Magnetic recording characteristics of a flexible disk with a non-magnetic underlayer

Journal of Magnetism and Magnetic Materials 120 (1993) 16-18 North-Holland

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Magnetic recording characteristics of a flexible disk with a non-magnetic underlayer Hideaki Komoda, Fumio Echigo, Masanaru Hasegawa and Nobuyuki Kaminaka Magnetic Devices Research Laboratory, Matsushita Electric Industrial Co. Ltd, 1006, Kadoma, Kadoma-shi, Osaka, 571 Japan

We have examined a flexible disk for high recording density (35 kFCI, 542 tpi), and with a maximum transfer rate of 4.5 MBPS. By setting the magnetic layer thickness at less than 0.4 p.m, the media coercivity at more than 120 kA/m, and applying it to a non-magnetic underlayer, we have realized useful flexible disk media wear resistance of 2 X 107 passes.

1. Introduction The development of high-density flexible disks is becoming increasingly urgent as the recording density of magnetic media increases rapidly as means of mass data storage. There are two ways to increase media recording density: to increase bit density, and to increase track density. Increasing the bit density involves some problems, however, such as bad overwrite modulation, poor resolution, etc. Using a thin magnetic layer is a good solution, but this creates other problems such as poor head pass endurance, and surface roughness. However, we have realized a useful medium consisting of a thin magnetic layer on a non-magnetic underlayer.

2. Experiment and results We prepared some media with non-magnetic underlayers. These media were constructed of magnetic layers of various thicknesses (0.23-0.6 p~m) made of iron metal particles. The non-magnetic underlayer was about 0.8 ttm thick and contained non-magnetic pigments of ot-Fe20 3 0.1 p~m in diameter and carbon black. The coercivity and residual magnetization of these media were about 120 k A / m (1500 Oe) and about 160 mT (1600 G). Figure 1 shows one such medium with the non-magnetic underlayer. For the measurements we used two types of ringshaped metal head, one a metal-in-gap (MIG) type with a sendust metal layer on both sides of the head gap, and the other a laminated amorphous metal (LAM) type (table 1). Correspondence to: Dr. H. Komoda, Magnetic Devices Research Laboratory, Matsushita Electric Industrial Co. Ltd, 1006, Kadoma, Kadoma-shi, Osaka, 571 Japan. Tel.: +81-6900-9621; telefax: + 81-6-906-7127.

We measured some magnetic recording characteristics on the assumption that these media were used for an unformatted capacity of 28 MB. The maximum recording density was 35 kFCI, and the track density was 542 tpi. The media revolution was 600 rpm. The modulation code was 2 - 7 run-length-limited code (the shortest flux turn period (3T) was 333 ns, and the longest (8T) 888 ns at a maximum data transfer of 4.5 MBPS). Figures 2 and 3 show the relationship between the magnetic layer thickness and the relative outputs of 8.8 and 35 kFCI at a write current of 20 mA, respectively. For each recording density the reference level was the output of single layer of 1.3 ixm. The relative output tended to be smaller in proportion to thickness of the magnetic layer, and that of 35 kFCI was more difficult to reduce than that of 8.8 kFCI. This caused a good resolution with thin media. Comparing the MIG and LAM heads, it was difficult to affect the relative output of the LAM head by altering the magnetic layer thickness because the ability of the LAM head to record high frequencies is poorer than that of the MIG head. Figure 4 shows the dependence of the overwrite modulation (OWM) on the magnetic layer thickness. The OWM is defined by erasability, which is determined from the ratio of the reproduced output of the original 8.8 kFCI signal to the reproduced output of the residual 8.8 kFCI signal after the 23.5 kFCI signal is overwritten. The OWM tended to be proportionally smaller as the thickness of the magnetic layer diminishes and as the head gap length widens. Thus the OWM depends on the thickness of the magnetic layer and the gap length. In fig. 4, we compare the OWM values on various media with coercivities of 125 and 130 k A / m . The data looked like apart of the relationship between the magnetic layer thickness and the OWM of 120 k A / m measured with a head gap lengths

0304-8853/93/$06.00 © 1993 - Elsevier Science Publishers B.V. All rights reserved

H. Komoda et aL / A flexible disk with a non-magnetic underlayer

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Fig. 1. Cross-sectional photograph of a flexible disk with a non-magnetic underlayer.

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Thickness of Magnetic Layer //an] Fig. 3. Relationship between the magnetic layer thickness and the relative output of 35 kFCI, measured at various head gap lengths. The reference level is the output of a single layer at 1.3 ~m.

Table 1 Head properties Type

MIG

LAM

LAM

MIG

G L [p,m] Tw [l~m] Turn

0.16 20 14×2

0.25 44 14×2

0.38 36 14x2

0.45 43 32X2

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of 0.45 p.m. Thus the O W M does not depend on the medium coercivity for a thin magnetic layer. An OWM of less than - 3 0 dB was achieved with a magnetic layer less than 0.4 i~m thick, using a head gap length of about 0.45 p.m. Figure 5 shows the relationship between resolution and the pattern peak shift for various head gap lengths. The resolution was defined by the reproduced output ratio, determined by the 35 and 13.3 kFCI signals under the same write current conditions. The pattern

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Fig. 5. Relationship between resolution and pattern peak shi~.

18

H. Komoda et al. / A flexible disk with a non-magnetic underlayer

peak shift was calculated from pulse intervals generated by reproducing 3 T - 8 T repeated signals. These wave forms were not compensated. The half window width of this format was 55.6 ns (T/2). The pattern peak shift at less than 55 ns was achieved using a resolution higher than 45%. Media wear resistance is one of the most essential prerequisites of flexible disks. In this experiment, the wear resistance of the reference medium (single layer of 1.3 ~m) was more than 2 x 107 passes on the same track at 5°C, 35%RH, but that of a single layer of 0.5 i~m was only 5 × 105 passes under the same conditions. The wear resistance of the media with a 0.41~m mag-

netic layer and non-magnetic underlayer was more than 2 x 107 passes under the same conditions. 3. Conclusion By setting the magnetic layer thickness at less than 0.4 ~m with a media coercivity of more than 120 k A / m , we have developed a flexible disk with high recording density (35 kFCI, 542 tpi), and a maximum transfer rate of 4.5 MBPS. With a non-magnetic underlayer, the thin magnetic layer achieved a wear resistance of 2 x 107 passes.