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Effects of substrate cooling in hard magnetic disk sputtering process on protective overcoat and magnetic layer properties
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Acta mater. Vol. 46, No. 11, pp. 3787±3791, 1998 # 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain S1359-6454(97)00462-X 1359-6454/98 $19.00 + 0.00
EFFECTS OF SUBSTRATE COOLING IN HARD MAGNETIC DISK SPUTTERING PROCESS ON PROTECTIVE OVERCOAT AND MAGNETIC LAYER PROPERTIES E. V. ANOIKIN1, M. M. YANG1, M. T. SULLIVAN1, J. L. CHAO1 and J. W. AGER III2 1 HMT Technology Corporation, Fremont, CA 94538, U.S.A. and 2Materials Sciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720, U.S.A.
(Received 4 April 1997; accepted 17 November 1997) AbstractÐAn in-situ substrate cooling procedure has been carried out in a hard magnetic disk sputtering process to reduce the disk temperature prior to a deposition of a protective overcoat. The eects of cooling on the tribological and magnetic performance of the media are examined and directly compared for the amorphous carbon (a-C), amorphous hydrogenated carbon (a-C:H) and amorphous nitrogenated carbon (a-C:N) overcoat compositions. The substrate cooling procedure is shown to promote sp3-bonding character and to improve the tribological properties of the overcoats. To achieve the best tribological and magnetic performance of the media, the magnetic layer and the protective overcoat should be optimized as a coupled system. # 1998 Acta Metallurgica Inc.
1. INTRODUCTION
2. EXPERIMENTAL PROCEDURE
Fabrication of hard disk magnetic media often requires a signi®cant substrate heating prior to a deposition of an underlayer and magnetic layers. Elevated temperatures are necessary to optimize magnetic properties of the media. However, excessive substrate temperature is known to adversely aect tribological properties of sputtered amorphous carbon (a-C) [1]. Structural studies of sputtered a-C ®lms showed that increasing the substrate temperature promotes the formation of sp2-bonded clusters instead of the desirable sp3-bonded atomic con®gurations [2]. Presently, amorphous hydrogenated carbon (a-C:H) and amorphous nitrogenated carbon (a-C:N) overcoat compositions are preferred by the hard disk media manufacturers [3, 4]. Based on the data published for a-C ®lms, lower substrate temperature may be bene®cial for the performance of a-C:H and a-C:N overcoats as well. In this work, we have introduced an in-situ substrate cooling procedure to the magnetic disk sputtering process to reduce the disk temperature before sputtering protective overlayers of various compositions on the same underlayer/magnetic layer structures. The eects of the cooling procedure on the tribological and magnetic properties of the media are examined and directly compared for the three most popular types of overcoat.
All the sputtering experiments were performed on an INTEVAC 250A d.c. magnetron stationary sputtering system [5]. Process diagram is shown in Fig. 1. A cooling station was installed between the magnetic layer sputtering chamber and the carbon chamber. Two series of disks (``cold'' and reference) were deposited in each particular experiment under the identical process conditions, except the cooling procedure was included in the process of depositing the ``cold'' disks. Cooling was achieved by placing the disk between the two closely spaced cold plates and rapidly pressurizing the chamber with He gas up to a total pressure of 10 Torr. Liquid nitrogen ¯ow (T = 77 K) was used to refrigerate the cold plates. Disk temperature prior to cooling was 02408C. Special care was taken to accurately measure the disk temperature drop as a result of cooling with a calibrated infrared sensor. Cooling procedure was found to reduce the disk temperature by 608C in our experiments without aecting a total system throughput. All disks were fabricated on mechanically textured 95 mm diameter AlMg/NiP substrates, using Cr underlayer and CoCrTa magnetic layer. Carbon overcoats were sputtered in pure Ar gas (a-C samples), Ar±N2 mixture (a-C:N samples) and Ar± H2 mixture (a-C:H samples). Overcoat composition was studied by Auger spectroscopy and 2.3 MeV He2+ Rutherford backscattering spectrometry
3787
3788
ANOIKIN et al.: EFFECT OF SUBSTRATE COOLING 3. RESULTS AND DISCUSSION
3.1. Composition/structure of the protective overcoat
Fig. 1. Process diagram of the magnetic hard disk sputtering.
(RBS). Atomic concentration was determined with an accuracy of 210%. Overcoat structure was probed by Raman spectroscopy using a 476-nm wavelength line of Ar+-laser at a power of 10± 20 mW for excitation. The collected Raman spectra were ®t by the two overlapping Gaussian bands known as D-band and G-band [6, 7]. The accuracy of the Gaussian parameters determination was estimated by ®tting the repeated spectral measurements on the same disk. As a result, the precision of the G-peak center frequency measurement was found to be 21.1 cmÿ1, while the peak intensity ratio of the D-band to the G-band ID/IG was determined with 20.04 accuracy. Overcoat thickness was ®xed at 120 AÊ in the experiments, as con®rmed by the pro®lometry, atomic force microscopy and ellipsometry. Remanent magnetic characteristics of the media (coercivity Hr, magnetization-thickness product Mrt and coercivity squareness Sr*) were measured non-destructively with a remanent moment magnetometer [5]. The disks were lubricated with a per¯uoropolyether lubricant layer. After lubrication, the disks were subject to 20 000 contact startstop (CSS) cycles. The head used was a 50% inductive proximity recording head with a sub-ambient pressure air bearing and a 3.5 g pre-load. In each cycle, the disk was accelerated to 4500 rpm at 1200 rpm/s, held at 4500 rpm for 3 s and then allowed to coast to 0 rpm. The disk would then remain idle for 3 s before the next cycle began. Tests were run in both ``hot/wet'' (308C/80% R.H.) and ``hot/dry'' (558C/10% R.H.) environments. Head-interface crash was de®ned as a 1 visible wear through the overcoat after 20,000 CSS cycles.
Table 1 presents the results of N and H atomic concentration measurements in the overcoats studied. Comparison of the ``cold'' and reference samples revealed some in¯uence of cooling on H concentration, most noticeable being the promotion of H incorporation in the ``cold'' a-C:H samples. Neither N concentration nor Auger depth pro®le of N in the a-C:N overcoat were aected by cooling. Figure 2 shows Raman spectra for the overcoats of various compositions. The spectra consist of the two broad overlapping bands (G-band centered at 1560±1580 cmÿ1 and D-band at 1370±1390 cmÿ1). Parameters of the bands determined by the Gaussian deconvolution of the spectra are reported in Table 2. Raman G-band position nG and peak intensity ratio of the D-band to the G-band ID/IG are considered to be the important indicators of the amorphous carbon structure and tribological properties [6, 7]. ``Cold'' samples showed consistently lower nG values than the reference ones. Ratio ID/IG decreased as a result cooling for the a-C:H and a-C:N samples and was not changed for the aC samples within the measurement uncertainty. The Raman spectra modi®cations thus point to the higher sp3 content and better tribological properties of the ``cold'' samples [6, 7]. Both Raman G-band shift DnG and ID/IG change due to cooling were the most pronounced for the a-C:H overcoat composition, indicating the enhanced sensitivity of this type of overcoat to a disk temperature during sputtering. 3.2. Tribological properties of the media Stiction and wear of the lubricated disks were studied by CSS cycling in hot/wet and hot/dry environments, respectively. The results are presented in Fig. 3 and Table 3. The cooling procedure was found to have a bene®cial in¯uence on the headdisk interface reliability for all three overcoat compositions studied. The results agree with the previous ®ndings of the lower stiction on a-C:H layers with higher H content [3] and the higher wear resistance of a-C layers sputtered at lower disk temperature [1]. 3.3. Magnetic properties of the media Table 4 presents the results of the magnetic measurements on the media samples with various Table 1. Composition of the protective overcoats Overcoat type a-C a-C:N a-C:H
Cooling
N, at.%
H, at.%
no yes no yes no yes
<1 <1 15.9 16.3 <1 <1
13.5 10.9 9.1 7.3 25.4 30.6
ANOIKIN et al.: EFFECT OF SUBSTRATE COOLING
3789
Fig. 2. Raman spectra of the overcoats studied.
overcoats. These results show that the remanent coercivity of the media was in¯uenced by the overcoat composition. Hr is clearly reduced in the media
with a-C:N overcoat as compared to the other overcoats on the same substrate/underlayer/magnetic layer structure. The cooling procedure was found to
Table 2. Results of the Gaussian deconvolution of the Raman scattering spectra Overcoat Type a-C a-C:N a-C:H
Cooling no yes no yes no yes
G-band center (cmÿ1) 1582.6 1578.9 1568.0 1563.4 1575.4 1566.9
D-band center (cmÿ1) 1384.0 1383.0 1378.5 1375.5 1385.7 1386.3
G-band width (cmÿ1)
D-band width (cmÿ1)
113 124 150 155 114 118
370 369 362 367 360 384
DnG=nG(reference) ÿ nG(``cold''); ID/IG Ð peak intensity ratio of the D-band to the G-band.
DnG (cmÿ1) 3.7 4.6 8.5
ID/IG 1.32 1.34 1.45 1.41 0.79 0.70
3790
ANOIKIN et al.: EFFECT OF SUBSTRATE COOLING
Fig. 3. Results of the CSS testing in the hot/wet environment.
partially recover the Hr in the a-C:N samples. In contrast, the cooling procedure resulted in the reduced Hr for the samples with a-C and a-C:H overcoats. Eects of cooling on Hr were therefore dependent on the type of overcoat used, indicating that an interaction existed between the magnetic
and overcoat layers. The cooling procedure had no eect on the Mrt and the Sr* of the media within the accuracy of the measurements. The exact origin of the cooling-induced coercivity change is not clear at the moment and it is under investigation.
Table 3. Results of the CSS testing in hot/dry environment
Table 4. Remanent magnetic characteristics of the media
Overcoat type a-C a-C:N a-C:H
Cooling no yes no yes no yes
Number of disks tested 12 12 15 15 15 15
Number of crashes 5 3 5 3 6 3
Overcoat Type a-C a-C:N a-C:H
Cooling
Hr (kA/m)
Mrt (mA)
no yes no yes no yes
181.12 175.60 165.36 169.76 180.80 171.20
21.0 20.6 21.6 21.2 21.5 21.3
Sr* 0.93 0.94 0.94 0.93 0.92 0.93
ANOIKIN et al.: EFFECT OF SUBSTRATE COOLING 4. CONCLUSIONS
An in-situ substrate cooling procedure was performed in the magnetic hard disk sputtering process to reduce the disk temperature before sputtering the protective overcoat. The cooling procedure was found to aect the properties of both the protective overcoat and the magnetic layer. Structural changes due to cooling were detected in the overcoats which indicate higher sp3 content and better tribological properties. CSS studies con®rmed the bene®cial in¯uence of cooling on the protective overcoat performance. Coercivity of the magnetic layer was also aected by the cooling procedure and the eects were dependent on the overcoat composition, indicating the magnetic layer-overcoat interactions. The cooling processing step is useful in improving the tribological properties of thin ®lm media. The overcoat and magnetic layers should be optimized as a coupled system to achieve the best tribological and magnetic performance.
3791
AcknowledgementsÐThe authors thank M. A. Russak for his support of this work and critical reading of the manuscript, Joga Khangura, Charles Lee, Guy Ng, Duc Duong and Rekha Desai of HMT Technology, Ray Goldberg and Steve Aragon of INTEVAC for their assistance. Brad Burrow of Philips Semiconductors is gratefully acknowledged for the Auger/RBS measurements. REFERENCES 1. Agarwal, S., Li, E. and Heiman, N., IEEE Trans. Magn., 1993, 29, 264. 2. Cho, N. H., Veirs, D. K., Ager, J. W. III, Rubin, M. D., Hopper, C. B. and Bogy, D. B., J. Appl. Phys., 1992, 71, 2243. 3. Wang, R.-H., Meeks, S. W., White, R. L. and Weresin, W. E., IEEE Trans. Magn., 1995, 31, 2919. 4. Yang, M. M., Chao, J. L. and Russak, M. A., IEEE Trans. Magn., 1997, 33, 3145. 5. Johnson, K. E., Thin ®lm media, Magnetic Disk Drive Technology, ed. K. G. Ashar, IEEE Press, 1997. 6. Cho, N.-H., Krishnan, K. M., Veirs, D. K., Rubin, M. D., Hopper, C. B., Bhushan, B. and Bogy, D. B., J. Mater. Res., 1990, 5, 2543. 7. Ager, J. W. III, IEEE Trans. Magn., 1993, 29, 259.
Acta mater. Vol. 46, No. 11, pp. 3787±3791, 1998 # 1998 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved Printed in Great Britain S1359-6454(97)00462-X 1359-6454/98 $19.00 + 0.00
EFFECTS OF SUBSTRATE COOLING IN HARD MAGNETIC DISK SPUTTERING PROCESS ON PROTECTIVE OVERCOAT AND MAGNETIC LAYER PROPERTIES E. V. ANOIKIN1, M. M. YANG1, M. T. SULLIVAN1, J. L. CHAO1 and J. W. AGER III2 1 HMT Technology Corporation, Fremont, CA 94538, U.S.A. and 2Materials Sciences Division, Lawrence Berkeley National Laboratory, University of California, Berkeley, CA 94720, U.S.A.
(Received 4 April 1997; accepted 17 November 1997) AbstractÐAn in-situ substrate cooling procedure has been carried out in a hard magnetic disk sputtering process to reduce the disk temperature prior to a deposition of a protective overcoat. The eects of cooling on the tribological and magnetic performance of the media are examined and directly compared for the amorphous carbon (a-C), amorphous hydrogenated carbon (a-C:H) and amorphous nitrogenated carbon (a-C:N) overcoat compositions. The substrate cooling procedure is shown to promote sp3-bonding character and to improve the tribological properties of the overcoats. To achieve the best tribological and magnetic performance of the media, the magnetic layer and the protective overcoat should be optimized as a coupled system. # 1998 Acta Metallurgica Inc.
1. INTRODUCTION
2. EXPERIMENTAL PROCEDURE
Fabrication of hard disk magnetic media often requires a signi®cant substrate heating prior to a deposition of an underlayer and magnetic layers. Elevated temperatures are necessary to optimize magnetic properties of the media. However, excessive substrate temperature is known to adversely aect tribological properties of sputtered amorphous carbon (a-C) [1]. Structural studies of sputtered a-C ®lms showed that increasing the substrate temperature promotes the formation of sp2-bonded clusters instead of the desirable sp3-bonded atomic con®gurations [2]. Presently, amorphous hydrogenated carbon (a-C:H) and amorphous nitrogenated carbon (a-C:N) overcoat compositions are preferred by the hard disk media manufacturers [3, 4]. Based on the data published for a-C ®lms, lower substrate temperature may be bene®cial for the performance of a-C:H and a-C:N overcoats as well. In this work, we have introduced an in-situ substrate cooling procedure to the magnetic disk sputtering process to reduce the disk temperature before sputtering protective overlayers of various compositions on the same underlayer/magnetic layer structures. The eects of the cooling procedure on the tribological and magnetic properties of the media are examined and directly compared for the three most popular types of overcoat.
All the sputtering experiments were performed on an INTEVAC 250A d.c. magnetron stationary sputtering system [5]. Process diagram is shown in Fig. 1. A cooling station was installed between the magnetic layer sputtering chamber and the carbon chamber. Two series of disks (``cold'' and reference) were deposited in each particular experiment under the identical process conditions, except the cooling procedure was included in the process of depositing the ``cold'' disks. Cooling was achieved by placing the disk between the two closely spaced cold plates and rapidly pressurizing the chamber with He gas up to a total pressure of 10 Torr. Liquid nitrogen ¯ow (T = 77 K) was used to refrigerate the cold plates. Disk temperature prior to cooling was 02408C. Special care was taken to accurately measure the disk temperature drop as a result of cooling with a calibrated infrared sensor. Cooling procedure was found to reduce the disk temperature by 608C in our experiments without aecting a total system throughput. All disks were fabricated on mechanically textured 95 mm diameter AlMg/NiP substrates, using Cr underlayer and CoCrTa magnetic layer. Carbon overcoats were sputtered in pure Ar gas (a-C samples), Ar±N2 mixture (a-C:N samples) and Ar± H2 mixture (a-C:H samples). Overcoat composition was studied by Auger spectroscopy and 2.3 MeV He2+ Rutherford backscattering spectrometry
3787
3788
ANOIKIN et al.: EFFECT OF SUBSTRATE COOLING 3. RESULTS AND DISCUSSION
3.1. Composition/structure of the protective overcoat
Fig. 1. Process diagram of the magnetic hard disk sputtering.
(RBS). Atomic concentration was determined with an accuracy of 210%. Overcoat structure was probed by Raman spectroscopy using a 476-nm wavelength line of Ar+-laser at a power of 10± 20 mW for excitation. The collected Raman spectra were ®t by the two overlapping Gaussian bands known as D-band and G-band [6, 7]. The accuracy of the Gaussian parameters determination was estimated by ®tting the repeated spectral measurements on the same disk. As a result, the precision of the G-peak center frequency measurement was found to be 21.1 cmÿ1, while the peak intensity ratio of the D-band to the G-band ID/IG was determined with 20.04 accuracy. Overcoat thickness was ®xed at 120 AÊ in the experiments, as con®rmed by the pro®lometry, atomic force microscopy and ellipsometry. Remanent magnetic characteristics of the media (coercivity Hr, magnetization-thickness product Mrt and coercivity squareness Sr*) were measured non-destructively with a remanent moment magnetometer [5]. The disks were lubricated with a per¯uoropolyether lubricant layer. After lubrication, the disks were subject to 20 000 contact startstop (CSS) cycles. The head used was a 50% inductive proximity recording head with a sub-ambient pressure air bearing and a 3.5 g pre-load. In each cycle, the disk was accelerated to 4500 rpm at 1200 rpm/s, held at 4500 rpm for 3 s and then allowed to coast to 0 rpm. The disk would then remain idle for 3 s before the next cycle began. Tests were run in both ``hot/wet'' (308C/80% R.H.) and ``hot/dry'' (558C/10% R.H.) environments. Head-interface crash was de®ned as a 1 visible wear through the overcoat after 20,000 CSS cycles.
Table 1 presents the results of N and H atomic concentration measurements in the overcoats studied. Comparison of the ``cold'' and reference samples revealed some in¯uence of cooling on H concentration, most noticeable being the promotion of H incorporation in the ``cold'' a-C:H samples. Neither N concentration nor Auger depth pro®le of N in the a-C:N overcoat were aected by cooling. Figure 2 shows Raman spectra for the overcoats of various compositions. The spectra consist of the two broad overlapping bands (G-band centered at 1560±1580 cmÿ1 and D-band at 1370±1390 cmÿ1). Parameters of the bands determined by the Gaussian deconvolution of the spectra are reported in Table 2. Raman G-band position nG and peak intensity ratio of the D-band to the G-band ID/IG are considered to be the important indicators of the amorphous carbon structure and tribological properties [6, 7]. ``Cold'' samples showed consistently lower nG values than the reference ones. Ratio ID/IG decreased as a result cooling for the a-C:H and a-C:N samples and was not changed for the aC samples within the measurement uncertainty. The Raman spectra modi®cations thus point to the higher sp3 content and better tribological properties of the ``cold'' samples [6, 7]. Both Raman G-band shift DnG and ID/IG change due to cooling were the most pronounced for the a-C:H overcoat composition, indicating the enhanced sensitivity of this type of overcoat to a disk temperature during sputtering. 3.2. Tribological properties of the media Stiction and wear of the lubricated disks were studied by CSS cycling in hot/wet and hot/dry environments, respectively. The results are presented in Fig. 3 and Table 3. The cooling procedure was found to have a bene®cial in¯uence on the headdisk interface reliability for all three overcoat compositions studied. The results agree with the previous ®ndings of the lower stiction on a-C:H layers with higher H content [3] and the higher wear resistance of a-C layers sputtered at lower disk temperature [1]. 3.3. Magnetic properties of the media Table 4 presents the results of the magnetic measurements on the media samples with various Table 1. Composition of the protective overcoats Overcoat type a-C a-C:N a-C:H
Cooling
N, at.%
H, at.%
no yes no yes no yes
<1 <1 15.9 16.3 <1 <1
13.5 10.9 9.1 7.3 25.4 30.6
ANOIKIN et al.: EFFECT OF SUBSTRATE COOLING
3789
Fig. 2. Raman spectra of the overcoats studied.
overcoats. These results show that the remanent coercivity of the media was in¯uenced by the overcoat composition. Hr is clearly reduced in the media
with a-C:N overcoat as compared to the other overcoats on the same substrate/underlayer/magnetic layer structure. The cooling procedure was found to
Table 2. Results of the Gaussian deconvolution of the Raman scattering spectra Overcoat Type a-C a-C:N a-C:H
Cooling no yes no yes no yes
G-band center (cmÿ1) 1582.6 1578.9 1568.0 1563.4 1575.4 1566.9
D-band center (cmÿ1) 1384.0 1383.0 1378.5 1375.5 1385.7 1386.3
G-band width (cmÿ1)
D-band width (cmÿ1)
113 124 150 155 114 118
370 369 362 367 360 384
DnG=nG(reference) ÿ nG(``cold''); ID/IG Ð peak intensity ratio of the D-band to the G-band.
DnG (cmÿ1) 3.7 4.6 8.5
ID/IG 1.32 1.34 1.45 1.41 0.79 0.70
3790
ANOIKIN et al.: EFFECT OF SUBSTRATE COOLING
Fig. 3. Results of the CSS testing in the hot/wet environment.
partially recover the Hr in the a-C:N samples. In contrast, the cooling procedure resulted in the reduced Hr for the samples with a-C and a-C:H overcoats. Eects of cooling on Hr were therefore dependent on the type of overcoat used, indicating that an interaction existed between the magnetic
and overcoat layers. The cooling procedure had no eect on the Mrt and the Sr* of the media within the accuracy of the measurements. The exact origin of the cooling-induced coercivity change is not clear at the moment and it is under investigation.
Table 3. Results of the CSS testing in hot/dry environment
Table 4. Remanent magnetic characteristics of the media
Overcoat type a-C a-C:N a-C:H
Cooling no yes no yes no yes
Number of disks tested 12 12 15 15 15 15
Number of crashes 5 3 5 3 6 3
Overcoat Type a-C a-C:N a-C:H
Cooling
Hr (kA/m)
Mrt (mA)
no yes no yes no yes
181.12 175.60 165.36 169.76 180.80 171.20
21.0 20.6 21.6 21.2 21.5 21.3
Sr* 0.93 0.94 0.94 0.93 0.92 0.93
ANOIKIN et al.: EFFECT OF SUBSTRATE COOLING 4. CONCLUSIONS
An in-situ substrate cooling procedure was performed in the magnetic hard disk sputtering process to reduce the disk temperature before sputtering the protective overcoat. The cooling procedure was found to aect the properties of both the protective overcoat and the magnetic layer. Structural changes due to cooling were detected in the overcoats which indicate higher sp3 content and better tribological properties. CSS studies con®rmed the bene®cial in¯uence of cooling on the protective overcoat performance. Coercivity of the magnetic layer was also aected by the cooling procedure and the eects were dependent on the overcoat composition, indicating the magnetic layer-overcoat interactions. The cooling processing step is useful in improving the tribological properties of thin ®lm media. The overcoat and magnetic layers should be optimized as a coupled system to achieve the best tribological and magnetic performance.
3791
AcknowledgementsÐThe authors thank M. A. Russak for his support of this work and critical reading of the manuscript, Joga Khangura, Charles Lee, Guy Ng, Duc Duong and Rekha Desai of HMT Technology, Ray Goldberg and Steve Aragon of INTEVAC for their assistance. Brad Burrow of Philips Semiconductors is gratefully acknowledged for the Auger/RBS measurements. REFERENCES 1. Agarwal, S., Li, E. and Heiman, N., IEEE Trans. Magn., 1993, 29, 264. 2. Cho, N. H., Veirs, D. K., Ager, J. W. III, Rubin, M. D., Hopper, C. B. and Bogy, D. B., J. Appl. Phys., 1992, 71, 2243. 3. Wang, R.-H., Meeks, S. W., White, R. L. and Weresin, W. E., IEEE Trans. Magn., 1995, 31, 2919. 4. Yang, M. M., Chao, J. L. and Russak, M. A., IEEE Trans. Magn., 1997, 33, 3145. 5. Johnson, K. E., Thin ®lm media, Magnetic Disk Drive Technology, ed. K. G. Ashar, IEEE Press, 1997. 6. Cho, N.-H., Krishnan, K. M., Veirs, D. K., Rubin, M. D., Hopper, C. B., Bhushan, B. and Bogy, D. B., J. Mater. Res., 1990, 5, 2543. 7. Ager, J. W. III, IEEE Trans. Magn., 1993, 29, 259.