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Microstructure evolution of cobalt coating electroless plated on SiC whisker during electroless plating and heat treatment
Surface & Coatings Technology 201 (2007) 6059 – 6062 www.elsevier.com/locate/surfcoat
Short communication
Microstructure evolution of cobalt coating electroless plated on SiC whisker during electroless plating and heat treatment J.T. Jiang, L. Zhen ⁎, C.Y. Xu, W.Z. Shao School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China Received 26 August 2006; accepted in revised form 20 October 2006 Available online 1 December 2006
Abstract SiCw/Co nanocomposite particles were prepared by electroless plating cobalt on SiC whiskers and the microstructure evolution of the plated coating was investigated by SEM and XRD. SEM images show that growth occurs on the surface of the clusters at the initial stage; as they grow larger, the clusters converge to form a continuous coating, which is actually stacking of cobalt clusters. After heat treated at 500 °C in a hydrogen atmosphere, the cobalt coating transforms from an amorphous to a crystalline state. The thermal stability of SiCw/Co composite is low because of the weak bonding between the substrate and the cobalt coating. The continuous coating aggregates to clusters through surface diffusion during heat treatment. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanocomposite; Electroless plating; Microstructure evolution; Growth behavior
1. Introduction SiC whisker is of great importance for its wide applications and it has been used as reinforcements in metal matrix composites and ceramic composites for many years [1,2]. SiC ceramic has found new applications in the electronic industry for its adjustable permittivity in recent years [3,4]. On the other hand, some researches have proved that ferromagnetic metal coatings plated on the dielectric substrate possess unique electromagnetic properties [5–8], for example, carbon fibers with FeCoNiP alloy coating plated on its surface have been used as high performance electromagnetic wave absorbers in GHz band [8]. Electroless plating is an easy process used to deposit metal or alloy film on a conductive, semiconductive or even insulated substrate, and especially, it can be used to deposit a metal coating on a fine substrate in the form of particles, fibers or whiskers. In the past decade or so, a lot of work has been done on how to deposit Ni or Co by electroless plating on fine substrates, including CNTs and carbon nanofibers [7,8], SiC
⁎ Corresponding author. Tel.: +86 451 8641 2133; fax: +86 451 8641 3922. E-mail address: [email protected] (L. Zhen). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.10.032
particles [9,10], ZrO2 particles [11]. Although electroless plating process has already been under study for many years, to the best of our knowledge, little work has been done on electroless plating metals on SiC whiskers and not much attention has been paid to the growth behavior of plated coating. This situation justifies our efforts to study the process of electroless plating metals on SiC whiskers, and the growth behavior of cobalt coating in particular. Furthermore, the thermal stability of nanoscaled metal structures at high temperature is of great significance, because heat treatment at high temperature is usually required to condition microstructures. Although much work has been done on the thermal stability of metal coatings plated on bulk substrates [12,13], it is obvious that the thermal stability of metal coating plated on whiskers needs further investigation [14]. Therefore, we adopted an electroless plating method to deposit cobalt on SiC whiskers and studied the microstructure evolution of metal coatings during electroless plating and heat treatments. 2. Experimental SiC whiskers of 0.1–1 μm in diameter were used as substrates and the morphology of raw SiC whiskers is shown in Fig. 1a).
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Fig. 1. SEM images of raw SiC whiskers and SiCw/Co nanocomposite powders a) raw SiC whiskers; b) at 15th minute; c) at 30th minute; d) at 60th minute; e) at the end of electroless plating; f) after heat treatment.
Raw SiC whisker was pretreated to obtain catalytic activity before electroless plating because of its chemical inertness. The pretreatment included coarsening, sensitizing–activating and drying. 2 g of pretreated whisker was then added into 400 mL preheated plating solution, which contains 25 g L− 1 cobaltous sulfate, 25 g L− 1 sodium phosphite, 50 g L− 1 sodium citrate and 25 g L− 1 boric acid. During plating, the temperature of the plating solution was kept in the range of 50–55 °C by water bathing. The pH value of the plating solution was kept at 8.5 by instilling 2 M NaOH solution. Measly samples were taken from the plating bath with a sucker at the 15th, 30th, 60th minute and at the end as well, and these samples were named successively as S15, S30 S60 and SE. All these samples were rinsed in deionized water for three times and dried in atmosphere in an oven at 80 °C overnight. Some final nanocomposite powder was heat
treated in hydrogen in an oven at 500 °C for 2 h and the heat treated sample was named as SH5. The morphologies of these nanocomposite particles were observed with a FE-SEM (Hitachi S-4700) and their phase compositions were XRD analyzed (Regaku D/max-γB, Cukα, graphite filter). 3. Results and discussion It can be seen from the SEM images of raw SiC whickers and samples taken at different process time shown in Fig. 1 that, fine cobalt clusters of 30–40 nm in diameter appeared on the surface of SiC whiskers at the 15th minute from the beginning of the reaction. In most of the areas, cobalt clusters distribute uniformly at an interval of about 100 nm as shown in region
J.T. Jiang et al. / Surface & Coatings Technology 201 (2007) 6059–6062
A. However, in few areas few clusters can be observed as shown in region B. It is well known that, electroless deposition occurs first on the surface of catalytic Pd nuclei which forms during sensitizing–activating treatment, so the distribution of initial cobalt clusters is determined by the distribution of Pd nuclei. At this process time, most of the cobalt clusters do not touch each other, which indicate their independent growth. The cobalt clusters grow up to about 60 nm in diameter in 30 min and they start to contact each other and conjoin together to form a continuous coating in the uniformly covered areas. However, there are some areas, as the region marked as B in Fig. 1c), where few clusters can be seen, which indicates that, no cobalt can be deposited in the initial cluster-free areas. The distribution of cobalt clusters at the 30th minute is similar to that observed at the 15th minute, which mean that there is still deposition of cobalt occurring on the surface of the cobalt clusters from the 15th to the 30th minute. A continuous cobalt coating with a rough surface instead of cobalt clusters is observed on SiC whiskers at the 60th minute. It can be easily understood that the cobalt coating at this stage is built up by stacking of cobalt clusters. Similar characteristics of stacking were also observed when copper is electroless plated on the surface of natural pollens or Al2O3 particles [15,16]. A similar evolution from clusters to a continuous coating similar to that in the current study was also observed when a thin copper, platinum or silver film is plated on a bulk substrate by a sputtering process. However, the size of clusters and the intervals between clusters observed during sputtering are much smaller than those observed in the present study [17,18]. Such difference may originate from the difference in the surface curvatures and electrical properties of the substrates. The Pd nuclei stems from the Pd2+ ions adsorbed on the surface while the whiskers are activated. Because of the large surface curvature of SiC whiskers and the electrostatic repulsion between the Pd2+ ions, the adsorption of the Pd2+ ions becomes very difficult, so that the subsequent distribution of Pd particles has a very low density. The metal ions translate into electroneutral atoms immediately after they are deposited by sputtering to the substrate and then the electrostatic repulsion is eliminated. Furthermore, the plane surface of bulk substrate is more suitable for the deposition of metal atoms than the convex surface of fine SiC whiskers. It can be seen from the SEM images of sample SE shown in Fig. 1e) that, in comparison with the morphologies observed at the early stage, some cobalt platelets appear on the surface of cobalt coating. Possibly, these platelets are caused by the nonuniform distribution of deposition sites. After the clusters start to contact each other, the stacking of cobalt clusters creates some lathy and jaggy boundary areas which are more suitable for deposition of cobalt atoms than plane and smooth areas on clusters, and so, cobalt atoms may be preferentially deposited in these boundary areas. Meanwhile, catalytic activity decreases in the plane and smooth areas because little cobalt is deposited to form a fresh layer catalytic with high catalytic activity, which further slows down the cobalt deposition in these areas. The selective and nonuniform deposition eventually leads to the formation of cobalt platelets.
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Fig. 2. XRD patterns of SiCw/Co nanocomposite particles, before and after heat treatment.
Some researchers believe that the formation of these flakes may be due to the rapid hydrogen release during electroless plating [15]. However, we did not see any flake cluster formed at the early stage when there is an intense hydrogen release, and this means that the formation of flakes is due to the nonuniform deposition rather than hydrogen release. It can be seen from Fig. 2 that, there is a great change in the microstructure of cobalt coating after heat treatment. Only weak broaden peaks can be observed in SE patterns, which reveals that the cobalt coating is amorphous. After heat treatment, sharp diffraction peaks corresponding to α-Co can be seen in XRD patterns of SH5, which indicates that the cobalt in coating has been transformed into a crystalline state through heat treatment. Morphological changes took place in cobalt coating during heat treatment at 500 °C. After the heat treatment, the continuous coating totally disappeared and cobalt clusters similar to those obtained at early stages of electroless plating can be seen all over the SiC whickers. This morphological transformation indicates that the cobalt coating is not stable enough to maintain its original structure at 500 °C. It is also found that, the size of the clusters is very close to those of the clusters obtained during electroless plating in some areas (area O), whereas the clusters formed during heat treatment are much larger in sizes in some other areas (area C). The existence of these larger clusters suggests that surface diffusion of cobalt atoms has taken place during heat treatment. A similar phenomenon of aggregation was also observed in a Ni–P coating deposited on the surface of silicon after heat treatment [12]. However, the examination of Ni–W–P coatings on steel substrate after heat treatment shows that, the nodular surface of the coating tends to be flat, which is quite different from what is observed in the present study [13]. The different behaviors mentioned above may be attributed to the difference in bonding strength between substrate and coating. The bonding strength between coating and substrate is very weak because the Pd particles are just adsorbed on the surface of SiC whiskers. The conjunction between coating and SiC whiskers at sites other than those of Pd particles is very weak mechanical bonding, too. Consequently, cobalt coating trends to
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aggregate to form clusters through surface diffusion to reduce surface energy when it is subjected to a high temperature [14]. 4. Conclusions We successfully prepared SiCw/Co nanocomposite particles by electroless plating and investigated the microstructure evolution of cobalt coating. The surface of the whickers is uniformly covered by insular cobalt clusters in the initial stage of electroless plating. These cobalt clusters grow larger and lager and eventually form a uniform cobalt coating. After the clusters contact, the deposition of cobalt preferentially takes place in the boundary areas, which result in the formation of cobalt flakes. These results reveal that, the growth behavior of the cobalt coating plated on a pretreated inert substrate is an evolution from clusters to a continuous coating. The cobalt coating transforms from an amorphous to a crystalline state and aggregates to clusters through surface diffusion to reduce the surface energy when the SiCw/Co nanocomposite is heat treated in hydrogen at 500 °C. Acknowledgement This work is funded by the Outstanding Young Scientist Foundation of Heilongjiang Province.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
D.Z. Wang, H.X. Peng, J. Liu, C.K. Yao, Wear 184 (1995) 187. C.Z. Huang, X. Ai, Mater. Res. Bull. 31 (1996) 951. A. Kassiba, M. Tabellout, Solid State Commun. 115 (2000) 389. J.J. Sun, J.B. Li, G.L. Sun, B. Zhang, Ceram. Int. 28 (2002) 741. I.T. Iakubov, A.N. Lagarkov, Sergi A. Maklakov, J. Magn. Magn. Mater. 258–259 (2003) 195. N.J. Tang, W. Zhong, H.Y. Jiang, Z.D. Han, W.Q. Zou, Y.W. Du, Solid State Commun. 132 (2004) 71. X. Shen, R.Z. Gong, Y. Nie, J.H. Nie, J. Magn. Magn. Mater. 288 (2005) 397. G.W. Xie, Z.B. Wang, Z.L. Cui, Y.L. Shi, Carbon 43 (2005) 3181. Y.J. Chen, M.S. Cao, Q. Xu, J. Zhu, Surf. Coat. Technol. 172 (2003) 90. M. Kang, J.M. Kim, J.W. Kim, Surf. Coat. Technol. 161 (2002) 79. G.W. Wen, Z.X. Guo, C.K.L. Davies, Scr. Mater. 43 (2000) 307. H.X. Li, H.Y. Chen, S.Z. Dong, J.S. Yang, Appl. Surf. Sci. 125 (1998) 115. F.B. Wu, S.K. Tien, J.G. Duh, J.H. Wang, Surf. Coat. Technol. 166 (2003) 60. I.S. Kim, S.K. Lee, Scr. Mater. 52 (2005) 1045. L.N. Xu, K.C. Zhou, H.F. Xu, Appl. Surf. Sci. 183 (2001) 58. G.P. Ling, Y. Li, Mater. Lett. 59 (2005) 1610. V. Podgursky, I. Costina, R. Franchy, Surf. Sci. 529 (2003) 419. J.C. Lin, C.P. Lee, Thin Solid Films 307 (1997) 96.
Short communication
Microstructure evolution of cobalt coating electroless plated on SiC whisker during electroless plating and heat treatment J.T. Jiang, L. Zhen ⁎, C.Y. Xu, W.Z. Shao School of Materials Science and Engineering, Harbin Institute of Technology, Harbin 150001, China Received 26 August 2006; accepted in revised form 20 October 2006 Available online 1 December 2006
Abstract SiCw/Co nanocomposite particles were prepared by electroless plating cobalt on SiC whiskers and the microstructure evolution of the plated coating was investigated by SEM and XRD. SEM images show that growth occurs on the surface of the clusters at the initial stage; as they grow larger, the clusters converge to form a continuous coating, which is actually stacking of cobalt clusters. After heat treated at 500 °C in a hydrogen atmosphere, the cobalt coating transforms from an amorphous to a crystalline state. The thermal stability of SiCw/Co composite is low because of the weak bonding between the substrate and the cobalt coating. The continuous coating aggregates to clusters through surface diffusion during heat treatment. © 2006 Elsevier B.V. All rights reserved. Keywords: Nanocomposite; Electroless plating; Microstructure evolution; Growth behavior
1. Introduction SiC whisker is of great importance for its wide applications and it has been used as reinforcements in metal matrix composites and ceramic composites for many years [1,2]. SiC ceramic has found new applications in the electronic industry for its adjustable permittivity in recent years [3,4]. On the other hand, some researches have proved that ferromagnetic metal coatings plated on the dielectric substrate possess unique electromagnetic properties [5–8], for example, carbon fibers with FeCoNiP alloy coating plated on its surface have been used as high performance electromagnetic wave absorbers in GHz band [8]. Electroless plating is an easy process used to deposit metal or alloy film on a conductive, semiconductive or even insulated substrate, and especially, it can be used to deposit a metal coating on a fine substrate in the form of particles, fibers or whiskers. In the past decade or so, a lot of work has been done on how to deposit Ni or Co by electroless plating on fine substrates, including CNTs and carbon nanofibers [7,8], SiC
⁎ Corresponding author. Tel.: +86 451 8641 2133; fax: +86 451 8641 3922. E-mail address: [email protected] (L. Zhen). 0257-8972/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.surfcoat.2006.10.032
particles [9,10], ZrO2 particles [11]. Although electroless plating process has already been under study for many years, to the best of our knowledge, little work has been done on electroless plating metals on SiC whiskers and not much attention has been paid to the growth behavior of plated coating. This situation justifies our efforts to study the process of electroless plating metals on SiC whiskers, and the growth behavior of cobalt coating in particular. Furthermore, the thermal stability of nanoscaled metal structures at high temperature is of great significance, because heat treatment at high temperature is usually required to condition microstructures. Although much work has been done on the thermal stability of metal coatings plated on bulk substrates [12,13], it is obvious that the thermal stability of metal coating plated on whiskers needs further investigation [14]. Therefore, we adopted an electroless plating method to deposit cobalt on SiC whiskers and studied the microstructure evolution of metal coatings during electroless plating and heat treatments. 2. Experimental SiC whiskers of 0.1–1 μm in diameter were used as substrates and the morphology of raw SiC whiskers is shown in Fig. 1a).
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Fig. 1. SEM images of raw SiC whiskers and SiCw/Co nanocomposite powders a) raw SiC whiskers; b) at 15th minute; c) at 30th minute; d) at 60th minute; e) at the end of electroless plating; f) after heat treatment.
Raw SiC whisker was pretreated to obtain catalytic activity before electroless plating because of its chemical inertness. The pretreatment included coarsening, sensitizing–activating and drying. 2 g of pretreated whisker was then added into 400 mL preheated plating solution, which contains 25 g L− 1 cobaltous sulfate, 25 g L− 1 sodium phosphite, 50 g L− 1 sodium citrate and 25 g L− 1 boric acid. During plating, the temperature of the plating solution was kept in the range of 50–55 °C by water bathing. The pH value of the plating solution was kept at 8.5 by instilling 2 M NaOH solution. Measly samples were taken from the plating bath with a sucker at the 15th, 30th, 60th minute and at the end as well, and these samples were named successively as S15, S30 S60 and SE. All these samples were rinsed in deionized water for three times and dried in atmosphere in an oven at 80 °C overnight. Some final nanocomposite powder was heat
treated in hydrogen in an oven at 500 °C for 2 h and the heat treated sample was named as SH5. The morphologies of these nanocomposite particles were observed with a FE-SEM (Hitachi S-4700) and their phase compositions were XRD analyzed (Regaku D/max-γB, Cukα, graphite filter). 3. Results and discussion It can be seen from the SEM images of raw SiC whickers and samples taken at different process time shown in Fig. 1 that, fine cobalt clusters of 30–40 nm in diameter appeared on the surface of SiC whiskers at the 15th minute from the beginning of the reaction. In most of the areas, cobalt clusters distribute uniformly at an interval of about 100 nm as shown in region
J.T. Jiang et al. / Surface & Coatings Technology 201 (2007) 6059–6062
A. However, in few areas few clusters can be observed as shown in region B. It is well known that, electroless deposition occurs first on the surface of catalytic Pd nuclei which forms during sensitizing–activating treatment, so the distribution of initial cobalt clusters is determined by the distribution of Pd nuclei. At this process time, most of the cobalt clusters do not touch each other, which indicate their independent growth. The cobalt clusters grow up to about 60 nm in diameter in 30 min and they start to contact each other and conjoin together to form a continuous coating in the uniformly covered areas. However, there are some areas, as the region marked as B in Fig. 1c), where few clusters can be seen, which indicates that, no cobalt can be deposited in the initial cluster-free areas. The distribution of cobalt clusters at the 30th minute is similar to that observed at the 15th minute, which mean that there is still deposition of cobalt occurring on the surface of the cobalt clusters from the 15th to the 30th minute. A continuous cobalt coating with a rough surface instead of cobalt clusters is observed on SiC whiskers at the 60th minute. It can be easily understood that the cobalt coating at this stage is built up by stacking of cobalt clusters. Similar characteristics of stacking were also observed when copper is electroless plated on the surface of natural pollens or Al2O3 particles [15,16]. A similar evolution from clusters to a continuous coating similar to that in the current study was also observed when a thin copper, platinum or silver film is plated on a bulk substrate by a sputtering process. However, the size of clusters and the intervals between clusters observed during sputtering are much smaller than those observed in the present study [17,18]. Such difference may originate from the difference in the surface curvatures and electrical properties of the substrates. The Pd nuclei stems from the Pd2+ ions adsorbed on the surface while the whiskers are activated. Because of the large surface curvature of SiC whiskers and the electrostatic repulsion between the Pd2+ ions, the adsorption of the Pd2+ ions becomes very difficult, so that the subsequent distribution of Pd particles has a very low density. The metal ions translate into electroneutral atoms immediately after they are deposited by sputtering to the substrate and then the electrostatic repulsion is eliminated. Furthermore, the plane surface of bulk substrate is more suitable for the deposition of metal atoms than the convex surface of fine SiC whiskers. It can be seen from the SEM images of sample SE shown in Fig. 1e) that, in comparison with the morphologies observed at the early stage, some cobalt platelets appear on the surface of cobalt coating. Possibly, these platelets are caused by the nonuniform distribution of deposition sites. After the clusters start to contact each other, the stacking of cobalt clusters creates some lathy and jaggy boundary areas which are more suitable for deposition of cobalt atoms than plane and smooth areas on clusters, and so, cobalt atoms may be preferentially deposited in these boundary areas. Meanwhile, catalytic activity decreases in the plane and smooth areas because little cobalt is deposited to form a fresh layer catalytic with high catalytic activity, which further slows down the cobalt deposition in these areas. The selective and nonuniform deposition eventually leads to the formation of cobalt platelets.
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Fig. 2. XRD patterns of SiCw/Co nanocomposite particles, before and after heat treatment.
Some researchers believe that the formation of these flakes may be due to the rapid hydrogen release during electroless plating [15]. However, we did not see any flake cluster formed at the early stage when there is an intense hydrogen release, and this means that the formation of flakes is due to the nonuniform deposition rather than hydrogen release. It can be seen from Fig. 2 that, there is a great change in the microstructure of cobalt coating after heat treatment. Only weak broaden peaks can be observed in SE patterns, which reveals that the cobalt coating is amorphous. After heat treatment, sharp diffraction peaks corresponding to α-Co can be seen in XRD patterns of SH5, which indicates that the cobalt in coating has been transformed into a crystalline state through heat treatment. Morphological changes took place in cobalt coating during heat treatment at 500 °C. After the heat treatment, the continuous coating totally disappeared and cobalt clusters similar to those obtained at early stages of electroless plating can be seen all over the SiC whickers. This morphological transformation indicates that the cobalt coating is not stable enough to maintain its original structure at 500 °C. It is also found that, the size of the clusters is very close to those of the clusters obtained during electroless plating in some areas (area O), whereas the clusters formed during heat treatment are much larger in sizes in some other areas (area C). The existence of these larger clusters suggests that surface diffusion of cobalt atoms has taken place during heat treatment. A similar phenomenon of aggregation was also observed in a Ni–P coating deposited on the surface of silicon after heat treatment [12]. However, the examination of Ni–W–P coatings on steel substrate after heat treatment shows that, the nodular surface of the coating tends to be flat, which is quite different from what is observed in the present study [13]. The different behaviors mentioned above may be attributed to the difference in bonding strength between substrate and coating. The bonding strength between coating and substrate is very weak because the Pd particles are just adsorbed on the surface of SiC whiskers. The conjunction between coating and SiC whiskers at sites other than those of Pd particles is very weak mechanical bonding, too. Consequently, cobalt coating trends to
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J.T. Jiang et al. / Surface & Coatings Technology 201 (2007) 6059–6062
aggregate to form clusters through surface diffusion to reduce surface energy when it is subjected to a high temperature [14]. 4. Conclusions We successfully prepared SiCw/Co nanocomposite particles by electroless plating and investigated the microstructure evolution of cobalt coating. The surface of the whickers is uniformly covered by insular cobalt clusters in the initial stage of electroless plating. These cobalt clusters grow larger and lager and eventually form a uniform cobalt coating. After the clusters contact, the deposition of cobalt preferentially takes place in the boundary areas, which result in the formation of cobalt flakes. These results reveal that, the growth behavior of the cobalt coating plated on a pretreated inert substrate is an evolution from clusters to a continuous coating. The cobalt coating transforms from an amorphous to a crystalline state and aggregates to clusters through surface diffusion to reduce the surface energy when the SiCw/Co nanocomposite is heat treated in hydrogen at 500 °C. Acknowledgement This work is funded by the Outstanding Young Scientist Foundation of Heilongjiang Province.
References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [14] [15] [16] [17] [18]
D.Z. Wang, H.X. Peng, J. Liu, C.K. Yao, Wear 184 (1995) 187. C.Z. Huang, X. Ai, Mater. Res. Bull. 31 (1996) 951. A. Kassiba, M. Tabellout, Solid State Commun. 115 (2000) 389. J.J. Sun, J.B. Li, G.L. Sun, B. Zhang, Ceram. Int. 28 (2002) 741. I.T. Iakubov, A.N. Lagarkov, Sergi A. Maklakov, J. Magn. Magn. Mater. 258–259 (2003) 195. N.J. Tang, W. Zhong, H.Y. Jiang, Z.D. Han, W.Q. Zou, Y.W. Du, Solid State Commun. 132 (2004) 71. X. Shen, R.Z. Gong, Y. Nie, J.H. Nie, J. Magn. Magn. Mater. 288 (2005) 397. G.W. Xie, Z.B. Wang, Z.L. Cui, Y.L. Shi, Carbon 43 (2005) 3181. Y.J. Chen, M.S. Cao, Q. Xu, J. Zhu, Surf. Coat. Technol. 172 (2003) 90. M. Kang, J.M. Kim, J.W. Kim, Surf. Coat. Technol. 161 (2002) 79. G.W. Wen, Z.X. Guo, C.K.L. Davies, Scr. Mater. 43 (2000) 307. H.X. Li, H.Y. Chen, S.Z. Dong, J.S. Yang, Appl. Surf. Sci. 125 (1998) 115. F.B. Wu, S.K. Tien, J.G. Duh, J.H. Wang, Surf. Coat. Technol. 166 (2003) 60. I.S. Kim, S.K. Lee, Scr. Mater. 52 (2005) 1045. L.N. Xu, K.C. Zhou, H.F. Xu, Appl. Surf. Sci. 183 (2001) 58. G.P. Ling, Y. Li, Mater. Lett. 59 (2005) 1610. V. Podgursky, I. Costina, R. Franchy, Surf. Sci. 529 (2003) 419. J.C. Lin, C.P. Lee, Thin Solid Films 307 (1997) 96.