MP tape for high density digital recording

Journal of Magnetism and Magnetic Materials 155 (1996) 80-82

4 ~ i Journalof magnetism 4 ~ i and magnetic materials

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MP tape for high density digital recording H.J. Richter *, R.J. Veitch BASF Magnetics GmbH, D-68004 Mannheim, Germany

Abstract Experimental particulate tapes using very fine highly advanced metal particles are shown to yield higher output than commercially available Hi8 metal evaporated tapes. Based on simple signal to noise considerations, we argue that MP tape is able to sustain almost the same areal recording density as ME tape, and that forseeable needs for information storage on tape can best be met with proven and trusted particulate coating technology.

1. Introduction When the first metal evaporated (ME) tape was released onto the market in 1989, the recording performance of ME tape was significantly better than that of metal particle tapes available at the time. It thus seemed that MP tape had been irrevocably overtaken by ME tape. Recently, very thin MP coatings have been reported to yield excellent recording properties [1]. The present paper presents experimental data for both thick and thin MP coatings. It is shown that the output of thick as well as thin MP tape can exceed that of commercially available ME tape. Furthermore, it is found that thin layer MP tape can yield narrower pulse widths than ME, as well as comparable overwrite performance. In Section 3 we discuss the density limits of these media based on signal to noise considerations.

2. Experimental results for advanced MP tape The output frequency spectra shown in Fig. 1 demonstrate that experimental MP tape can exceed the performance of commercially available ME tape. Typical magnetic data for our MP tapes in this quality range are as follows: remanent magnetization M r = 340 k A / m (i.e. comparable to ME tape), static coercivity H c = 185 k A / m , squareness S = 0.9, switching field distribution SFD = 0.25 and orientation factor ~ 2.5. The tape thickness was ~ 1 p~m, i.e. it is not a thin layer MP tape. For this tape, we

* Corresponding author. Present address: Seagate Recording Media, 47010 Kato Road, Fremont, CA 94538, USA. Fax: + 1510-651-7916; email: hans [email protected].

used advanced metal particles of categories MP1 and MP2, as shown in Table 1. The recording current for each tape was optimized at f = 6.5 MHz (wavelength A = 500 nm). Note that the difference between ME and MP is almost constant over most of the spectrum mainly due to the use of a narrow gap record head. For comparison, Fig. 1 also shows data from a Hi8 MP tape of average quality. The improved performance of our new tapes is essentially due to the increased magnetization and coercivity (Hi8 MP: M r = 200 k A / m , H~ = 125 k A / m ) and the utilization of smaller particles which lead to a better head to tape contact. Fig. 2 shows the pulse widths PWso and pulse amplitudes for MP tapes with comparable magnetic data as functions of the coating thickness. Note that the electrical gap length g of the record head used is larger (236 nm) than that used for the measurements shown in Fig. 1 (170 nm). Therefore, a frequency spectrum measured for the sample with the thick magnetic coating is not parallel to that of ME tape measured using the head with a 236 nm gap length (not shown). The maximum output at short wavelength is virtually independent of the layer thickness. As can be inferred from Fig. 2, this is due to the combined effect of the decreases in PWs0 and the pulse amplitude. For comparison, data for ME tape are also given. From the results it can be concluded that both PWs 0 and the pulse amplitude of MP tape can be adjusted by a proper choice of layer thickness to match ME tape. In a thick magnetic coating, the medium can not considered to be saturated up to a sharply defined 'depth of recording'. Due to the switching field distribution, there will be a zone of partial magnetization. The state of magnetization in this zone is strongly dependent on demagnetizing fields. This can lead to a poor overwrite performance, particularly if a very long wavelength has to

0304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSD10304-8853(95)00695-8

H.J. Richter, R.J. Veitch / Journal of Magnetism and Magnetic Materials 155 (1996) 80 82 ~,~, 40 " . - ~

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be overwritten by a short one. Fig. 3 demonstrates that a suitable reduction in layer thickness improves the overwrite performance o f M P tapes to a degree comparable to M E tapes.

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The fundamental physical medium parameter which limits the m a x i m u m obtainable recording density is the particle size [2]. Following a recent analysis o f rigid disk media [3], we assume that at least 1000 panicles are needed to define a bit to ensure a sufficient signal to noise ratio. The dimensions of a bit are characterized by its bit length BL and the track width W. The depth o f the bit is taken to be B L / 2 . Table I presents data on particles for the various media. For M P tape, we examined five types o f particles: the first two represent the state of the art available now (1995), and the other three are projections which we believe will result from future developments. The crucial parameter for future developments of M P is the thickness o f the oxide shell, d~, needed to passivate the metallic core o f the particles. The onset o f superparamagnetism prevents the use of arbitrarily small particles. As a criterion for magnetic stability we assume that the energy barrier to be o v e r c o m e at magnetization reversal has to be larger than 150kT ( k = B o l t z m a n n ' s constant, T = temperature in K). W e analyzed barium ferrite and M E tape using the same scheme. For more details, see Ref. [4]. The consequence of particle size for bit aspect ratio A and the achievable areal density D is shown in Fig. 4. Using the bit dimensions (length BL, width W, and depth B L / 2 ) together with the limiting panicle sizes from Table 1, and the assumption that at least 1000 particles are needed to define a bit, we can calculate the threshold lines for each medium, i.e. the bit aspect ratio as function o f the areal storage density. This means that the media have to be operated on the left-hand side of these lines. At decreasing bit length, the particle length can cause signal degradation (see also Ref. [5]). We therefore limited the m i n i m u m bit length by the condition BL >_ 0.75L ( L = panicle length; see Table I ). This condition prevents the utilization o f very

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Fig. 1. Measured output spectra for experimental MP tape, commercial Hi8 MP tape of average quality and commercial Hi8 ME tape. Head to medium velocity: 3.17 m / s , writing gap length 170 nm, reading gap length 150 nm, output level optimized at 6.5 MHz.

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Table 1 Particle data for the various media. L = particle length, D = particle diameter, d, = thickness of the oxide shell in the case of MP, V = volume of the metallic core in the case of MP, Vp = total particle volume, AW = energy barrier to be overcome at magnetization reversal. MP3-5 are hypothetical Medium

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H.J. Richter, R.J. Veitch / Journal of Magnetism and Magnetic Materials 155 (1996) 80-82

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large aspect ratios along with high areal densities, The threshold lines have to be interpreted in conjunction with system developments. Up to now, these more or less reflect the signal to noise ratio, which depends on the bit dimensions as [2]

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In Fig. 4 the triangles represent actual data for digital systems. Using Eq. (1), the dashed line gives a fit enabling us to estimate future system developments. We conclude from Fig. 4 that (a) the length loss is not a critical issue, and (b) the recording density limit of MP tape is almost as high as that of ME tape.

ment for a high output is relaxed. In such a case, barium ferrite is also a candidate for a high-density recording medium. If inductive reading is utilized, barium ferrite cannot compete with the other two, simply because its output level is too low due to its low magnetization. In Section 2 we discussed the effects of layer thickness on the recording behaviour of MP tape. The most striking feature of thin layers concerns the improved overwrite capability. This helps the system manufacturer, who can dispense with erase heads. The price for this is paid by the medium manufacturer, since thin magnetic coatings require a more complicated coating technology [1]. In the end, the system and tape manufacturers will have to figure out who has to do the unpleasant job. There is no conclusive physical argument in favour of thin magnetic coatings; rather, it is a question of system design whether thin magnetic coatings are required. Thin coatings may be preferred for consumer applications in order to minimize the equipment cost and power consumption, while professional machines will use heavy-duty thick layer coatings. Based on these arguments and the experimental data shown here, we believe that MP coatings can satisfy prospective demands for storage capacity.

Acknowledgements We would like to express our thanks to all our colleagues in the research and development departments of BASF who contributed to the present work.

References 4. Discussion As can be seen from Fig. 4, the ultimate recording densities to be achieved with both ME and MP tape are comparable. In Fig. 4 it is tacitly assumed that the magnetic information can actually be read, i.e. that medium noise dominates over system noise. We realize that this may not be possible using inductive reading technology. If magnetoresistive reading elements are used, the require-

[1] H. Inaba, K. Ejiri, N. Abe, K. Masaki and H. Araki, IEEE Trans. Magn. 29 (1993) 3607. [2] J.C. Mallinson, The Foundations of Magnetic Recording (Academic Press, San Diego, CA, 1987). [3] T. Yogi and T.A. Nguyen, IEEE Trans. Magn. 29 (1993) 307. [4] H.J. Richter and R.J. Veitch, IEEE Trans. Magn. 31 (1995) 2883. [5] H.J. Richter and R.J. Veitch, J. Magn. Magn. Mater. 155 (1996) 335 (these proceedings).