 O H3PO4 H3PO4
70 15 25 40
strates). All the substrates were dipped in sulphuric acid (10 vol% in water) and rinsed in demineralized water prior deposition in order to activate the surface. Stiff substrates are normally used in planar devices (silicon wafers, printed circuit boards, etc.). The study of electrodeposition onto stiff substrates would allow to integrate the plated materials in small sensors and devices. During the electrodeposition process, agitation was provided to the electrolyte by means of a magnetic stirrer.
3. Results and discussion Fig. 1 shows the surface of CoP samples plated onto different substrates at different temperatures. The
Fig. 1. Optical micrographs of the surface of the samples. (a) and (b) are grown at room temperature on stiff substrates and (c) and (d) on flexible substrates. (e) and (f) are grown onto stiff substrates at 60 and 80 1C, respectively.
ARTICLE IN PRESS I. Lucas et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 1513–1516
2Hþ þ 2e ! H2 : The composition of the samples plated from electrolytes with pH less than 1.0 is Co80P20, measured by energy-dispersive spectroscopy. When increasing the pH above this value, the phosphorous content decreases up to 10%. This dependence of composition on pH has been previously reported by Fukunaka et al. [7]. X-ray diffraction experiments have confirmed that all the samples are amorphous. Magnetic characterization was carried out by means of a vibrating sample magnetometer. The hysteresis loops of some of them are plotted in Fig. 2. All the samples are ferromagnetic and isotropic. In Fig. 3, the variation of coercivity and total magnetic moment of the samples as a function of the pH is shown. Below pH 1.0, the slight increment of the total magnetic moment can be related to an increase in the efficiency of the electrodeposition process due to the reduction of [H2þ ] concentration. Nevertheless, the increment of magnetic moment is small which means that efficiency is practically independent of pH for these pH values. The increment of magnetic moment above this value of pH is mainly due to the change in composition. A reduction in the phosphorous content produces an increase in the saturation magnetization and, therefore, in the total magnetic moment of the film [8].
pH = 0.3 pH = 1.0 pH = 1.4
0.8
M / Ms
0.4 0.0 -0.4 -0.8
-1200
-800
-400
0 400 Hc(Oe)
800
1200
Fig. 2. Hysteresis loops of samples plated onto flexible substrate from electrolytes with different pH values.
140
Hc Ms
350
120
300
80
200
60
150
40
100
20
50
mmax (memu)
100
250
Hc (Oe)
surface of the samples plated onto stiff substrates is completely full of cracks and fractures (see Figs. 1a and b). These fractures, that were not observed by Djokic, disappeared when using flexible substrates (as they did) (Figs. 1a and b). Nevertheless, in these samples, there are bubbles and striations, probably due to hydrogen evolution during the plating process. The bubbles and striations observed in samples plated on flexible substrates disappeared when plating at higher temperatures (Figs. 1e and f). Electrodeposition at room temperature involves high internal stresses in the samples. Flexible substrates are able to absorb part of the stress and stiff substrates are not. Due to this, samples plated at room temperature on the latter fracture. The internal stresses and bubbles may be caused by hydrogen evolution, which is particularly intense in CoP electrodeposition. An increase in the temperature of the electrolyte enhances hydrogen diffusion in the sample and reduces the internal stress. The increase of temperature makes it possible to plate onto stiff substrates using the electrolyte proposed by Djokic´. In order to investigate the effect of hydrogen evolution in the magnetic properties of Co–P alloys, a set of samples was plated onto flexible substrate from electrolytes with different pH values (from 0.3 to 1.4). When pH decreases, the [H2þ ] concentration in the electrolyte increases and also the hydrogen evolution following the chemical reaction:
1515
0
0
-20 0.4
0.6
0.8
1.0
1.2
1.4
pH Fig. 3. Coercivity and total magnetic moment of samples plated onto flexible substrate from electrolytes with different pH values.
Regarding the coercivity, it is noticed that the values obtained are higher than the ones obtained in CoP electrodeposited at 80 1C [9]. There is also an increment in the coercivity when the pH is reduced and the hydrogen evolution enhanced. As we previously showed, hydrogen induces stress in the samples, which produces an increase in the coercivity. The small increase of coercivity when the pH increases above 1.0 can be due to the reported change in composition. Previous measurements done in CoP samples plated at high temperature showed that a decrease in the phosphorous content produces an increase in the coercive force [10] as observed in our samples.
4. Conclusions To sum up, electrodeposition at room temperature is not suitable when using stiff substrates because
ARTICLE IN PRESS 1516
I. Lucas et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 1513–1516
samples fracture. These fractures can be avoided using flexible substrates or increasing the temperature. There is also a clear relationship between the pH of the electrolyte and the coercive force of the plating samples: coercivity increases when pH decreases. We claim that these effects are due to hydrogen evolution during electrodeposition. The hydrogen produces high internal stresses in the plated alloys, which cause problems with the adhesion of the films and also the increase in the coercivity.
Acknowledgement This work has been partially supported by the Spanish Ministry of Education and Science under the projects MAT2000-0330-P4, MAT2001-3554-C02 and TIC200204132-C02.
References [1] L. Perez, O´ de Abril, M.C. Sa´nchez, C. Aroca, E. Lo´pez, P. Sa´nchez, J. Magn. Magn. Mater. 215–216 (2000) 337. [2] J.P. Sinnecker, L.A. Souza de Oliveira, J. Appl. Phys. 87 (2000) 4825. [3] L. Perez, C. Aroca, P. Sa´nchez, E. Lo´pez, M.C. Sa´nchez, Sensors Actuators A 109 (2004) 208. [4] W. Ruythooren, E. Beyne, J.-P. Celis, J- De Boeck, IEEE Trans. Magn. 38 (2002) 3498. [5] A. Brenner, Electrodeposition of Alloys, Academic Press Inc., New York, 1963. [6] S.S. Djokic´, J. Electrochem. Soc. 146 (1999) 1824. [7] F. Fukunaka, S. Aiwaka, Z. Asaki, J. Electrochem. Soc. 141 (1994) 1783. [8] A. Garcı´ a-Arribas, J.M. Barandiara´n, y J. Herreros, J. Magn. Magn. Mater. 131 (1994) 129. [9] J.M. Riveiro, M.C. Sa´nchez-Trujillo, IEEE Trans. Magn. MAG-16 (1980) 1426. [10] G. Rivero, I. Navarro, P. Crespo, E. Pulido, A. Garcı´ aEscorial, A. Hernando, M. Va´zquez, M. Vallet, y J. Gonza´lez-Calbet, J. Appl. Phys. 69 (1991) 54540.
Journal of Magnetism and Magnetic Materials 290–291 (2005) 1513–1516 www.elsevier.com/locate/jmmm
Magnetic properties of CoP alloys electrodeposited at room temperature I. Lucasa, L. Pereza,, C. Arocab, P. Sa´nchezb, E. Lo´peza, M.C. Sa´ncheza a
Dpt. Fı´sica de Materiales, Facultad de C.C. Fı´sicas, Universidad Complutenese de Madrid, Ciudad Universitaria s/n. 28040 Madrid, Spain b ISOM & Dpt. Fı´sica Aplicada, E.T.S.I. Telecomunicacio´n, U.P.M. 28040 Madrid, Spain Available online 21 December 2004
Abstract CoP alloys have been electrodeposited at room temperature from electrolytes with different pH values and their magnetic properties have been studied. Cracks and fractures appear when using stiff substrates, showing that high internal stresses, due to hydrogen evolution, are involved in the electrodeposition process. Samples electrodeposited onto flexible substrates do not show cracks on the surface. We also report an increment in the coercivity of the alloys when the pH of the electrolyte decreases, and therefore, the hydrogen evolution and the internal stresses increase. r 2004 Elsevier B.V. All rights reserved. PACS: 75.50.Kj; 81.15.Pq Keywords: Amorphous systems—metallic glasses; Electrodeposition; Stress-internal
1. Introduction In the last few years there has been increasing interest in CoP electrodeposited alloys due to their excellent properties as soft magnetic materials (low coercivity and high permeability) [1] in addition to the GMI that they exhibit [2]. These alloys have been recently used in integrated sensors and inductors [3,4]. These alloys are normally electroplated from electrolytes at 60–80 1C [5], which is an important drawback for the integration of CoP alloys in small systems because high temperature degrades the photoresist used in some lithographic processes. In 1998, Djokic´ reported the possibility of plating CoP alloys at room temperature [6]. This work opened Corresponding author. Tel.: +34 91 394 4747; +34 91 394 4547. E-mail address: lucas.perez@fis.ucm.es (L. Perez).
fax:
new possibilities for the fabrication of these materials. Nevertheless, in the cited paper there are no magnetic measurements of the electrodeposited alloys. In this paper, we report the magnetic properties of CoP alloys electrodeposited at room temperature. We show the influence of the stiffness of the substrate in the adhesion of the thin films to them and also the dependence of the coercivity on the hydrogen evolution during the plating process.
2. Electrodeposition of samples Samples were prepared by galvanostatic electrodeposition from an electrolyte with the composition proposed by Djokic´ [6] (see Table 1). The pH of the electrolyte was adjusted to different values (between 0.4 and 1.4) by adding sodium hydroxide or sulphuric acid. The
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.11.563
ARTICLE IN PRESS 1514
I. Lucas et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 1513–1516
current density used for plating all the samples was 100 mA cm2 and the plating time was 10 min. A cobalt foil was used as anode. As cathode, two different kinds of substrates were used: electropolished copper foils (flexible substrates similar to the ones used by Djokic´) and sputtered Si/Ta/Cu wafers (stiff sub-
Table 1 Composition of the electrolyte used for the electrodeposition of the CoP alloys, based on a previous work by Djokic´ [6] Chemical
Concentration (g/l)
CoSO4 6H2 O CoCl2 6H2 O H3PO4 H3PO4
70 15 25 40
strates). All the substrates were dipped in sulphuric acid (10 vol% in water) and rinsed in demineralized water prior deposition in order to activate the surface. Stiff substrates are normally used in planar devices (silicon wafers, printed circuit boards, etc.). The study of electrodeposition onto stiff substrates would allow to integrate the plated materials in small sensors and devices. During the electrodeposition process, agitation was provided to the electrolyte by means of a magnetic stirrer.
3. Results and discussion Fig. 1 shows the surface of CoP samples plated onto different substrates at different temperatures. The
Fig. 1. Optical micrographs of the surface of the samples. (a) and (b) are grown at room temperature on stiff substrates and (c) and (d) on flexible substrates. (e) and (f) are grown onto stiff substrates at 60 and 80 1C, respectively.
ARTICLE IN PRESS I. Lucas et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 1513–1516
2Hþ þ 2e ! H2 : The composition of the samples plated from electrolytes with pH less than 1.0 is Co80P20, measured by energy-dispersive spectroscopy. When increasing the pH above this value, the phosphorous content decreases up to 10%. This dependence of composition on pH has been previously reported by Fukunaka et al. [7]. X-ray diffraction experiments have confirmed that all the samples are amorphous. Magnetic characterization was carried out by means of a vibrating sample magnetometer. The hysteresis loops of some of them are plotted in Fig. 2. All the samples are ferromagnetic and isotropic. In Fig. 3, the variation of coercivity and total magnetic moment of the samples as a function of the pH is shown. Below pH 1.0, the slight increment of the total magnetic moment can be related to an increase in the efficiency of the electrodeposition process due to the reduction of [H2þ ] concentration. Nevertheless, the increment of magnetic moment is small which means that efficiency is practically independent of pH for these pH values. The increment of magnetic moment above this value of pH is mainly due to the change in composition. A reduction in the phosphorous content produces an increase in the saturation magnetization and, therefore, in the total magnetic moment of the film [8].
pH = 0.3 pH = 1.0 pH = 1.4
0.8
M / Ms
0.4 0.0 -0.4 -0.8
-1200
-800
-400
0 400 Hc(Oe)
800
1200
Fig. 2. Hysteresis loops of samples plated onto flexible substrate from electrolytes with different pH values.
140
Hc Ms
350
120
300
80
200
60
150
40
100
20
50
mmax (memu)
100
250
Hc (Oe)
surface of the samples plated onto stiff substrates is completely full of cracks and fractures (see Figs. 1a and b). These fractures, that were not observed by Djokic, disappeared when using flexible substrates (as they did) (Figs. 1a and b). Nevertheless, in these samples, there are bubbles and striations, probably due to hydrogen evolution during the plating process. The bubbles and striations observed in samples plated on flexible substrates disappeared when plating at higher temperatures (Figs. 1e and f). Electrodeposition at room temperature involves high internal stresses in the samples. Flexible substrates are able to absorb part of the stress and stiff substrates are not. Due to this, samples plated at room temperature on the latter fracture. The internal stresses and bubbles may be caused by hydrogen evolution, which is particularly intense in CoP electrodeposition. An increase in the temperature of the electrolyte enhances hydrogen diffusion in the sample and reduces the internal stress. The increase of temperature makes it possible to plate onto stiff substrates using the electrolyte proposed by Djokic´. In order to investigate the effect of hydrogen evolution in the magnetic properties of Co–P alloys, a set of samples was plated onto flexible substrate from electrolytes with different pH values (from 0.3 to 1.4). When pH decreases, the [H2þ ] concentration in the electrolyte increases and also the hydrogen evolution following the chemical reaction:
1515
0
0
-20 0.4
0.6
0.8
1.0
1.2
1.4
pH Fig. 3. Coercivity and total magnetic moment of samples plated onto flexible substrate from electrolytes with different pH values.
Regarding the coercivity, it is noticed that the values obtained are higher than the ones obtained in CoP electrodeposited at 80 1C [9]. There is also an increment in the coercivity when the pH is reduced and the hydrogen evolution enhanced. As we previously showed, hydrogen induces stress in the samples, which produces an increase in the coercivity. The small increase of coercivity when the pH increases above 1.0 can be due to the reported change in composition. Previous measurements done in CoP samples plated at high temperature showed that a decrease in the phosphorous content produces an increase in the coercive force [10] as observed in our samples.
4. Conclusions To sum up, electrodeposition at room temperature is not suitable when using stiff substrates because
ARTICLE IN PRESS 1516
I. Lucas et al. / Journal of Magnetism and Magnetic Materials 290–291 (2005) 1513–1516
samples fracture. These fractures can be avoided using flexible substrates or increasing the temperature. There is also a clear relationship between the pH of the electrolyte and the coercive force of the plating samples: coercivity increases when pH decreases. We claim that these effects are due to hydrogen evolution during electrodeposition. The hydrogen produces high internal stresses in the plated alloys, which cause problems with the adhesion of the films and also the increase in the coercivity.
Acknowledgement This work has been partially supported by the Spanish Ministry of Education and Science under the projects MAT2000-0330-P4, MAT2001-3554-C02 and TIC200204132-C02.
References [1] L. Perez, O´ de Abril, M.C. Sa´nchez, C. Aroca, E. Lo´pez, P. Sa´nchez, J. Magn. Magn. Mater. 215–216 (2000) 337. [2] J.P. Sinnecker, L.A. Souza de Oliveira, J. Appl. Phys. 87 (2000) 4825. [3] L. Perez, C. Aroca, P. Sa´nchez, E. Lo´pez, M.C. Sa´nchez, Sensors Actuators A 109 (2004) 208. [4] W. Ruythooren, E. Beyne, J.-P. Celis, J- De Boeck, IEEE Trans. Magn. 38 (2002) 3498. [5] A. Brenner, Electrodeposition of Alloys, Academic Press Inc., New York, 1963. [6] S.S. Djokic´, J. Electrochem. Soc. 146 (1999) 1824. [7] F. Fukunaka, S. Aiwaka, Z. Asaki, J. Electrochem. Soc. 141 (1994) 1783. [8] A. Garcı´ a-Arribas, J.M. Barandiara´n, y J. Herreros, J. Magn. Magn. Mater. 131 (1994) 129. [9] J.M. Riveiro, M.C. Sa´nchez-Trujillo, IEEE Trans. Magn. MAG-16 (1980) 1426. [10] G. Rivero, I. Navarro, P. Crespo, E. Pulido, A. Garcı´ aEscorial, A. Hernando, M. Va´zquez, M. Vallet, y J. Gonza´lez-Calbet, J. Appl. Phys. 69 (1991) 54540.