- Email: [email protected]
Pd multilayer perpendicular magnetic recording media by the addition of an underlayer
Journal of Magnetism and Magnetic Materials 235 (2001) 40–44
Improvement of signal to noise ratio for Co/Pd multilayer perpendicular magnetic recording media by the addition of an underlayer T. Onouea,*, T. Asahib, K. Kuramochia, J. Kawajia, T. Hommaa,c, T. Osakaa,b,c a
Department of Applied Chemistry, School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan b Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan c Advanced Research Institute of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan
Abstract Signal and noise characteristics of Co/Pd multilayer perpendicular magnetic recording media are discussed based upon MFM analysis. The noise of the Co/Pd multilayer media is originated mainly from irregularities in the magnetic transition regions, and can be drastically suppressed by increasing the thickness of the carbon underlayer although a large number of reversed magnetic domains between the magnetic transitions still exist. The fine magnetic clusters formed in the medium with a thick carbon underlayer result in clear magnetic transition boundaries of recorded bits in the high recording density region. By examining the profile of the M2H loops, the decrement in the a value, which is defined as the slope of M2H loop at the value of coercivity, improves the signal to noise (S/N) ratio for the Co/Pd multilayer media. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Multilayers; Perpendicular magnetic recording; Amorphous carbon underlayer
1. Introduction To realize ultra high-density perpendicular magnetic recording, the CoCr-based alloys have been investigated as a magnetic layer of perpendicular magnetization [1,2]. Compared with CoCrbased alloy longitudinal magnetic recording media, CoCr-based alloy perpendicular magnetic *Corresponding author. Tel.: +81-3-5286-3202; fax: +81-33205-2074. E-mail address: [email protected] (T. Onoue).
recording media still show relatively high noise in the signal-recorded state as well as in DC-erased state because of the formation of reversed magnetic domains between magnetic transitions [2]. Moreover, the CoCr-based alloy media with a low squareness ratio (SQR) exhibit signal decay especially in the low recording density region, which is a critical issue in perpendicular magnetic recording media [3]. To suppress the formation of reversed magnetic domains and to improve thermal decay, media with SQR of unity [1] and a large negative
0304-8853/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 2 9 6 - 7
41
T. Onoue et al. / Journal of Magnetism and Magnetic Materials 235 (2001) 40–44
nucleation field (Hn ) [4], such as Co/Pd multilayer media, have been proposed for perpendicular magnetic recording [4–6]. Recently, we have successfully developed a Co/Pd multilayer film possessing a negative Hn of B5000 Oe and a small a value, defined as the slope of M2H loop at the value of coercivity, of B2 by adding an appropriate underlayer [7,8]. In this study, the signal and noise characteristics of the Co/Pd multilayer perpendicular magnetic recording media were investigated based upon MFM analysis, with an emphasis on the media with an additional carbon underlayer.
3. Results and discussion 3.1. Correlation of D50 with magnetic cluster size Figs. 1(a)–(c) show the dependence of signal, noise, and S=N ratio on the carbon underlayer thickness, respectively. The S=N ratio improved with increasing thickness of the carbon underlayer in the entire recording density range examined in this study. It is clearly seen that increasing the thickness of carbon underlayer increases the readback output and decreases the integrated medium noise, resulting in the marked improvement in the S=N ratio as shown in Fig. 1(c). Figs. 2 and 3 show MFM images of ACdemagnetized state (2(a) and 3(a)) and recorded patterns (2(b) and 3(b)) of 300 kFRPI for the C5 and C30 samples, respectively. In the AC-demagnetized state, the size of magnetic clusters decreased with increasing thickness of the carbon underlayer [8]. The C30 sample has fine magnetic clusters in the AC-demagnetized state and exhibits clear recorded transitions in the signal-recorded state. On the other hand, the C5 sample exhibits large magnetic clusters in the AC-demagnetized state, and large magnetic domains that connect neighboring bits cause degradation of the S=N ratio. Fig. 4 shows the dependence of D50 value on the magnetic cluster size estimated from MFM images in the AC-demagnetized state. The cluster size obtained in the AC-demagnetized state shows a close relation with the D50 value, which increases with decreasing magnetic cluster size. These results show that fine magnetic clusters in the ACdemagnetized state are required to clearly form magnetic recorded patterns in the high density recording region.
2. Experimental Co/Pd multilayer media consisting of [Co(0.2 nm)/Pd(0.8 nm)]20/C(X nm, X ¼ 1260 nmÞ were prepared by DC-magnetron sputtering on 2.5 in diameter glass substrates. The magnetic properties of the media used in the present work are shown in Table 1. The SQR of the media was equal to unity within experimental errors. The R=W characteristics were investigated by using a merged MR head for recording (gap length gl ¼ 0:28 mm, track width Tw ¼ 1:45 mm) and readback (shield-to-shield gap length gs-s=0.2 mm, Tw ¼ 0:9 mm), where the linear velocity was 6.35 m/s. The medium noise, corrected for the system noise, was evaluated as an integrated value up to 50 MHz. The recorded signal patterns were observed by using a magnetic force microscopy (MFM, Nanoscope III), with a standard probe coated with a Co alloy film and a lift height of B30 nm.
Table 1 Co/Pd perpendicular magnetic recording media used in the present work Samples
Thickness of C (nm)
Hc (Oe)
Hn (Oe)
a (dimensionless)
C0 C5 C10 C30 C60
0 5 10 30 60
1900 2000 4200 5200 5200
1600 1700 4000 4500 4200
12.1 11.9 9.6 5.3 3.8
42
T. Onoue et al. / Journal of Magnetism and Magnetic Materials 235 (2001) 40–44
Fig. 2. MFM images of (a) AC demagnetized state and (b) the recorded patterns of 300 kFRPI for the C5 sample.
Fig. 3. MFM images of (a) AC demagnetized state and (b) the recorded patterns of 300 kFRPI for the C30 sample.
Fig. 4. Dependence of D50 on magnetic cluster size.
3.2. Reduction of medium noise Fig. 1. Dependence of (a) readback signal output, (b) integrated medium noise and (c) S=N ratio on carbon underlayer thickness at 200 kFRPI (K), 300 kFRPI (m), and 400 kFRPI (~).
Figs. 5(a)–(c) show MFM images of the (a) C5, (b) C30 and (c) C60 samples at the recording density of 20 kFRPI. For the C5 sample shown in Fig. 5(a), few reversed magnetic domains were observed between the magnetic transitions,
T. Onoue et al. / Journal of Magnetism and Magnetic Materials 235 (2001) 40–44
43
Fig. 5. MFM images of recorded patterns at 20 kFRPI for the samples (a) C5, (b) C30, and (c) C60.
although the magnetic transition boundaries were quite irregular. However, with an increase in the thickness of carbon underlayer, the number of reversed magnetic domains increased and the magnetic transition boundaries became sharper. In the case of the media possessing the SQR of unity and a negative Hn [1,4], the formation of reversed magnetic domains was prevented. However, although C60 possesses a much higher value of Hn than C5, the C60 sample also has a large number of reversed magnetic domains between transitions as shown in Fig. 5(c). A small a value implies that a large external field is required to reverse the magnetic domains completely, hence, the presence of such reversed domains suggests that the recording head field is insufficient. However, the MFM image of the C60 sample in the DC-magnetized state (by an external field of 15 kOe) showed reversed magnetic domains. In this case, defects or inhomogeneities within the Co/Pd multilayer may cause the formation of reversed magnetic domains. It appears that a high Hn may not always suppress the formation of reversed magnetic domains between magnetic transitions. Fig. 6 shows noise spectra of the C5, C30, and C60 media at the recording density of 20 kFRPI, which corresponds to the density of the recorded patterns imaged by MFM in Fig. 5. Although a large number of reversed magnetic domains between magnetic transitions were observed for the C60 sample, it exhibited the lowest noise among the three media. These results indicate that the major source of the noise of the Co/Pd multilayer media is the irregularities at the transition regions rather than the reversed mag-
Fig. 6. Noise spectra for C5, C30, and C60 media at the recording density of 20 kFRPI. Sys indicates the system noise.
Fig. 7. Dependence of S=N ratio on a for Co/Pd media with various underlayers.
netic domains between magnetic transitions. The irregular transitions can be attributed to the strong lateral exchange coupling present in Co/Pd multilayer media [8]. Therefore, the low noise obtained by increasing the thickness of carbon underlayer is attributed to the suppression of irregularities at the transition regions. Fig. 7 shows the relationship between the S=N ratio and the a value for the Co/Pd media with various underlayers, i.e., carbon, silicon and palladium. The S=N ratio increases linearly with decreasing a value for all of underlayer materials.
44
T. Onoue et al. / Journal of Magnetism and Magnetic Materials 235 (2001) 40–44
It is anticipated that the a value becomes one when the exchange coupling between magnetic grains is zero [1]. In this study, the smallest value of a obtained was 3.8 for the media with C underlayer of 60 nm in thickness, which would still be too large to achieve an excellent S=N ratio. It is important to decrease the a value to suppress the irregularities at the magnetic transition boundaries.
4. Conclusion Signal and noise characteristics of Co/Pd multilayer perpendicular magnetic recording media were investigated using MFM observations. It was found that the magnetic cluster size obtained in the AC-demagnetized state is closely related to the D50 value for the Co/Pd media. Although a large number of reversed magnetic domains were observed between the magnetic transitions for the C60 sample, it exhibited the lowest medium noise in all the media studied. The noise of the Co/Pd multilayer media primarily originates from the irregularities of magnetic transition boundaries. The finer magnetic clusters of the media with a thicker carbon underlayer lead to the distinct formation of transition boundaries in the high recording density region. Furthermore, reduction
of the a value results in the improvement of S=N ratio for Co/Pd multilayer media.
Acknowledgements This work was financially supported by a grantin-aid for the Future Project, ‘‘Ultra High-density Magnetic Recording’’ of the Japan Society for the Promotion of Science (JSPS-RFTF97R14401) and a grant from the Storage Research Consortium. The authors thank Dr. Y. Okinaka for his useful comments on their manuscript.
References [1] N. Honda, J. Ariake, K. Ouchi, S. Iwasaki, IEEE Trans. Magn. 34 (1998) 1651. [2] Y. Hirayama, M. Futamoto, K. Ito, Y. Honda, Y. Maruyama, IEEE Trans. Magn. 33 (1997) 996. [3] Y. Hirayama, K. Ito, Y. Honda, M. Futamoto, J. Magn. Soc. Japan 21 (S2) (1997) 297. [4] L. Wu, S. Yanese, N. Honda, K. Ouchi, IEEE Trans. Magn. 33 (1997) 3094. [5] G.-L. Chen, J. Appl. Phys. 87 (2000) 6887. [6] W. Peng, R.H. Victoria, J.H. Judy, K. Gao, J.M. Sivertsen, J. Appl. Phys. 87 (2000) 6358. [7] T. Onoue, J. Kawaji, K. Kuramochi, T. Asahi, T. Osaka, Dig. PMRC 2000 (2000) 41. [8] T. Asahi, K. Kuramochi, J. Kawaji, T. Onoue, T. Osaka, Dig. PMRC 2000 (2000) 43.
Improvement of signal to noise ratio for Co/Pd multilayer perpendicular magnetic recording media by the addition of an underlayer T. Onouea,*, T. Asahib, K. Kuramochia, J. Kawajia, T. Hommaa,c, T. Osakaa,b,c a
Department of Applied Chemistry, School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan b Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan c Advanced Research Institute of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan
Abstract Signal and noise characteristics of Co/Pd multilayer perpendicular magnetic recording media are discussed based upon MFM analysis. The noise of the Co/Pd multilayer media is originated mainly from irregularities in the magnetic transition regions, and can be drastically suppressed by increasing the thickness of the carbon underlayer although a large number of reversed magnetic domains between the magnetic transitions still exist. The fine magnetic clusters formed in the medium with a thick carbon underlayer result in clear magnetic transition boundaries of recorded bits in the high recording density region. By examining the profile of the M2H loops, the decrement in the a value, which is defined as the slope of M2H loop at the value of coercivity, improves the signal to noise (S/N) ratio for the Co/Pd multilayer media. r 2001 Elsevier Science B.V. All rights reserved. Keywords: Multilayers; Perpendicular magnetic recording; Amorphous carbon underlayer
1. Introduction To realize ultra high-density perpendicular magnetic recording, the CoCr-based alloys have been investigated as a magnetic layer of perpendicular magnetization [1,2]. Compared with CoCrbased alloy longitudinal magnetic recording media, CoCr-based alloy perpendicular magnetic *Corresponding author. Tel.: +81-3-5286-3202; fax: +81-33205-2074. E-mail address: [email protected] (T. Onoue).
recording media still show relatively high noise in the signal-recorded state as well as in DC-erased state because of the formation of reversed magnetic domains between magnetic transitions [2]. Moreover, the CoCr-based alloy media with a low squareness ratio (SQR) exhibit signal decay especially in the low recording density region, which is a critical issue in perpendicular magnetic recording media [3]. To suppress the formation of reversed magnetic domains and to improve thermal decay, media with SQR of unity [1] and a large negative
0304-8853/01/$ - see front matter r 2001 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 2 9 6 - 7
41
T. Onoue et al. / Journal of Magnetism and Magnetic Materials 235 (2001) 40–44
nucleation field (Hn ) [4], such as Co/Pd multilayer media, have been proposed for perpendicular magnetic recording [4–6]. Recently, we have successfully developed a Co/Pd multilayer film possessing a negative Hn of B5000 Oe and a small a value, defined as the slope of M2H loop at the value of coercivity, of B2 by adding an appropriate underlayer [7,8]. In this study, the signal and noise characteristics of the Co/Pd multilayer perpendicular magnetic recording media were investigated based upon MFM analysis, with an emphasis on the media with an additional carbon underlayer.
3. Results and discussion 3.1. Correlation of D50 with magnetic cluster size Figs. 1(a)–(c) show the dependence of signal, noise, and S=N ratio on the carbon underlayer thickness, respectively. The S=N ratio improved with increasing thickness of the carbon underlayer in the entire recording density range examined in this study. It is clearly seen that increasing the thickness of carbon underlayer increases the readback output and decreases the integrated medium noise, resulting in the marked improvement in the S=N ratio as shown in Fig. 1(c). Figs. 2 and 3 show MFM images of ACdemagnetized state (2(a) and 3(a)) and recorded patterns (2(b) and 3(b)) of 300 kFRPI for the C5 and C30 samples, respectively. In the AC-demagnetized state, the size of magnetic clusters decreased with increasing thickness of the carbon underlayer [8]. The C30 sample has fine magnetic clusters in the AC-demagnetized state and exhibits clear recorded transitions in the signal-recorded state. On the other hand, the C5 sample exhibits large magnetic clusters in the AC-demagnetized state, and large magnetic domains that connect neighboring bits cause degradation of the S=N ratio. Fig. 4 shows the dependence of D50 value on the magnetic cluster size estimated from MFM images in the AC-demagnetized state. The cluster size obtained in the AC-demagnetized state shows a close relation with the D50 value, which increases with decreasing magnetic cluster size. These results show that fine magnetic clusters in the ACdemagnetized state are required to clearly form magnetic recorded patterns in the high density recording region.
2. Experimental Co/Pd multilayer media consisting of [Co(0.2 nm)/Pd(0.8 nm)]20/C(X nm, X ¼ 1260 nmÞ were prepared by DC-magnetron sputtering on 2.5 in diameter glass substrates. The magnetic properties of the media used in the present work are shown in Table 1. The SQR of the media was equal to unity within experimental errors. The R=W characteristics were investigated by using a merged MR head for recording (gap length gl ¼ 0:28 mm, track width Tw ¼ 1:45 mm) and readback (shield-to-shield gap length gs-s=0.2 mm, Tw ¼ 0:9 mm), where the linear velocity was 6.35 m/s. The medium noise, corrected for the system noise, was evaluated as an integrated value up to 50 MHz. The recorded signal patterns were observed by using a magnetic force microscopy (MFM, Nanoscope III), with a standard probe coated with a Co alloy film and a lift height of B30 nm.
Table 1 Co/Pd perpendicular magnetic recording media used in the present work Samples
Thickness of C (nm)
Hc (Oe)
Hn (Oe)
a (dimensionless)
C0 C5 C10 C30 C60
0 5 10 30 60
1900 2000 4200 5200 5200
1600 1700 4000 4500 4200
12.1 11.9 9.6 5.3 3.8
42
T. Onoue et al. / Journal of Magnetism and Magnetic Materials 235 (2001) 40–44
Fig. 2. MFM images of (a) AC demagnetized state and (b) the recorded patterns of 300 kFRPI for the C5 sample.
Fig. 3. MFM images of (a) AC demagnetized state and (b) the recorded patterns of 300 kFRPI for the C30 sample.
Fig. 4. Dependence of D50 on magnetic cluster size.
3.2. Reduction of medium noise Fig. 1. Dependence of (a) readback signal output, (b) integrated medium noise and (c) S=N ratio on carbon underlayer thickness at 200 kFRPI (K), 300 kFRPI (m), and 400 kFRPI (~).
Figs. 5(a)–(c) show MFM images of the (a) C5, (b) C30 and (c) C60 samples at the recording density of 20 kFRPI. For the C5 sample shown in Fig. 5(a), few reversed magnetic domains were observed between the magnetic transitions,
T. Onoue et al. / Journal of Magnetism and Magnetic Materials 235 (2001) 40–44
43
Fig. 5. MFM images of recorded patterns at 20 kFRPI for the samples (a) C5, (b) C30, and (c) C60.
although the magnetic transition boundaries were quite irregular. However, with an increase in the thickness of carbon underlayer, the number of reversed magnetic domains increased and the magnetic transition boundaries became sharper. In the case of the media possessing the SQR of unity and a negative Hn [1,4], the formation of reversed magnetic domains was prevented. However, although C60 possesses a much higher value of Hn than C5, the C60 sample also has a large number of reversed magnetic domains between transitions as shown in Fig. 5(c). A small a value implies that a large external field is required to reverse the magnetic domains completely, hence, the presence of such reversed domains suggests that the recording head field is insufficient. However, the MFM image of the C60 sample in the DC-magnetized state (by an external field of 15 kOe) showed reversed magnetic domains. In this case, defects or inhomogeneities within the Co/Pd multilayer may cause the formation of reversed magnetic domains. It appears that a high Hn may not always suppress the formation of reversed magnetic domains between magnetic transitions. Fig. 6 shows noise spectra of the C5, C30, and C60 media at the recording density of 20 kFRPI, which corresponds to the density of the recorded patterns imaged by MFM in Fig. 5. Although a large number of reversed magnetic domains between magnetic transitions were observed for the C60 sample, it exhibited the lowest noise among the three media. These results indicate that the major source of the noise of the Co/Pd multilayer media is the irregularities at the transition regions rather than the reversed mag-
Fig. 6. Noise spectra for C5, C30, and C60 media at the recording density of 20 kFRPI. Sys indicates the system noise.
Fig. 7. Dependence of S=N ratio on a for Co/Pd media with various underlayers.
netic domains between magnetic transitions. The irregular transitions can be attributed to the strong lateral exchange coupling present in Co/Pd multilayer media [8]. Therefore, the low noise obtained by increasing the thickness of carbon underlayer is attributed to the suppression of irregularities at the transition regions. Fig. 7 shows the relationship between the S=N ratio and the a value for the Co/Pd media with various underlayers, i.e., carbon, silicon and palladium. The S=N ratio increases linearly with decreasing a value for all of underlayer materials.
44
T. Onoue et al. / Journal of Magnetism and Magnetic Materials 235 (2001) 40–44
It is anticipated that the a value becomes one when the exchange coupling between magnetic grains is zero [1]. In this study, the smallest value of a obtained was 3.8 for the media with C underlayer of 60 nm in thickness, which would still be too large to achieve an excellent S=N ratio. It is important to decrease the a value to suppress the irregularities at the magnetic transition boundaries.
4. Conclusion Signal and noise characteristics of Co/Pd multilayer perpendicular magnetic recording media were investigated using MFM observations. It was found that the magnetic cluster size obtained in the AC-demagnetized state is closely related to the D50 value for the Co/Pd media. Although a large number of reversed magnetic domains were observed between the magnetic transitions for the C60 sample, it exhibited the lowest medium noise in all the media studied. The noise of the Co/Pd multilayer media primarily originates from the irregularities of magnetic transition boundaries. The finer magnetic clusters of the media with a thicker carbon underlayer lead to the distinct formation of transition boundaries in the high recording density region. Furthermore, reduction
of the a value results in the improvement of S=N ratio for Co/Pd multilayer media.
Acknowledgements This work was financially supported by a grantin-aid for the Future Project, ‘‘Ultra High-density Magnetic Recording’’ of the Japan Society for the Promotion of Science (JSPS-RFTF97R14401) and a grant from the Storage Research Consortium. The authors thank Dr. Y. Okinaka for his useful comments on their manuscript.
References [1] N. Honda, J. Ariake, K. Ouchi, S. Iwasaki, IEEE Trans. Magn. 34 (1998) 1651. [2] Y. Hirayama, M. Futamoto, K. Ito, Y. Honda, Y. Maruyama, IEEE Trans. Magn. 33 (1997) 996. [3] Y. Hirayama, K. Ito, Y. Honda, M. Futamoto, J. Magn. Soc. Japan 21 (S2) (1997) 297. [4] L. Wu, S. Yanese, N. Honda, K. Ouchi, IEEE Trans. Magn. 33 (1997) 3094. [5] G.-L. Chen, J. Appl. Phys. 87 (2000) 6887. [6] W. Peng, R.H. Victoria, J.H. Judy, K. Gao, J.M. Sivertsen, J. Appl. Phys. 87 (2000) 6358. [7] T. Onoue, J. Kawaji, K. Kuramochi, T. Asahi, T. Osaka, Dig. PMRC 2000 (2000) 41. [8] T. Asahi, K. Kuramochi, J. Kawaji, T. Onoue, T. Osaka, Dig. PMRC 2000 (2000) 43.