Double-layer particulate magnetic recording media

M,M

Journal of Magnetism and Magnetic Materials 120 (1993) 11-15 North-Holland

Double-layer particulate magnetic recording media Dennis E. Speliotis Advanced Development Corporation, 8 Ray Avenue, Burlington, MA 01803, USA

Double-layer media employ two layers with different magnetic characteristics, with the objective of optimizing performance and/or reducing the overall cost. The idea has recently been applied to longitudinally oriented thin top layers of Ba-ferrite (BF) or metal particle (MP) coated over thicker Co-iron oxide (COFe) or non-magnetic (NM) underlayers resulting in significantly improved recording performance for high recording density video, digital audio and data applications.

1. Introduction The concept of double-layer particulate coatings as a vehicle to optimize the performance of magnetic tape is an old one, but recently it is finding some very promising applications. Typically, the concept involves the employment of a thinner, higher coercivity top layer (closest to the head), and a thicker, lower coercivity or non-magnetic, a n d / o r lower cost, bottom layer. The optimal recording depth at a wavelength (A) is about A/3. Therefore, the thickness of the top layer should be approximately AmtJ3 , where A~n is the minimum recorded wavelength. A t the band edge of Hi-8 mm video (100 kcfi equivalent density) we have A / 3 - - 0 . 1 7 I~m. A single-layer coating of 0.17 p,m would be very difficult to achieve with respect to ~niformity, surface smoothness and durability, and furthermore it would be highly deficient with respect to chrominance signal output (for video) or imbedded servo signal output (for helical scan data recording), since both o~lthese signals are recorded at much longer wavelengths'. For such high-density applications, a double-layer coating offers very significant advantages, plus the possibility of lower cost by using less expensive particles for the bottom layer. One of the most promising candidates for very high-density data, video, and digital audio recording is particulate Ba-ferrite (BF). Compared to metal particle (MP) tapes, BF tapes show superior high-density signal and frequency response characteristics but inferior low-density output. Consequently, double-coated tapes consisting of a thin BF top layer and a thick Co-3/ (VHS-type) bottom layer offer the promise of optimized signal and frequency response across the entire

Correspondence to: Dr. D.E. Speliotis, Advanced Development Corporation, 8 Ray Avenue, Burlington, MA 01803, USA.

spectrum, in addition to lower cost. Alternatively, a thin MP layer can be coated on top of a thick Co-y or non-magnetic (titanium oxide) layer to provide improved performance and lower-cost tapes. In this paper we report on the magnetic properties and the recording performance characteristics of various double-layer tapes.

2. Experimental The magnetic properties were measured in a DMS 1660 dual V S M / T o r q u e magnetometer at a peak field of 12 kOe. Figure l(a) shows the longitudinal hysteresis and remanence loops of a double layer B F / C o F e tape, and fig. l(b) shows an expanded plot of the loops near the coercive points. The switching of the different coercivity layers can be clearly seen. This tape consists of a 0.6 p.m BF layer with coercivity of about 1400 Oe on top of a 2.4 i~m CoFe layer (SVHS) with coercivity of about 950 Oe. At long wavelengths (5 ~m) this double-coated tape matches the signal output of Hi-8 mm MP tapes, whereas a thick (3 I~m) single layer BF tape of the same coercivity has a signal output of - 2.5 dB compared with Hi-8 mm MP tape at 5 {xm wavelength. At short wavelengths, the double-coated BF tape distinctly outperforms the Hi-8 mm MP tapes with respect to signal output and SNR. Recording measurements were made using a Media Logic ML4500 system which employs 8 mm mechanisms and MIG heads with a gap length of 0.25 ~m. The relative head-to-tape velocity is 3.18 m / s . Figure 2 shows the relative output of the double-layer BF tape described above as a function of recording frequency, and compares it with a thick, single-layer BF tape and with a typical Hi-8 MP tape. Double coating boosts the output signal across the entire band. The longitudinal squareness of the BF coatings in this case is only 0.67, and significant additional improvement can be ob-

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

12 (a)

D.E. Speliotis / Double-layer magnetic recording media '.~ ~

Table 1 Magnetic properties of the top MP layer of double-coated MP/TiO 2 and MP/CoFe (VHS) tapes (longitudinal/ transverse/perpendicular)

" (b)

S Fig. 1. (a) Longitudinal hysteresis loop and remanence curve of double-layer BF/C~Fe tape. (b) Expanded plot of (a) near the coercive point.

tained by increasing it (by about 2 dB at low frequencies and 5 - 8 dB at high frequencies). Instead of a BF top coating one may use an MP top coating with the underlayer being either lower-coercivity CoFe (VHS or SVI-IS type) or completely non-magnetic (NM) such as titanium oxide. Table 1 lists the magnetic properties of the top layers of two such tapes, with the subscript 'r' denoting remanence parameters. The parameters shown in table 1, are the magnetic layer thickness t, the saturation induction Bs, the hysteresis loop squareness SQ (SQ = B r / B s, where B r is the remanent induction), the coercivity H c, the

ttop [Ixm] Bs [G] SQ H c [Oe] S* OR (SO L/SQ r ) H r [Oe] SFDr SR75

MP/TiO 2

MP/CoFe (VHS)

0.45 3000 0.83/0.37/0.24 1635/1148/904 0.67 2.24 1743/1892 0.35 0.27

0.6 2850 0.77/0.44/0.27 1578/1330/978 0.67 1.75 1699/1843 0.34 0,27

coercive squareness S*, the orientation ratio O R in the plane of the tape, the remanence coercivity H r, the switching field distribution SFD r, and the initial slope of the remanence loop SR75. The hysteretic properties were measured along the three principal directions: longitudinal (L), transverse (T), and perpendicular (P). The M P / T i O 2 tape has a thinner MP layer with somewhat higher magnetization, squareness, coercivity, and orientation ratio compared with the top layer of the M P / C o F e (VHS) tape. Their respective recording per-

Relatlve 81gnel O u t p u t (dB)

3

¢x 2

1

0

-1 -2 --~ -3 0

i 2

i 4 Frequency ( M H z )

HI8MP

6

8

Fig. 2. Relative signal output [dB] of a single-layer BF tape and a double-layer BF/CoFe (SVHS) tape with Hi-8 mm MP tape as a reference.

D.E. Speliotis / Double-layermagnetic recording media

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Signal Output (uVp-p) 800 -~-

Double Layer MP/TIO2

600

400

200

0 0

i

r

i

I

f

1

2

3 Frequency (MHz)

4

6

Fig. 3. Frequency response of double-coated M P / T i O 2 and MP/CoFe (VHS) tapes.

Slgn&l Output (uVp-p) 800 A

Double Layer MP/TI02

700 600 500 400 300 200 100 0 0

I

r

I

I

r

1

2

3 Frequency (MHz)

4

6

Fig. 4. Frequency response of double-layer M P / T i O 2 tape and ME tapes.

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D.E. Speliotis / Double-layer magnetic recording media Table 2 Recording parameters of 8 mm tapes

formances are shown in fig. 3, with the M P / T i O 2 tape showing a significantly better response at short wavelengths compared to the M P / C o F e (VHS) tape. We also compared the recording performance of a double coated M P / T i O 2 tape with that of metalevaporated (ME) tape, and the results are plotted in fig. 4. The thinner ME tape has a flatter response, with lower output at long wavelengths and a higher output at short wavelengths compared to the M P / T i O 2 tape. Some recent M P / T i O 2 tapes with a thinner MP layer (0.3 ~m) and with higher magnetization (3400 G) outperform standard ME tapes across the entire band, as shown in fig. 5. There is some serious concern, however, regarding the environmental stability of such thin and very high magnetization MP coatings which employ very small metal particles with high surface areas [1]. Table 2 shows a compilation of certain important recording parameters for various 8 mm tapes. The numbers attached to the parameters designate frequency in MHz (except, of course, for D50 and PWs0). The parameter d + a designates the combined spacing and magnetization transition length, and is obtained from the frequency response curve [2]. The other parameters in table 2 include the resolution R, defined as the ratio of the signals at 4 and 1.33 MHz, the density Ds0 at which the output signal is 6 dB below

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Parameter

Hi8MP

BF/CoFe

MP/TiO 2

ME

R(4/1.33) Ds0 [kfci] SNR 4 [dB]

57.3 54.5 47.6 39.2 0.14 190

67.3 77 48.2 51.2 0.09 200

64.2 71.3 50.8 36.4 0.11 180

83 89 49.5 26.2 0.09 140

PS 4 [ns]

d + a [p,m] PWs0 [ns]

the signal output at low densities, the signal-to-noise ratio SNR (measured as rms signal to rms noise with a 10 kHz bandwidth), the peakshift PS representing the percentage change in the spacing of the signal peaks of two successive 'ones' (with respect to the written spacing) using the MFM pattern 1010110101, and the width PWs0 at half the amplitude of the isolated pulse. The peak shift (PS 4) data represent the average value of several million samples. The value for the B F / C o F e tape is high because of some waveform distortion due to higher perpendicular component in this tape, as a result of the imperfect longitudinal alignment of the BF platelets. The high-end tails of the peak shift histogram, however, extend further for the other three tapes in table 2, and it is precisely these tails which would tend to cause decoding errors.

Relative Output (dB)

j~

10

Ol 0

0

0

I

2

0

o

0

0

0

I

0

I

4 6 Frequency (MHz) MP (Reference)

o

MP/TiO2

0

0

0

L

8

10

A ME

Fig. 5. Relative signal output of MP (reference), double-coated MP/TiO 2, and ME 8 mm tapes versus frequency.

D.E. Speliotis / Double-layermagnetic recording media 3. Conclusions Double-coated BF tapes can match or outperform the best MP tapes across the entire spectrum of frequencies used for high-performance video, audio, and data recording while at the same time providing immunity to environmental corrosion. On the other hand, double-coated MP media, and particularly those using a non-magnetic underlayer offer improved recording performance compared with thick MP media, but extreme care must be taken not to expose such tapes to

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adverse environmental conditions for prolonged periods of time. It is a pleasure to thank J.P. Judge for his expert assistance with the magnetic and recording measurements. References [1] A. Djalali et al., J. Electrochem. Soc. (Sept. 1991) 2504. [2] D. Speliotis and K. Peter, J. Magn. Soc. Jpn. 15 (1991) 407.