High SNR perpendicular recording media

~ ELSEVIER

Journal of Magnetism and Magnetic Materials 134 (1994) 304-309

~

journalof magnetism and magnetic materials

High SNR perpendicular recording media Masayoshi Shinohara a,*, Hiroaki Wakamatsu a, Isatake Kaitsu a, Ikuya Tagawa h, Yoshihisa Nakamura b "File Memory Laboratory, Fujitsu Laboratories Ltd., Atsugi, 10-1 Morinosato-wakamiya, Atsugi 243-01, Japan, b Research Institute of Electrical Communication, Tohoku University, 2-1-1 Katahira, Aoba, Sendai, 980, Japan

Abstract

We investigated the effect of a Ti interlayer in perpendicular double-layer disks to give a typical columnar structure and a good crystal orientation. Coercivity in thinner Co-Cr with Ti is larger than that without Ti. However, we found the Ti layer was not good for high density recording because of its spacing loss. The effects of Ta in a Co-Cr layer on coercivity are also studied particularly in thin film. The effect of Ta is similar to that of a Ti interlayer in coercivity but very superior in recording characteristics. We also measured signal-to-noise ratio at very high density and got 16 dB of SNR at a density of 2 Gbit/in 2 using the C o - C r - T a film.

I. Introduction

A c o b a l t - c h r o m i u m layer with titanium on a substrate has been reported [1] to have good crystal orientation and magnetic properties. We studied the effects of the Ti interlayer on the magnetic properties and crystal structures of C o Cr films on a nickel-iron underlayer and their recording characteristics at different spacing between the head and disk. We clarified that the coercivity of C o - C r with Ti exceeds that of C o - C r without Ti and at relatively large h e a d / d i s k spacing such as 0.15 Ixm, the Ti layer increases head output. While at small spacing such as contact (less than 0.05 I~m), the Ti layer has no such effect.

* Corresponding author.

We discussed the output and density characteristics using a magnetic circuit model of the head and disk and finite element method (FEM) analysis. Many groups have reported that C o - C r / N i Fe double-layer media have good recording characteristics in combination with single-pole thinfilm heads [2,3,4]. C o - C r with tantalum has been reported to have good vertical crystal orientation and good magnetic characteristics [5,6]. We confirmed that the effect of Ta is similar to that of a Ti interlayer in coercivity, and that Ta increases the coercivity in very thin film thickness without interlayer. We investigated the relationship between recording characteristics and Ta content in C o - C r / N i - F e double-layer media using singlepole thin-film heads for various spacing. C o - C r with Ta has good overwrite ( O / W ) characteristics and a high SNR (signal-to-noise ratio). We

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M. Shinohara et al. /Journal

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of Magnetism and Magnetic Materials 134 (1994) 304-309

evaluated Co-Cr-Ta films of high density recording characteristics with head-contact conditions, and obtained a SNR of 16 dB at a density of 2.0 Gbit/ in’.

“I s

WithTi

2,

2 Y 1

2. Experiments 2.1. Preparation of samples

OOl,,-,

Fig. 1 shows a sectional view of the disk. We prepared double-layer disks with and without a Ti interlayer between the Ni-Fe and Co-Cr. Ti and Co-Cr were deposited by sputtering and Ni-Fe was prepared by electrical plating. Co-Cr films were 0.15 pm thick. Ti layers were 0.02 p,rn thick, but ranged from 0 to 0.5 km in some cases. The Ni-Fe underlayer was 2 pm thick and permeability was about 2000. 2.2. Magnetic properties and film structures

We measured the magnetic properties of the films and analyzed the crystal structure using scanning transmission electron microscopy (STEM). Typical magnetic properties of Co-Cr films measured by a vibrating sample magnetometer (VSM) are as follows; coercivity (H, I) in perpendicular direction is 1600 Oe, H,,, in horizontal direction is 500 Oe and saturation magnetization 4~A4, is 5000 G. These films were removed from the underlayer by etching before measurement. H, I is larger for films with Ti than without. To clarify why, we investigated the

J

Recording layer

Fig. 1. Double-layer disk structure.

0.20 Co-Cr

thickness

(mm)

Fig. 2. Relationship between H, I and Co-Cr thickness.

H,,

distribution in film thickness by etching the film and using a Kerr effect B-H measurement system. Fig. 2 shows the relationship between Co-Cr thickness and H, I with and without Ti measured by the Kerr effect. The H, I of Co-Cr with Ti is larger than that of Co-Cr without Ti, particularly in a very thin range (less than 0.1 km). We observed the Co-Cr films by STEM. The grain size with Ti (40-50 nm) is larger than that without (20-30 nm). Fig. 3 shows STEM cross sections of the films. Columnar structures appear more commonly in films with Ti than without. Co-Cr appears to grow more easily in a good orientation on the Ti surface, so the H,, in thinner Co-Cr with Ti is larger than that without it. 2.3. Read/write

characteristics

We studied the effects of the Ti interlayer on recording characteristics at different spacings between the head and disk using a single-pole-type head. Media H,, ranges from 500 to 1800 Oe. We used two types of single-pole thin-film heads, a flying head (0.15 pm spacing) and a contact head [7] (0.05 pm spacing including a disk protective layer). Figs. 4 and 5 show the relationship of normalized output (E,) and H, I by VSM in media with and without Ti. We confirmed previous reports 141that higher output was due to a higher H,, . With the flying head, output depends on H,, and is independent of the Ti layer. With the

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M. Shinohara et al. /Journal of Magnetism and Magnetic Materials 134 (1994) 304-309

CoCr

Ti

NiFe

!

With Ti interlayer

Without Ti interlayer

t

0.1 ~m

Fig. 3. STEM cross sections of the Co-Cr films.

contact head, however, output saturates at H~. above 1000 Oe, and the output for the disk without Ti interlayer is higher than that with it. As the head efficiencies of the contact type and the flying type head are very different, we did not discuss the absolute outputs in Figs. 4 and 5.

as 0.05 I~m, output does not increase with increasing Hc 1. This difference is explained by an operating model [8] using the permeance factor of the recording layer in a magnetic circuit consisting of head and disk.

3.1. Analysis by FEM simulation 3. Discussion

Reading efficiency is better in Co-Cr without Ti than with Ti at small spacing. In large spacing such as 0.15 txm, output is approximately in proportion to H c ±, however, in small spacing such

We estimated how the Ti layer affected recording density D50 by using FEM [9,10]. Fig. 6 shows the used FEM model that includes media parameters of Co-Cr thickness, He, Ni-Fe thickness, permeability, interlayer of Ti thickness,

1.0 1.0

Contact head Flying head

~ 0,5

0-'~ =

I 0 D::: w :hh~, Ti I

0

1' ' Coercivity (kOe)

2

Fig. 4. Relationship between output and H c • for flying head.

I

1

I

2

Coercivity (kOe)

Fig. 5. Relationship between output and H c • for contact head,

M. Shinohara et aL /Journal of Magnetism and Magnetic Materials 134 (1994) 304-309

307

3000

Head (Main Pole)

)

2000 :

~

I

~

~

,

d

Recording

,

7"

layer

1000

~~5~'~-Interlayer

i

~'~CoCr ~ CoCrTa

Fig. 6. F E M model.

0 spacing and head parameters of pole thickness. Fig. 7 shows measured and calculated relationship between Df0 and Ti interlayer thickness. Calculated curves agreed with the measured curves. We see the thinner the Ti, the larger the Df0. It is thought that the distribution of the magnetic field from the head pole spreads between the pole and N i - F e underlayer. As the thinner Ti decreases the space, the magnetic field distribution of the head spreads less, resulting in an increased reading efficiency and Df0.

3.2. Ta effect in crystal structure and recording characteristics The effect of Ta additives into a C o- C r layer is similar to that of Ti interlayers in terms of coercivity but very superior in recording charac-

120,

40

= t ~ c r contacthead

Measuredby Flying head

~

2O 0

•

i

-

=

-

i

•

i

0.3

Fig. 8. Relationship between film thickness and H c ± with and without Ta.

teristics. A Ta content of 2 to 5 at% produces a high He± of over 1700 Oe. Fig. 8 shows the relationship between C o - C r film thickness and Hc ± with and without Ta in cases of substrate temperature of 150 and 250°C. These Co-Cr films had no interlayer. The H c ± of Co-Cr with Ta is high in very thin films (about 0.05 p,m) and so H~ II becomes small. We then investigated the crystal structure of the films using TEM. We see that Co-Cr with Ta typically has a pillar structure (diameter is about 200 ,~) from the initial film formation stage comparing that without Ta. This confirmed that the anisotropy is a result of the micro pillar shape structure of C o - C r film.

3.3. Areal recording density characteristics in the region of Gbit / in 2

lO0~Measured by Contacthead ~ 80 ~ ~ t ~

011 012 Thickness(/am)

•

i

10 20 30 40 50 Ti layer thickness (nm)

•

60

Fig. 7. Measured and calculated relationship between Df0 and Ti interlayer thickness•

We used a contact head with pole thickness and width of 0.4 and 1 ~m respectively and an estimated spacing between head and recording layer of 0.03 ~m where the spacing is the sum of disk protective film thickness, disk surface roughness and the recess of the head pole. Each head has a N i - Z n ferrite return yoke and a 33-turn coil. The main pole material is Co-Zr. Fig. 9 shows the relationship between the overwrite ( O / W ) characteristics by flying type head with the same magnetic motive force of 0.8 ampere

308

M. Shinohara et al. / Journal of Magnetism and Magnetic Materials 134 (1994) 304-309 -2(] t ~

30

CoCrTa 0.13 i,tm Hc,l_ (Kerr) 1 800 Oe

CoCrTa : 0.09 I,tm NiFe

~,25 ~3C

Contact head Tw : 1 ~trn (17 kTPI)

"O

z o~

2O ...................

-4C

10

j ....................

16.1dB

*O" 1 5 !--

Ta content (at.*/.) Fig. 9. Relationship between O/W and Ta content.

: 7 lira, F = 2000

0

~"~k~

1

2

3

Areal r e c o r d i n g d e n s i t y (Gbit/in 2) Fig. 11. SNR characteristics vs. recording density.

turn, and Ta content in the films having almost the same H c ± (1800 Oe) and film thickness (0.13 p.m). O / W is defined as the attenuation of a high frequency (9 MHz) signal overwritten by one of low frequency (2 MHz). Ta improves the O / W characteristics considerably. In perpendicular recording, it is a more severe condition to overwrite with low frequency than with high frequency, because the recorded pattern is more stable magnetically in higher density. Fig. 10 shows typical output characteristics in h e a d - d i s k

Hc± (Oel HC.L/ He,'/

Thickness

(I.tm)

Bs (G)

Co78CrlrTas

2000

6

0.13

4500

C076Cr24

1750

3.5

0.15

3500

A

E

0.5

E :1

0.4

EL EL

>

EL

o= E o

03

0.2 Tw :. 1.0 pm Tm : 0.4 pm v : 13 m/s

0.1 0

0

I

I

I

0.5

1.0

1.5

Recording density (Gbit/in2)

Fig. 10. Output characteristics vs. recording density.

2.0

contact for C o - C r with and without Ta vs. recording density. The normalized output values of 0.49 txVpp in Fig. 10 is very high in comparison with conventional data [1,4]. C o - C r with Ta films has high output in all recording density ranges in comparison with films without Ta. We think that C o - C r - T a film has a less magnetically poor layer, which does not contribute to the output, than C o - C r (without Ta) as seen in Fig. 8, so the spacing between head main pole and underlayer becomes small and output becomes high. In other words, the effect of Ta is to decrease the spacing loss. The SNR characteristics of C o - C r with Ta are superior to those without Ta, producing high output and high Ds0. The noise level of C o - C r without Ta was almost the same as that of with Ta. Fig. 11 shows the relationship between SNR and recording density for 0.09 Ixm thick C o - C r Ta. The head pole width was 1 Ixm, so the track density corresponds to 17 kTPI assuming a guard band of 0.5 p.m. We see total SNR value (total noise includes head noise and circuit noise) of 16 dB at 2.0 G b i t / i n 2 (90 kFCI:120 kBPI by 1 / 7 coding). In this case the media SNR was 23.8 dB. We think that a recording density of 2.0 G b i t / i n 2 is possible by combining perpendicular and contact recording and new data decoding in near future.

M. Shinohara et al. /Journal of Magnetism and Magnetic Materials 134 (1994) 304-309

4. Conclusions Co-Cr has a good orientation on the Ti surface, so the H c ± in thinner Co-Cr with Ti interlayer is larger than that without it. Co-Cr with Ti interlayer has relatively large output in large spacing comparing with non-interlayer, but less output in small spacing because of interlayer spacing loss. C o - C r - T a typically has a pillar structure from the initial film formation stage without interlayer. C o - C r - T a has a large Hc±/Hcl I ratio. Ta improves the O / W characteristics significantly. A SNR of 16 dB is possible at an areal density of 2.0 G b i t / i n 2 at a flux reversal density of 90 kFCI (120 kBPI by 1/7 coding) in perpendicular contact recording.

Acknowledgment We thank Dr. Yoshimasa Miura of Fujitsu Laboratories Ltd. for his guidance and encouragement. We would like to thank Mr. Gotoh for

309

observing the cross-sections of the films and recorded patterns using TEM.

References [1] K. Kobayashi, J. Toda and T. Yamamoto, Fujitsu Sci. Tech. J. 19 (1983) 99. [2] J. Tachibana, K. Ouchi and Y. Nakamura, J. Magn. Soc. Jpn. 16, No. 2 (1992). [3] Y. Maeda and M. Asahi, IEEE Trans. Magn. 23, No. 5 (1987). [4] K. Kiuchi, H. Wakamatsu, F. Suzuki and H. Takagi, IEEE Trans. Magn. 24 (1988) 6657. [5] M. Naoe and M. Matsuoka, J. Appl. Phys. 57 (1985) 4019. [6] H. Tamai and H. Hayasaka, IEEE Trans. Magn. 24 (1988) 2347. [7] Y. Nakamura, K. Ouchi, S. Yamamoto and I. Watanabe, IEEE Trans. Magn. 26 (1990) 2436. [8] K. Kobayashi, Analysis of Output Voltage in Perpendicular Recording, J. Magn. Soc. Jpn. 12 (1988) 479 (in Japanese). [9] I. Tagawa and Y. Nakamura, Proceedings of Perpendicular Magnetic Recording Conference '89 (PMRC'89), pp. 97-101. [10] I. Tagawa and Y. Nakamura, IEEE Trans. Magn. 26 (1991).