Perpendicular recording technology on rigid disks

journal of magnetism and magnetic materials

j411• ELSEVIER

Journal of Magnetism and Magnetic Materials 134 (1994) 223-227

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Perpendicular recording technology on rigid disks T . S u e n a g a *, H . T a k e n o Research Center, Denki Kagaku Kogyo K.K., 3-5-1Asahimachi, Machida-City, Tokyo, Japan

Abstract We have studied a flying single-pole head and CoCrTa/NiFe double magnetic recording. The media consisted of electro-plated Ni-Fe underlayer and sputtered Co-Cr-Ta recording layer. The 'pancake' type head formed four-layered 60 turns, the pole width is 7 ~m, and the flying height is 0.075 Ixm. We have confirmed that a combination of such medium and head is capable of achieving 300 Mbits/in 2.

1. Introduction In recent years, the speed of improvement in the recording density of longitudinal rigid disks is remarkable. In fact, products at the 200 M b i t s / i n 2 level are already offered on the market. Moreover, for next generation products, a performance of 500 M b i t s / i n 2 has already been studied. On the other hand, the performance of perpendicular recording which uses single-pole-type heads and double-layered perpendicular media has enough output level and signal to noise ratio [1].

2. Perpendicular recording media A lot of work has been reported regarding perpendicular media, most of which generally use a NiFe underlayer on the surface of P E T sub-

* Corresponding author.

strate and a CoCr perpendicular recording layer on top. In other words, these studies have been made with floppy disks or tapes as fundamental ideas. Composition of perpendicular recording media is basically the same as that of conventional longitudinal media except that it has a NiFe layer instead of a NiP layer. 2.1. Permalloy underlayer

It has been common to deposit a permalloy underlayer by sputtering. The thickness of the layer by that method is usually 0.5-1.0 Ixm. We introduced the electro-plating method utilizing a plating bath composed of sulfate. The reasons for it are, first of all, this method gives as good through-put as that of NiP deposition in longitudinal recording media production (electroplating shows better through-put when the number of media processed at a time is smaller), and media made by this method have better noise characteristics. The important question is why it shows superior noise characteristics.

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T. Suenaga, H. Takeno / Journal of Magnetism and Magnetic Materials 134 (1994) 223-227

Grain size was studied and compared between layers made by plating and sputtering. Fig. 1 shows the TEM image and X-ray diffraction pattern of each. Although the TEM image in Fig. 1 shows different grain size between plated and sputtered layer, we have verified that the TEM image of the early stage of deposition shows a quite similar grain size between them. We assume that the nonuniformity of grain size in the layer affects the media noise. We also studied crystallization by X-ray diffraction and obtained a result showing that a plated layer has small crystal structure while a sputtered layer has large grain size crystals. The plated layer has 5-6 nm size, and the sputtered layer has 20-30 nm size. The grain size of crystals was calculated from the half width of the peak shown in Fig. 1, and it matches the above TEM image observation results. It has also been known that the thicker the permalloy layer (about 5 0.m minimum), the more effective it is against external magnetic field [2]. The permalloy underlayer is polished before being textured. This is because even perpendicular recording media require texture roughness of

Table 1 Media/head parameters

Media Magnetic layer Thickness H e ( _L)

Co78Cr17Ta 5 (at%) 0.11 Ixm 2000 Oe

Hc (II) Ms (II) S (II)

850 Oe 370 emu/cc 0.33

Head Pole material Track width Thickness Coil Flying height

Co 77Zr6 Nb17 (wt%) 7 Ixm 0.27 Ixm 60 turns 0.09 ~m at 10 m / s

3 Ixm (Ra) or less on conventional NiP substrates for lower flying height to achieve higher recording density. In this case, we textured the media surface to meet flying height at 0.1 t~m maximum. The roughness is smaller than 3 nm(Ra) on conventional NiP substrate. From the result of asperity curves, we confirmed that the minimum flying height can be as low as 0.03 ~m with the textured permalloy substrates.

2.2. Recording media Specification of a perpendicular recording medium is shown in Table 1. To achieve higher perpendicular coercivity, Hc( _L), we studied V, B, Ta and so on for CoCrMx composition and selected 5 at% Ta with which the largest H c of 2000 Oe by VSM ( ~ 2300 Oe Kerr) was obtained.

2.3. Protective layer

if SPUTTERED ,i

~

- PLATED ~A.A~

5S.8~

Fig. 1. TEM image and X-ray diffraction of plated and sputtered NiFe.

Carbon is most commonly used for the protective layer of magnetic disks, but a lot of studies to modify it have been made to improve durability which is required for lower flying height. DLC layer [3] and hydrogenated carbon have been reported to be the most effective. The mechanism of their durability has not been understood yet; perpendicular recording media also require the same CSS durability as conventional media. We tested various types of carbon from the viewpoint of wear resistance. We conducted drag

225

T. Suenaga, H. Takeno /Journal of Magnetism and Magnetic Materials 134 (1994) 223-227

't'4.4.64

tests [A1203-TiC heads, 0.3 m / s constant], measured wear quantity with an ELiPSo-meter and calculated wear rate. Judging from these results, we decided to use hydrogenated carbon with 20% C 2 H 4 contents. The thickness was set at 25 nm. As for lubricant, a fluorine containing lubricant with hydroxide on its end group is being used.

-Zg.

L

075

(I.320

nun

O,WO

B, 3~9

3. Perpendicular magnetic head The perpendicular magnetic head is basically a single-pole type. The cross section and head parameters are shown in Table 1 and Fig. 2. Fig. 2 shows a coil formed with four layers, 60 turns, and shield structure formed with ferrite around the back yoke. This head has strong structural characteristics against an external magnetic field. The winchester type is also important for contact s t a r t - s t o p technology, so we have developed a head taking the adhesion resistance into consideration, which forms small b u m p s to decrease the contact area between media surface and head slider. Fig. 3 shows the b u m p form (30-50 nm height) by Zygo. In the head process, the pole recession must be controlled. The head has b e e n formed with 70% slider and has used Hutchinson type 13 suspension which has flying height of 0.075 ~ m / 8 . 6 m / s . In order to reduce velocity dependency, we m a d e a cross-cut in the slider rails. Fig. 3 also shows the structure of the crosscut.

Fig. 2. Cross section of single-pole head.

i . . . . . . . . . Ikalo

i . . . . . . . . . O. too

i . . . . . . . . . O.NO

i • , ,

0.310

Fig. 3. Head overviewand structure of bump on slider.

We have evaluated the flyability of the above head and have confirmed good pitching, rolling, and stabilized flying position. The mechanism of 'Barkhausen noise' in thin film head has b e e n studied [4,5], but its mechanism has not been clarified yet. However, one of the mechanisms is related to change of magnetic domain wall when switching from write to read mode. Regarding write-after-noise, we have investigated our head and c o m p a r e d it to conventional thin film head, We have counted write-after-noise pulse at 50 I~V threshhold level; conditions were set at 10 ~s for write m o d e and 100 ~s for read windows. Fig. 4 shows the result. This shows that write-after-noise of our head is relatively smaller

T. Suenaga, H. Takeno /Journal of Magnetism and Magnetic Materials 134 (1994) 223-227

226

100000

THai Number :100000

30000

10000

Long.

8f-

Threshold : 50uV Delay Time : 5us

TFH

Window: 100us

tO

o .m O z

1000 300

~

30

104

0 2'

Peru. TFH I

6

ISOLATED SIGNAL

|

|

8

,o

,'4

,o

Fig. 5. Reproduced wave forms.

Track Width(um)

Fig. 4. Write noise of actual head.

than that of conventional T F H using permalloy. The reason for this is not fully understood, but we assume that the level of optimization of annealing within the magnetic field has contributed to it.

4. Read-write characteristics

dia with perpendicular ones, a drive can achieve 300 M b i t s / i n 2 (100 k B P I / 8 0 kfci, 3000 TPI) which would lead to the storage of 300, 150 and 75 MB on 3.5, 2.5 and 1.8" disks, respectively. Of course, the H D D capacity can reach 20-30% more when combined with other methods such as Z B R or PRML.

5. CSS characteristics

We have evaluated read and write characteristics of our perpendicular heads and media. The results are shown in Table 2, and Fig. 5. In Table 2, LF is 45 kfci, and H F is 60 kfci. It can be observed from these results that the head amplitude is as strong as 480 IxV/at 60 kfci. The results of other parameters such as O / W , S / N , Res. were also satisfactory. Reproduced wave forms can be said to be equivalent to those of conventional longitudinal recording media. The single-pole-type perpendicular-recording head used here has 7 txm pole width. We have also estimated TPI with this head. The perpendicular head only needs 1.5 l~m guard band while the conventional type calls for 3 p,m; hence perpendicular recording with pole width of 7 ~ m is satisfactory to achieve 3000 TPI. In other words, by replacing conventional heads and me-

Perpendicular recording studies in this report focused on the flying type head. Though Censtor Corp. in USA has been developing contact type heads (Micro-Flex HeadTR), the mainstream of head onto media surface loading/unloading systems is still of the CSS type. Even with perpendicular recording, we cannot disregard CSS characteristics and stiction characteristics. Recently, industry requires as many as hundreds of thousand of CSS test cycles. We have tested up to 30 k CSS cycles. The result was as good as the conventional type. The coefficient of friction obtained was at most 0.6(-) after 30 k CSS cycles. Stiction was not observed at a low R P M drag test, either. This is considered to be the effect of hydrogenated carbon explained earlier and small bumps on the slider surface.

Table 2 Parametric results of R / W performance

6. Discussion

LF (IxVpp)

HF (ixVpp)

Res. (%)

SNR (dB)

O/W (dB)

PW50 (p,m)

699

484

69.2

29.4

- 28.5

0.5

How much will recording density be increased by lowering the flying height to near contact level using our latest heads? The relationship between

T. Suenaga, H. Takeno /Journal of Magnetism and Magnetic Materials 134 (1994) 223-227 a0o~"

140 120

~10o O

to 80 ra

150

I-

60 I'40

0.025

0.05

0.075

0.1

0.125

FLYING HEIGHT 0Jm)

Fig. 6. Flying height dependency.

D50, normalized amplitude, and flying height is shown in Fig. 6. Approximately 400 Mbits/in 2 can be achieved at slightly lower than 0.05 Ixm flying height. From the asperity characteristics explained earlier in this report, a flying height of 0.05 ~m is a feasible level with aluminum substrates. We made a forecast of increasing recording density trend, from the test results in this report. We can say that current perpendicular technologies promise achieving 300 Mbits/in 2 while 200 Mbits/in 2 is achieved by longitudinal recording technologies. Perpendicular recording will be even more effective in the future, and we believe without any innovation in circuit technology, 1-2 Gbits/in 2 drives could be introduced onto the market within in this century.

227

7. Conclusion We have shown the capability of storing 300 Mbits/in 2 with magnetic recording technologies utilizing a flying single-pole perpendicular-magnetic head without using special technology, such as prml and so on. Furthermore, it is possible to obtain a performance of 400 Mbits/in 2 at a flying height of about 0.05 ~m. We believe that a flying-type perpendicular-recording head combined with MR elements will appear for the 'Gbits/in 2' class recording in the near future.

Acknowledgement The authors would like to thank Mr. Richard Anderson of Censtor Corporation for discussions during the development of the perpendicular heads.

References [1] V. Nakamura and K. Ise, J. Magn. Soc. Jpn. 15 Suppl. No. $2 (1991) 185. [2] M. Shimokoshi et al., Digests, 15th Annual Conf. on Magn. Jpn. (1991) 31pA-14. [3] H.-C. Tsai and D.B. Bogy, J. Vac. Sci. Technol. A5 (1987). [4] K.B. Klaassen and J.C.L. van Peppen, I E E E Trans. Magn. 26 (1990) 1697. [5] K.B. Klaassen et al., I E E E Trans. Magn. 25 (1989) 3212.