3596 IEEE TRANSACTIONS ON MAGNETICS, VOL. 32, NO. 5 , SEPTEMBER 1996

HSato A.Kikuchi Kao Corporation, Haga, Tochigi, 321-34, JAPAN FUJITSU LIMITED, Kita-owaribe, Nagano, 381, JAPAN

J .Nakai H.Mitsuya KOBE STEEL, Kobe, Hyogo, 651-22, JAPAN Fuji Electric Co., Ltd, Tsukama, Matsumoto, Nagano, 390, JAPAN

TShimatsu and M.Tak&as?i Dept. of Electronics Engng., Faculty of Engng., Tohoku University, Sendai, 980, JAPAN

Absimct - The effects of grain size on intergranular magnetostatic coupling and on recording property are discussed for the Co&3l0,sTq and Co&r17Ta5 thin f i b media fabricated under the ultra clean sputtering process (UC-process). S/N, increases not only with the increment in H,&grain but also

with the decrement in the grain diameter in both CoCrTa media. In the media with high Hc/Hkgrain value more than 0.35, S/N, drastically increases with decrease in the grain diameter, which corresponds to the decrease in the dispersion of magnetic field strength from recorded bits. To maintain h i h H,/Hkgrain value more than 0.35 even in the media with ;mall grain size below 10 nm, the selection of alloy composition which gives a small 4nMJ

value of less than 1.0 is required

I. INTRODUCTION

Ultra clean sputtering process (UC-process) has been proposed to be a key technology to realize low intergranular exchange coupling resulting in low media noise [l]. In the media fabricated under the UC-process, the reduction of ferromagnetic grain size was also found to be effective for the decrement in media noise [2]. Whlle, the reduction of grain size is generally suggested to increase intcrgranular magnetostatic coupling, therefore, some critical grain size to satisfy high SIN, in media should be experimentally

In tlus study, the recording property was discussed in connection with grain size and intergranular coupling for the thn film media fabricated under the UC-process. The effects of grain size on intergranular magnetostatic coupling and on recording property were discussed.

- determined.

a. EXPERIMENTAL PROCEDURE

The Cogg,~Cr10,5Taq /Cr and Co78@r17TagiCr thin film media were fabricated under the UC-process with a specialized production type sputtering machine (ILC3013 : ANELVA) [l]. The films were deposited onto non-textured NiPiAl substrates (R, < 1 nm). The value of tB, of the media was fixed at 100 - 120 Gpm, which corresponds to thickness of about 16 nm in Co8s.sCr10 sTa4, 28 nm in Co&r17Ta5. The substrate temperatwe was kept at 250 "C during the film deposition. The

dry-etching time of substrate surface just prior to ffim deposition was fixed at 0 or 10 seconds, which correspond to 0 or about 0.2 nm in etched depth.

The degree of intergranular coupling was evaluated by the rotational hysteresis loss analysis [3]. The microstructure was examined by transmission elecfron microscopy (TEM). The recording properties were evaluated using a magnetoresisitive head, having the gap length of 0.28 pm. The signal frequency was 16.8 MHz. The medianoise was measuredusing a spectrum analyzer and calculated by integrating the readback noise spectrum from 0 to 33.6 MHz. The spacing between the head and the media was 0.08 - 0.10 pm. Recorded bit patterns of recorded track were examined by magnetic force microscopy ( M F W

111. RESULTS AND DISCUSSION

A. Recording performance In Fig. 1, the values of readback signal to media noise ratio

SIN, in the C%5.5CrI0,5Taq and Cq8Crl7Tas m d a fabricated under several fabrication conditions (with and without dry- etching treatment) are plotted as functions of normalized coercive force Hc/Hkgain and grain diameter. Here, &gain was evaluated from the magnetic field where rotational hysteresis

I

Manuscript received February 18, 19% Fig. 1 The values of SIN, at 69.7 kFCI in Cog3 media plotted as functions^ of H,/HkWn and grain diameter.

5Ta4 and Co7&r17Tag

0018-9464196$05.00 0 1996 JEEE

loss just vanishes [3]. The value of Hc/Hkgrain correspnds to the degree of intergranular coupling, and takes the ideal maximum of about 0.5 in the case of isotropic media with no intergranular coupling. This value increases with decreasing the degree of intergranular magnetic coupling.

In both media, SIN, increases not only with the increment in Hc/Hkgrain but also with the decrement in the grain diameter. This increment of SIN, mainly corresponds to the decrement of media noise. Especially, for the Co&r17Tas media with the high Hc/Hkgrain value more than 0.35 (shown in mark), S/Nm drastically increases with decreasing the grain diameter, and takes the maximum value of 27.8 dB. Therefore, in the media with low intergranular coupling, the reduction in ferromagnetic grain diameter is found to be most effective for the improvement of SIN,.

B. Egect of grain size on media noise The realized low intergranular coupling mentioned above is

strongly correlated to the change of microstructure of filoos. In Fig. 2, the TEM images of the Co78Cr17Tag media e imark) with grain diameter of 12 nm and 22 nm, respectively, are shown. As seen in both media, the grain boundaries are clearly obslerved, and each grain is found to be homogeneously separated each other, which suggests the sufficient reduction of intergranular exchange coupling even in the media with very small grain diameter of 12 nm.

I

- - (a) D=22nm 10nm (b) D=12nm 10nm

Hc/Hkgrarn = 0.37 H$Ikgra in = 0.35

Fig. 2 HR-TEM images of the C ~ C r 1 7 T a 5 media

In order to verify the results for the recording performance depending on grain diameter, recorded bit patterns in the Cqgr17Tag media @ mark) were observed by MFM directly. For a recording density of 150 kFCI, figure 3 shows MFM images of recorded bit patterns and average profiles from IMFM image in the media with low intergranular exchange coupling (. mark). Here, the transitions are observed as bright and dark lines in MFM image. The average profile from MFM image means the profile of MFM signal averaged in the 2.2 ym length range across the track width.

In the media with large grain diameter (D = 22 nm), it is found that the bits at the transitions coalesce to form loatlized patches. On the other hand, in the media with small grain diameter (D = 12 nm), the transitions are well defined and exhibit very little zigzag across the track. Furthermore, from the average profiles, it is found that the dispersion of the strength of MFM signals at transibons in the media with small grain diameter (D

Grain diameter = 22 nm Hc/Hkgrain = 0.37 SM, = 25 268 - . .

3 IAverage pfofile ' ' 1 m - - E ol u) ._

e 0 1 2 3 4 5

Distance @m)

Grain diameter = 12 nm HC/Hkgra'n = 0 35 SIN, = 27 8 dB -

3

m v - E P z z

0 1 2 3 4 5 Distance @m)

Fig. 3 MFM images and average profiles of recorded bit patterns in the c ~ ~ C r ~ ~ T a 5 media with low intergranular exchange coupling (Recording density = 150 kFCI)

= 12 nm) is much smaller than that in the media with large gains. This means that, in the media with low intergranular exchange coupling, the reduction of grain size is effective for decreasing the dispersion of magnetic field strength at bit trasitions.

To clarify the effect of grain size on media noise, in Fig. 4, media noise and coefficient of variance on MFM signals at transitions are plotted against the grain diameter. Here, the coefficient of variance on MFM signals at transitions is defined as the ratio of standard deviation of MFM signals to averaged MFM signal at transitions. This value of coefficient of variance corresponds to the degree of dispersion of magnetic field strength at transitions, which decreases in accordance with decreasing the dispersion of magnetic field strength.

With the decrement in grain diameter, the coefficient of variance on MFM signals at transitions decreases in the media with the recording density of 100 kFCI and 150 kFCI. Especially, in the case of high recording density of 150 kFCI, the coefficient of variance on MFM signals at transitions

11

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7

z 5 - ._ $ 3 e - .- m 8

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4

2

0

Y ' C078Cr17Ta5 150 kFCl HdHkgra" = 0.35 - 0.37 (H, = 2.1 - 2.4 kOe)

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0 10 20 30 Grain diameter (nm)

Fig. 4 Media noise and coefficient of variance on MFM signals at transitions plotted against the grain diameter (Recording density = 100, 150 WCI)

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drastically decreases with decreasing grain diameter. Furthermore, the value of media noise shows similar dependence on the grain &meter in the case of the coefficient of variance on MFM signals at transitions. Therefore, in the media with low intergranular exchange coupling, the reduction of grain size to about 10 nm makes an important role to reduce the dispersion of magnetic field strength at transitions, resulting in the decrease of media noise.

c o 7 8 ~ r 1 7 T ~ , 3 6

C l . O ) ~ ~ f 1.03

- 1.27 o=ZZD=O-

-

(1.3)l AI1< ' O 8 5 sCr,O gTa4 . -

C. Eflect of grain size on intergranular mgnetostglic coupling Significantly

decreases with the reduction of grain size in the Coa.5Cr10.5Tq media, as shown in Fig. 1. To verify the decrease in Hc/Hkgrain due to the reduction of grain size, HR-TEM images of the C0&3~~,5Taq media with the grain diameter of 11 nm and 24 nm are shown in Fig. 5. Every grain is clearly separated by a segregated grain boundary in spite of the grain size, which suggests that the intergranular exchange coupling is sufficiently reduced. Therefore, the decrease in &/&gain with the reduction of grain size seems to be mainly caused by the increase in the intergranular magnetostatic coupling.

In order to &scuss the effect of intergranular magnetostatic coupling on Hc/Hk*ain, not only the grain diameter, but also the ratio of 4nM, to Hkgrain should be taken into account. 4nM,/ HkgGn means the relative strength of intergranular magnetostatic

In contrast to the Cc~Crl7Ta5 media,

(a) D=24nm 10nm H,Mkgrain = 0.38

__.

(b) D = 1 1 nm 10 nm H,/Hkgra"' = 0.25

Fig. 5 HR-TEM images of the Cog5 5Cr10 5Ta4 media

i

coupling [4]. In Fig. 6, the values of HC/Hkgraln in both C08s.&!rlo~Ta4

and Co7&r17Ta5 media fabricated onto the substrate surface with dry-etching treatment are plotted against the grain diameter. Here, the value of 4 n M s / H k F a i n shows about 1.34 (Hkgraln = 5.7 He, M, = 605 emdcc) for the C%5,&rl0.5Tq media with the grain diameter of 11 nm and about 0.94 (&grain = 6.0 kOe, M, = 449 emdcc) for the C98Cr17Ta5 media with the grain diameter of 12 nm.

As seen, in the Co7&rI7Tag media with small 476Ms/Bkgrain value of about 1.0, &/Hkgrain maintains relatively high value above 0.35 even at small grab diameter of 12 nm. However, in the Cog5.5Cr10.5Taq media with high 4nMs/Hkgra in value of about 1.3, the value of Hc/Hkgrain remarkably decreases from 0.38 to 0.25 with decreasing the grain diameter from 24 to 11 nm. This indicates that, in the Cogg,5Crl0,5Taq media, the reduction of grain size strongly affects the strength of magnetOStatiC coupling due to the higher vdue of 4 a s / H k F a i n . Therefore, to realize high value of Hc/Hkgain even in the media with small grains, the selection of alloy composition which gives a small 4Zh&/Hkgrain value less than 1.0 is required.

IV. SUNL1cIARY

The effects of grain size on intergranular magnetostatic coupling and on recording property are discussed for the co85.5crl0,5Tq and C9&r17Ta5 thin film media fabricated under the UC-process.

(1) SINm increases not only with the increment in Hc/Hkgrain but also with the decrement in the grain diameter in both CoCrTa media. (2) h the media with hgh %/&grain value more than 0.35, Si N, drastically increases with decreasing the grain &ameter, which corresponds to the decrease in the dispersion of magnetic field strength from recorded bits. (3) To maintain high H,/Hkgraln value more than 0.35 even in the media with small grain size below 10 11111, the selection of alloy composition which gives a small 4nMSlHkgrain value of less than 1.0 is required.

As the results,

ACKNOWLEDGMENT

This study has been made by Tohoku University in cooperation with 13 companies in Japan (ANELVA, KOBE STEEL, FUJITSU, Asahi Komag, Fuji Electric R & D, ALPS ELEXYRIC, Hitacbi Metals, Hoya, JAPAN ENERGY, TEIJIN, NIPPON SHEET GLASS, KAO and MITSUBISHI MATERIALS).

REERENCES

[l] T.Shimatsn and M.Takahashi, J. Materials Chemistry and Physics, 41, 134

[21 AKikuchi, S.Kawakita, J. Nakai, TShimatsu and M.Takahashi, J. Appl. Phys., 79, 1 1996 [3] M.Takahashi et. al., IEEE trans. Magn., 28, 3285 1992 [4] For example, H-Neal Bertram and R.Arias, J. Appl. Phys., 71, 3439 1992

1995