- Email: [email protected]
Electroless-deposited soft magnetic underlayer on silicon disk substrate for double-layered perpendicular magnetic recording media
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 287 (2005) 292–297 www.elsevier.com/locate/jmmm
Electroless-deposited soft magnetic underlayer on silicon disk substrate for double-layered perpendicular magnetic recording media T. Osakaa,b,c,, T. Asahib, T. Yokoshimac, J. Kawajia a
Major in Applied Chemistry, Graduate School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo, 169-8555, Japan b Major in Nano-Science and Nano-Engineering, Graduate School of Science and Engineering, Waseda University, 513 Wasedatsurumaki-cho, Shinjuku-ku, Tokyo 162-0041, Japan c Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, 2-8-26 Nishiwaseda, Shinjuku-ku, Tokyo 169-0051, Japan Available online 4 November 2004
Abstract A novel fabrication process of a soft magnetic underlayer (SUL) for a double-layered perpendicular magnetic recording medium was presented. The CoNiFeB SUL was deposited on a silicon disk substrate using an electroless deposition. The Ni seed layer for the electroless deposition was prepared by an electrochemical process. The surface of the deposited SUL was subjected to a chemical mechanical polishing to be flattened, and Ra value of the SUL was less than 0.4 nm. A magnetic domain structure greatly depended on the electroless deposition condition. Particularly, the control of an agitation speed during electroless deposition is much effective for the suppression of distinct domain walls appearing in CoNiFeB underlayers. r 2004 Published by Elsevier B.V. PACS: 75.50.Ss; 82.45.Aa Keywords: Perpendicular magnetic recording medium; Soft magnetic underlayer; Electroless deposition; Silicon disk
1. Introduction Corresponding author. Major in Applied Chemistry,
Graduate School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo, 169-8555, Japan. Tel.: +81 3 5286 3202; fax: +81 3 3205 2074. E-mail address: [email protected] (T. Osaka). 0304-8853/$ - see front matter r 2004 Published by Elsevier B.V. doi:10.1016/j.jmmm.2004.10.086
A double-layered perpendicular magnetic recording medium, which consists of a perpendicular magnetic recording layer and a soft magnetic underlayer (SUL), is an important candidate as the medium for realizing perpendicular magnetic
ARTICLE IN PRESS T. Osaka et al. / Journal of Magnetism and Magnetic Materials 287 (2005) 292–297
recording with an ultra-high recording density [1]. Therefore, developments of both the recording layer and the SUL of a double-layered perpendicular magnetic recording medium are much significant to fabricate a high-performance perpendicular magnetic recording medium. In the case of the recording layer, a magnetic thin film possessing fine granular crystal grains with strong perpendicular magnetic anisotropy has attracted much attention. In the case of the SUL, a large value of Bs t (Bs saturation magnetic flux density; t thickness of the SUL) is required for making the best use of the writing ability of a magnetic recording head. Various sputter-deposited SULs with a large value of Bs t have been investigated. However, a practical application of a sputterdeposited soft magnetic film to the SUL is rather difficult because the SUL with a relatively large amount of Fe element should be deposited to be more than a few hundred nanometers in thickness. Therefore, not only a soft magnetic thin film with high Bs but also a high cost-effective method for depositing a thick film has been required. ‘Plating’ methods have great advantages of thick film deposition and mass productivity compared with sputtering methods, and useful data for plating soft magnetic thin films with high Bs have been accumulated so far [2–4]. Especially, electroless deposition is a promising candidate among fabrication processes of the SUL [5–8] because the plating method realizes good uniformity of a thickly deposited film and does not need any external power supply. On the other hand, distinct domain walls in an SUL cause a serious problem, namely, the occurrence of ‘spike noise’ in the read–write characteristics. Usually, the hard magnetic layer called a pinning layer is used for suppressing the distinct domain walls. We recently developed a CoNiFeB SUL without distinct domain walls using an electroless deposition under appropriate bath and deposition conditions. In the CoNiFeB SUL, we did not need any pinning layer fortunately. However, the sputterdeposited Cu/Ti layer was indispensable as a seed layer on the glass substrate [7]. In other words, we had to introduce the dry processes to prepare the CoNiFeB SUL. That was a demerit of the new
293
fabrication method for the electroless SUL without distinct domain walls. We have tried to develop more useful fabrication methods for the electroless CoNiFeB SUL, which consists of wet processes only. Previously, we reported that metal seeds for electroless deposition were readily formed on a silicon substrate using an electrochemical process [9]. In addition, a silicon wafer disk substrate with a diameter smaller than 1 in is available at a low cost and shows a superior smooth surface. In this study, we propose a novel method of all wet processes for fabricating an electroless CoNiFeB SUL by using the silicon substrate.
2. Experimental A 2.5-in Si(1 0 0) single crystal disk was used as a substrate. First, Ni metal seeds were chemically deposited, whose bath compositions and operating conditions are shown in Table 1 [9], prior to the electroless deposition of the CoNiFeB SUL. The bath compositions and operating conditions for the CoNiFeB SUL appeared in our previous work [4], where the composition ratio of magnetic elements in wt% was set to be Co: Ni:Fe=72:13:15. The bath was agitated by means of a rotating substrate method for uniform deposition. When rotating the sample, a magnetic field can be applied in the direction parallel to the sample surface. After electroless deposition of the CoNiFeB SULs, they were subjected to the chemical mechanical polishing (CMP). The polished CoNiFeB SUL became 500–900 nm in thickness. The crystalline structure was investigated with X-ray diffraction (XRD) and Table 1 Bath compositions and operating conditions for chemical deposition of Ni 3
Chemicals
Concentration/mol dm
(NH4)2SO4 NiSO4 6H2O
0.50 0.06
pH Temperature
8 adjusted with NH4OH 85 1C
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transmission electron microscopy (TEM). The film composition of the SUL was analyzed with an Xray fluorescence. The surface roughness of the SUL was measured using a laser microscope and an atomic force microscope. Magnetic properties were measured with a vibrating sample magnetometer. Two-dimensional magnetic domain patterns on the whole disk were observed with an optical surface analyzer to confirm magnetic domain walls. Local magnetic domain patterns were observed using magnetic force microscope (MFM) and a Bitter technique.
3. Results and discussion 3.1. Chemically deposited Ni seed layer To fabricate the electroless-deposited SUL, the substrate surface should be catalyzed. It was reported that the Ni metal clusters acting as a catalyzed surface could be formed with an electrochemical process [9]. The bath compositions and operating conditions were further optimized to prepare the continuous Ni seed layer in this study. Fig. 1(a) shows a cross-sectional TEM image of the Ni seed layer. As shown in Fig. 1(a) the continuous film was successfully formed and the smooth surface was obtained. Figs. 1(b2) and (b2) are micro-beam diffraction patterns of the surface region of the Ni layer and the interface region between the Ni layer and Si substrate, respectively, which revealed that the surface region was well crystallized, while the microstructure of the interface region indicated an amorphous state. Fig. 1(c) shows the film composition of Ni and Si in the Ni layer on the Si substrate in the direction perpendicular to the film surface, where the analyzing points of Ni and Si contents are indicated in the cross-sectional TEM image. From Figs. 1 (a)–(c), the Ni content increased with an increase in the layer thickness and the microstructure of the Ni layer was changed from an amorphous-like phase to a fine crystalline phase as the layer thickness increased. Judging from our previous work that the Ni seed layer was formed with an electrochemical process such as a displacement plating method and that Si would be
Fig. 1. Cross-sectional TEM image (a); microdiffraction patterns (b1) and (b2); and film composition (c) of a chemically deposited Ni seed layer on Si substrate. The images of (b1) and (b2) are the patterns of surface region of the Ni layer and interface region between the Ni layer and Si substrate, respectively. The analyzing points of Ni and Si contents are indicated in the TEM image (a).
oxidized into Si–O at the wafer surface [8], it can be said that both the oxidation of Si and the reduction of Ni ion occurred near the substrate surface at the initial stage of the reaction, and thereby the amorphous phase consisting of Si–O and Ni was formed. As above-mentioned, the Ni seed layer with good crystallinity was obtained at the top region of the layer surface. It is known that Ni metal exhibits a catalysis for the chemical reaction of the reducing agent (dimethylamineborane) used in the electroless deposition of CoNiFeB [10]. Therefore, the Ni layer prepared in this study is expected to be a useful seed layer for the electroless deposition of the CoNiFeB film.
ARTICLE IN PRESS T. Osaka et al. / Journal of Magnetism and Magnetic Materials 287 (2005) 292–297
3.2. Electroless-deposited CoNiFeB underlayer Fig. 2 shows magnetic domain images and M–H hysteresis loops for the CoNiFeB SULs deposited with different operating conditions. The CoNiFeB SULs with a uniform thickness were obtained using an agitation by rotating the substrate. From XRD analysis, reflection peaks coming from a FCC structure were observed for each SUL. Fig. 2(a) shows the image and M–H hysteresis loop of the SUL prepared with low agitation speed and without an external magnetic field. Fig. 2(b) shows the results of the SUL prepared with the optimized agitation speed and without the external magnetic field. The characteristics of the M–H loops are roughly the same and isotropic magnetic properties in the in-plane direction were observed for
Fig. 2. Kerr images and M–H loops of CoNiFeB SULs deposited on different conditions; low agitation speed without external magnetic field (a), optimized agitation speed without external magnetic field (b), optimized agitation speed under external magnetic field applied (c).
295
both SULs. The value of Hc is about 4 Oe. Comparing these magnetic domain images, a crucial difference was observed. The SUL deposited with the optimized speed contained no domain wall, while the SUL with low agitation speed possessed distinct domain walls. Fig. 2(c) shows the magnetic domain image and M–H hysteresis loop of the SUL prepared with the optimized agitation speed, which was the same condition as the case of Fig. 2(b), and under the external magnetic field applied. Comparing the M–H loop of Fig. 2(b) with that of Fig. 2(c), characteristics of both loops were greatly different. Although magnetic anisotropy of the SUL of Fig. 2(b) was not observed in the in-plane directions, the SUL of Fig. 2(c) exhibited an easy axis along a circumferential direction and a hard axis along a radial direction, respectively. The value of Hc in both directions for the SUL of Fig. 2(c) decreased to less than 1 Oe. The Bs value of the SUL of Fig. 2(c) is larger by 1 kG than that of Fig. 2(b), which would be attributable to a little difference in the film composition between them. On the other hand, magnetic domain patterns for the SULs of Figs. 2(b) and (c) are roughly the same and the SULs of both were likely to contain no distinct domain wall, independent of the external magnetic field applied during deposition. Figs. 3(a) and (c) show MFM images and Figs. 3(b) and (d) represent the optical micrographs obtained with a Bitter technique for the CoNiFeB SULs deposited under the external magnetic field, applied or unapplied. The SUL deposited without the field indicated micromagnetic domains (Figs. 3(a) and (b)), but the SUL deposited under the field applied hardly showed micromagnetic domains (Figs. 3(c) and (d)). The marked domain boundaries shown in Fig. 3(d) did not intrinsically exist in the SUL and were induced by the sample preparation. It was reported that the SUL deposited under the appropriate agitation conditions and the external magnetic field applied did not indicate distinct domain walls [5,6]. Therefore, both appropriate conditions of the agitation and external magnetic field are much effective for the deposition of the SULs without distinct domain walls.
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Fig. 3. MFM images (a), (c); optical micrographs with a Bitter technique (b), (d) of CoNiFeB films. (a) and (b) are the results of the SULs deposited without external magnetic fields; (c) and (d) are those deposited under external magnetic fields applied.
3.3. Noise characteristics of CoNiFeB/Ni/silicon medium Noise properties of the SULs were investigated. As-deposited SULs indicated large surface roughness of more than 50 nm. Therefore, the SULs were polished with CMP, and the Ra value of the SULs were reduced to less than 0.4 nm. The ringtype head was used for this investigation and the spacing between the SUL and top of the head is 30 nm. The DC noise of the SUL deposited without an external magnetic field was higher than that of the SUL deposited under the external field applied. Especially, the higher DC noise of the SUL deposited without the external field was observed at the lower frequency. It is suggested that micromagnetic domains caused high DC noise of the SUL. On the other hand, the spike noise was often detected in the case of the SULs deposited under the external magnetic field applied, but they do not show a distinct domain wall as shown in
Fig. 2 (c). The relationship among plating conditions, magnetic domain structures and the occurrence of spike and DC noises for the electrolessdeposited SUL is now being investigated.
4. Conclusion An electroless-deposited CoNiFeB soft magnetic underlayer on a silicon disk substrate for double-layered perpendicular magnetic recording media was developed. The fabrication method for the CoNiFeB underlayer consisted of wet processes only, where the Ni seed layer was electrochemically deposited on the Si substrate. Magnetic domain structures of the CoNiFeB underlayer greatly depended on the electroless deposition condition. The control of an agitation speed during the electroless deposition is much effective for the suppression of distinct domain walls appearing in CoNiFeB underlayers. The
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spike noise of the underlayer deposited with the appropriate speed was successfully suppressed, while the DC noise caused by micromagnetic domains was observed. The SUL deposited with the appropriate speed and under the external magnetic field applied does not show distinct domain walls nor micromagnetic domains, but it often exhibits spike noise.
Acknowledgements This work was carried out at the ‘‘Center for Practical Nano-Chemistry’’ in the 21C-COE Program, MEXT, Japan. The work was financially supported by grants-in-aid of the Special Coordination Funds for Promoting Science and Technology, and the Center of Excellence Research from MEXT. The authors thank Professor K. Hono, National Institute for Materials Science, for useful TEM data. The authors also thank Mr. T. Tsumori, Shin-Etsu Chemical Co. for his fruitful
297
discussion and Mr. M. Ohmori, Showa Denko K.K, and H. Sakai, Showa Denko HD. K.K, for their cooperation. J.K. acknowledges the Research Fellowship of the Japan Society for the Promotion of Science for Young Scientists.
References [1] S. Iwasaki, Y. Nakamura, IEEE Trans. Magn. 13 (1977) 1272. [2] T. Osaka, et al., Nature 392 (1998) 6474. [3] T. Yokoshima, et al., IEEE Trans. Magn. 40 (2004) 2332. [4] T. Yokoshima, et al., J. Electroanal. Chem. 491 (2000) 197. [5] T. Asahi, et al., Novel preparation method for perpendicular double layered media composed of Co/(Pt, Pd) multilayer/soft magnetic underlayer with high Bs value, Digest of the 25th Annual Conference on Magnetic Society of Japan, 2001 pp. 130. [6] S. Saito, et al., J. Magn. Soc. Japan 23 (2004) 289. [7] T. Asahi, et al., IEEE Trans. Magn. 40 (2004) 2356. [8] H. Uwazumi, et al., IEEE Trans. Magn. 40 (2004) 2392. [9] N. Takano, et al., J. Electrochem. Soc. 146 (1999) 1407. [10] I. Ohno, et al., J. Electrochem. Soc. 132 (1985) 2323.
Journal of Magnetism and Magnetic Materials 287 (2005) 292–297 www.elsevier.com/locate/jmmm
Electroless-deposited soft magnetic underlayer on silicon disk substrate for double-layered perpendicular magnetic recording media T. Osakaa,b,c,, T. Asahib, T. Yokoshimac, J. Kawajia a
Major in Applied Chemistry, Graduate School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo, 169-8555, Japan b Major in Nano-Science and Nano-Engineering, Graduate School of Science and Engineering, Waseda University, 513 Wasedatsurumaki-cho, Shinjuku-ku, Tokyo 162-0041, Japan c Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, 2-8-26 Nishiwaseda, Shinjuku-ku, Tokyo 169-0051, Japan Available online 4 November 2004
Abstract A novel fabrication process of a soft magnetic underlayer (SUL) for a double-layered perpendicular magnetic recording medium was presented. The CoNiFeB SUL was deposited on a silicon disk substrate using an electroless deposition. The Ni seed layer for the electroless deposition was prepared by an electrochemical process. The surface of the deposited SUL was subjected to a chemical mechanical polishing to be flattened, and Ra value of the SUL was less than 0.4 nm. A magnetic domain structure greatly depended on the electroless deposition condition. Particularly, the control of an agitation speed during electroless deposition is much effective for the suppression of distinct domain walls appearing in CoNiFeB underlayers. r 2004 Published by Elsevier B.V. PACS: 75.50.Ss; 82.45.Aa Keywords: Perpendicular magnetic recording medium; Soft magnetic underlayer; Electroless deposition; Silicon disk
1. Introduction Corresponding author. Major in Applied Chemistry,
Graduate School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo, 169-8555, Japan. Tel.: +81 3 5286 3202; fax: +81 3 3205 2074. E-mail address: [email protected] (T. Osaka). 0304-8853/$ - see front matter r 2004 Published by Elsevier B.V. doi:10.1016/j.jmmm.2004.10.086
A double-layered perpendicular magnetic recording medium, which consists of a perpendicular magnetic recording layer and a soft magnetic underlayer (SUL), is an important candidate as the medium for realizing perpendicular magnetic
ARTICLE IN PRESS T. Osaka et al. / Journal of Magnetism and Magnetic Materials 287 (2005) 292–297
recording with an ultra-high recording density [1]. Therefore, developments of both the recording layer and the SUL of a double-layered perpendicular magnetic recording medium are much significant to fabricate a high-performance perpendicular magnetic recording medium. In the case of the recording layer, a magnetic thin film possessing fine granular crystal grains with strong perpendicular magnetic anisotropy has attracted much attention. In the case of the SUL, a large value of Bs t (Bs saturation magnetic flux density; t thickness of the SUL) is required for making the best use of the writing ability of a magnetic recording head. Various sputter-deposited SULs with a large value of Bs t have been investigated. However, a practical application of a sputterdeposited soft magnetic film to the SUL is rather difficult because the SUL with a relatively large amount of Fe element should be deposited to be more than a few hundred nanometers in thickness. Therefore, not only a soft magnetic thin film with high Bs but also a high cost-effective method for depositing a thick film has been required. ‘Plating’ methods have great advantages of thick film deposition and mass productivity compared with sputtering methods, and useful data for plating soft magnetic thin films with high Bs have been accumulated so far [2–4]. Especially, electroless deposition is a promising candidate among fabrication processes of the SUL [5–8] because the plating method realizes good uniformity of a thickly deposited film and does not need any external power supply. On the other hand, distinct domain walls in an SUL cause a serious problem, namely, the occurrence of ‘spike noise’ in the read–write characteristics. Usually, the hard magnetic layer called a pinning layer is used for suppressing the distinct domain walls. We recently developed a CoNiFeB SUL without distinct domain walls using an electroless deposition under appropriate bath and deposition conditions. In the CoNiFeB SUL, we did not need any pinning layer fortunately. However, the sputterdeposited Cu/Ti layer was indispensable as a seed layer on the glass substrate [7]. In other words, we had to introduce the dry processes to prepare the CoNiFeB SUL. That was a demerit of the new
293
fabrication method for the electroless SUL without distinct domain walls. We have tried to develop more useful fabrication methods for the electroless CoNiFeB SUL, which consists of wet processes only. Previously, we reported that metal seeds for electroless deposition were readily formed on a silicon substrate using an electrochemical process [9]. In addition, a silicon wafer disk substrate with a diameter smaller than 1 in is available at a low cost and shows a superior smooth surface. In this study, we propose a novel method of all wet processes for fabricating an electroless CoNiFeB SUL by using the silicon substrate.
2. Experimental A 2.5-in Si(1 0 0) single crystal disk was used as a substrate. First, Ni metal seeds were chemically deposited, whose bath compositions and operating conditions are shown in Table 1 [9], prior to the electroless deposition of the CoNiFeB SUL. The bath compositions and operating conditions for the CoNiFeB SUL appeared in our previous work [4], where the composition ratio of magnetic elements in wt% was set to be Co: Ni:Fe=72:13:15. The bath was agitated by means of a rotating substrate method for uniform deposition. When rotating the sample, a magnetic field can be applied in the direction parallel to the sample surface. After electroless deposition of the CoNiFeB SULs, they were subjected to the chemical mechanical polishing (CMP). The polished CoNiFeB SUL became 500–900 nm in thickness. The crystalline structure was investigated with X-ray diffraction (XRD) and Table 1 Bath compositions and operating conditions for chemical deposition of Ni 3
Chemicals
Concentration/mol dm
(NH4)2SO4 NiSO4 6H2O
0.50 0.06
pH Temperature
8 adjusted with NH4OH 85 1C
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transmission electron microscopy (TEM). The film composition of the SUL was analyzed with an Xray fluorescence. The surface roughness of the SUL was measured using a laser microscope and an atomic force microscope. Magnetic properties were measured with a vibrating sample magnetometer. Two-dimensional magnetic domain patterns on the whole disk were observed with an optical surface analyzer to confirm magnetic domain walls. Local magnetic domain patterns were observed using magnetic force microscope (MFM) and a Bitter technique.
3. Results and discussion 3.1. Chemically deposited Ni seed layer To fabricate the electroless-deposited SUL, the substrate surface should be catalyzed. It was reported that the Ni metal clusters acting as a catalyzed surface could be formed with an electrochemical process [9]. The bath compositions and operating conditions were further optimized to prepare the continuous Ni seed layer in this study. Fig. 1(a) shows a cross-sectional TEM image of the Ni seed layer. As shown in Fig. 1(a) the continuous film was successfully formed and the smooth surface was obtained. Figs. 1(b2) and (b2) are micro-beam diffraction patterns of the surface region of the Ni layer and the interface region between the Ni layer and Si substrate, respectively, which revealed that the surface region was well crystallized, while the microstructure of the interface region indicated an amorphous state. Fig. 1(c) shows the film composition of Ni and Si in the Ni layer on the Si substrate in the direction perpendicular to the film surface, where the analyzing points of Ni and Si contents are indicated in the cross-sectional TEM image. From Figs. 1 (a)–(c), the Ni content increased with an increase in the layer thickness and the microstructure of the Ni layer was changed from an amorphous-like phase to a fine crystalline phase as the layer thickness increased. Judging from our previous work that the Ni seed layer was formed with an electrochemical process such as a displacement plating method and that Si would be
Fig. 1. Cross-sectional TEM image (a); microdiffraction patterns (b1) and (b2); and film composition (c) of a chemically deposited Ni seed layer on Si substrate. The images of (b1) and (b2) are the patterns of surface region of the Ni layer and interface region between the Ni layer and Si substrate, respectively. The analyzing points of Ni and Si contents are indicated in the TEM image (a).
oxidized into Si–O at the wafer surface [8], it can be said that both the oxidation of Si and the reduction of Ni ion occurred near the substrate surface at the initial stage of the reaction, and thereby the amorphous phase consisting of Si–O and Ni was formed. As above-mentioned, the Ni seed layer with good crystallinity was obtained at the top region of the layer surface. It is known that Ni metal exhibits a catalysis for the chemical reaction of the reducing agent (dimethylamineborane) used in the electroless deposition of CoNiFeB [10]. Therefore, the Ni layer prepared in this study is expected to be a useful seed layer for the electroless deposition of the CoNiFeB film.
ARTICLE IN PRESS T. Osaka et al. / Journal of Magnetism and Magnetic Materials 287 (2005) 292–297
3.2. Electroless-deposited CoNiFeB underlayer Fig. 2 shows magnetic domain images and M–H hysteresis loops for the CoNiFeB SULs deposited with different operating conditions. The CoNiFeB SULs with a uniform thickness were obtained using an agitation by rotating the substrate. From XRD analysis, reflection peaks coming from a FCC structure were observed for each SUL. Fig. 2(a) shows the image and M–H hysteresis loop of the SUL prepared with low agitation speed and without an external magnetic field. Fig. 2(b) shows the results of the SUL prepared with the optimized agitation speed and without the external magnetic field. The characteristics of the M–H loops are roughly the same and isotropic magnetic properties in the in-plane direction were observed for
Fig. 2. Kerr images and M–H loops of CoNiFeB SULs deposited on different conditions; low agitation speed without external magnetic field (a), optimized agitation speed without external magnetic field (b), optimized agitation speed under external magnetic field applied (c).
295
both SULs. The value of Hc is about 4 Oe. Comparing these magnetic domain images, a crucial difference was observed. The SUL deposited with the optimized speed contained no domain wall, while the SUL with low agitation speed possessed distinct domain walls. Fig. 2(c) shows the magnetic domain image and M–H hysteresis loop of the SUL prepared with the optimized agitation speed, which was the same condition as the case of Fig. 2(b), and under the external magnetic field applied. Comparing the M–H loop of Fig. 2(b) with that of Fig. 2(c), characteristics of both loops were greatly different. Although magnetic anisotropy of the SUL of Fig. 2(b) was not observed in the in-plane directions, the SUL of Fig. 2(c) exhibited an easy axis along a circumferential direction and a hard axis along a radial direction, respectively. The value of Hc in both directions for the SUL of Fig. 2(c) decreased to less than 1 Oe. The Bs value of the SUL of Fig. 2(c) is larger by 1 kG than that of Fig. 2(b), which would be attributable to a little difference in the film composition between them. On the other hand, magnetic domain patterns for the SULs of Figs. 2(b) and (c) are roughly the same and the SULs of both were likely to contain no distinct domain wall, independent of the external magnetic field applied during deposition. Figs. 3(a) and (c) show MFM images and Figs. 3(b) and (d) represent the optical micrographs obtained with a Bitter technique for the CoNiFeB SULs deposited under the external magnetic field, applied or unapplied. The SUL deposited without the field indicated micromagnetic domains (Figs. 3(a) and (b)), but the SUL deposited under the field applied hardly showed micromagnetic domains (Figs. 3(c) and (d)). The marked domain boundaries shown in Fig. 3(d) did not intrinsically exist in the SUL and were induced by the sample preparation. It was reported that the SUL deposited under the appropriate agitation conditions and the external magnetic field applied did not indicate distinct domain walls [5,6]. Therefore, both appropriate conditions of the agitation and external magnetic field are much effective for the deposition of the SULs without distinct domain walls.
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Fig. 3. MFM images (a), (c); optical micrographs with a Bitter technique (b), (d) of CoNiFeB films. (a) and (b) are the results of the SULs deposited without external magnetic fields; (c) and (d) are those deposited under external magnetic fields applied.
3.3. Noise characteristics of CoNiFeB/Ni/silicon medium Noise properties of the SULs were investigated. As-deposited SULs indicated large surface roughness of more than 50 nm. Therefore, the SULs were polished with CMP, and the Ra value of the SULs were reduced to less than 0.4 nm. The ringtype head was used for this investigation and the spacing between the SUL and top of the head is 30 nm. The DC noise of the SUL deposited without an external magnetic field was higher than that of the SUL deposited under the external field applied. Especially, the higher DC noise of the SUL deposited without the external field was observed at the lower frequency. It is suggested that micromagnetic domains caused high DC noise of the SUL. On the other hand, the spike noise was often detected in the case of the SULs deposited under the external magnetic field applied, but they do not show a distinct domain wall as shown in
Fig. 2 (c). The relationship among plating conditions, magnetic domain structures and the occurrence of spike and DC noises for the electrolessdeposited SUL is now being investigated.
4. Conclusion An electroless-deposited CoNiFeB soft magnetic underlayer on a silicon disk substrate for double-layered perpendicular magnetic recording media was developed. The fabrication method for the CoNiFeB underlayer consisted of wet processes only, where the Ni seed layer was electrochemically deposited on the Si substrate. Magnetic domain structures of the CoNiFeB underlayer greatly depended on the electroless deposition condition. The control of an agitation speed during the electroless deposition is much effective for the suppression of distinct domain walls appearing in CoNiFeB underlayers. The
ARTICLE IN PRESS T. Osaka et al. / Journal of Magnetism and Magnetic Materials 287 (2005) 292–297
spike noise of the underlayer deposited with the appropriate speed was successfully suppressed, while the DC noise caused by micromagnetic domains was observed. The SUL deposited with the appropriate speed and under the external magnetic field applied does not show distinct domain walls nor micromagnetic domains, but it often exhibits spike noise.
Acknowledgements This work was carried out at the ‘‘Center for Practical Nano-Chemistry’’ in the 21C-COE Program, MEXT, Japan. The work was financially supported by grants-in-aid of the Special Coordination Funds for Promoting Science and Technology, and the Center of Excellence Research from MEXT. The authors thank Professor K. Hono, National Institute for Materials Science, for useful TEM data. The authors also thank Mr. T. Tsumori, Shin-Etsu Chemical Co. for his fruitful
297
discussion and Mr. M. Ohmori, Showa Denko K.K, and H. Sakai, Showa Denko HD. K.K, for their cooperation. J.K. acknowledges the Research Fellowship of the Japan Society for the Promotion of Science for Young Scientists.
References [1] S. Iwasaki, Y. Nakamura, IEEE Trans. Magn. 13 (1977) 1272. [2] T. Osaka, et al., Nature 392 (1998) 6474. [3] T. Yokoshima, et al., IEEE Trans. Magn. 40 (2004) 2332. [4] T. Yokoshima, et al., J. Electroanal. Chem. 491 (2000) 197. [5] T. Asahi, et al., Novel preparation method for perpendicular double layered media composed of Co/(Pt, Pd) multilayer/soft magnetic underlayer with high Bs value, Digest of the 25th Annual Conference on Magnetic Society of Japan, 2001 pp. 130. [6] S. Saito, et al., J. Magn. Soc. Japan 23 (2004) 289. [7] T. Asahi, et al., IEEE Trans. Magn. 40 (2004) 2356. [8] H. Uwazumi, et al., IEEE Trans. Magn. 40 (2004) 2392. [9] N. Takano, et al., J. Electrochem. Soc. 146 (1999) 1407. [10] I. Ohno, et al., J. Electrochem. Soc. 132 (1985) 2323.