in2 magnetic recording

ELSEVIER

Journal of Magnetism and Magnetic Materials 134 (1994) 298-303

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journalof magnetism

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materials

magnetic

Dual magnetic layer medium for 2 G b / i n 2 magnetic recording Masaaki

Futamoto

*, Y o s h i b u m i M a t s u d a , N o b u y u k i Yukio Honda

Inaba, Mikio Suzuki,

Central Research Laboratory, Hitachi, Ltd., Kokubunji, Tokyo 185, Japan

Abstract Low-noise media composed of CoCrPt/CoCrPtSi dual-layered magnetic films have been developed for ultra-high density longitudinal recording. The optimum thickness ratio between the two magnetic layers has been determined to be 1 : 1 for a total thickness of 30 nm. The reduction of medium noise is attributed to enhancement of magnetic isolation between crystals owing to the segregation of nonmagnetic elements in crystal grains along with the physical crystal gain isolation achieved by employing a high Ar pressure during medium formation. When the medium is combined with a narrow-track dual-element head, a high areal density of 2 G b / i n z is achieved.

1. Introduction Recent investigations [1-4] have shown the possibility of extending areal densities over 1 G b / i n 2 by improving the component technologies of rigid disk drives. Noise reduction in the recording medium is very important to increase the" areal density. To reduce the medium noise, the magnetic coupling among crystal grains forming a recording medium must be decreased. This can be done either by separating the crystal grains physically or through magnetic separation by enhancing the segregation of nonmagnetic phase at grain boundaries [5]. Another factor which should be considered in designing a high S / N system is the medium magnetization value, which is related to the reproduced voltage. We employed a longitudinal recording medium with dual-magnetic layer structure which consists of Co-based mag-

* Corresponding author. Fax: + 81-423-27-7710.

netic layers with different magnetization values. By varying the thickness ratio of the layers, it is possible to adjust the magnetization value while investigating other properties necessary for highdensity magnetic recording. We have achieved an areal density of 2 G b / i n 2 [6,7] using a dual-magnetic layered medium structure optimized to yield a high S / N output, in combination with a newly developed 1-micron-trackwidth inductive-write/ M R - r e a d dual-element head [8], an optical position detection system [9], a dual-stage actuator, a data channel with extended class-IV partial response signalling and a Viterbi detector [10]. In the present paper, we report on the magnetic and structural properties of dual-layered media together with the recording characteristics.

2. Experimental procedure Thin films were deposited using a dc magnetron sputtering system on 3.5 inch diameter

0304-8853/94/$07.00 © 1994 Elsevier Science B.V. All rights reserved SSDI 0304-8853(94)00307-D

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M. Futamoto et al. /Journal of Magnetism and Magnetic Materials 134 (1994) 298-303

plain glass substrates. Cr was used as the underlayer. Two sputter targets were used to form dual-layer structures with respective magnetic layer compositions of CoCr14.sPt8.4Si2.9 (M s = 493 e m u / c c ) and CoCra.7Pt2t.t (M s = 912 emu/cc). Carbon was sputter-deposited 10 nm thick on the magnetic layer as an overcoat. Ar pressure for Cr deposition was varied between 15 and 50 mTorr while it was fixed at 15 mTorr for the magnetic layer deposition. The substrate temperature and the sputtering power density were fixed at 150°C and 1.6 W / c m 2. The magnetic and structural properties were investigated using vibrating sample magnetometer (VSM), X-ray diffraction, scanning electron microscope (SEM), transmission electron microscope (TEM) and magnetic force microscope (MFM) techniques. The r e a d / w r i t e characteristics were evaluated using both an MIG head and an inductive-write/magnetoresistive-read dualelement head.

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3.1. Magnetic properties The physical separation of crystal grains in a film is enhanced by employing a high Ar pressure during film deposition. Since the magnetic crystal grain distribution depends on that of the Cr underlayer, the influence of Ar pressure during Cr deposition on the magnetic properties of C / C o C r P t S i / C r / g l a s s media has been investigated. The results are shown in Fig. 1. The in-plane coercivity Hc and the in-plane remanent magnetization M r are nearly constant up to Ar pressures around 30 mTorr. They begin to decrease beyond this point, while vertical H~ and vertical M r begin to increase. X-ray diffraction showed that the dominant growth orientation of Cr film was (ll0)bcc and did not change up to an Ar pressure of 50 mTorr. On the other hand, the dominant diffraction of the magnetic layer was (101)hcp for Ar pressures below 30 mTorr. When the Ar pressure is greater than 30 mTorr, the intensity of (002)h~p reflection begins to increase. The increase in the vertical H~ or vertical M r is due to

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Fig. 1. Effect of Ar pressure during Cr underlayer deposition on the magnetic properties.

the increase in the c-axis oriented growth of the magnetic layer. Therefore, the maximum Ar pressure for maintaining good in-plane magnetic properties while enhancing the crystal grain isolation is determined to be 28 mTorr, just below the critical pressure of 30 mTorr. Dual-magnetic layer structure is employed to adjust the magnetization value and to seek a further possibility to improve the medium S / N ratio in addition to the grain isolation. The magnetic properties of dual-magnetic layer media, C(10 n m ) / C o C r P t / C o C r P t Si/Cr(150 nm)/glass, which were prepared employing the Ar sputtering pressure of 28 mTorr for Cr deposition are shown in Fig. 2 as functions of CoCrPt layer thickness, t~p, when the total magnetic layer thickness is kept at 30 nm. M r • tc~p increases linearly as the thickness of the CoCrPt layer increases. However, the in-plane H c and the S* values vary non-linearly. The variations of low-frequency output So, medium noise Nd, and S o / N d are shown as functions of CoCrPt layer

M. Futamoto et aL /Journal of Magnetism and Magnetic Materials 134 (1994) 298-303

300

thickness in Fig. 3. Here, S O is the peak-to-peak reproduced voltage of a signal written at 1 kFCI and N d is the integrated-rms media-noise voltage measured for medium recorded at 75 kFCI. The S o / N d values are normalized with the S o / N d of the single-layer medium of C / C o C r P t / C r . With increasing thickness of the CoCrPt layer, the signal output S o increases linearly, corresponding to the linear increase of M r • t. On the contrary, N d remains almost constant up to around tcep = 15 nm, and then increases greatly. Consequently, a high S o / N d value as well as a high output is achieved when the CoCrPt thickness is 15 nm. Several 6M plots were measured for the duallayered structure for various thickness ratios of the two magnetic layers. The data, shown in Fig. 4, suggest that the magnetostatic interaction and exchange coupling become minimum when the two magnetic layers are stacked at an equal thickness of 15 nm. The correlation between the mag-

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netic interaction behavior and the 6 M plots is discussed in the reference [11]. Microscopic structural analysis of this type of medium is required to determine the reason for the reduced magnetic interaction.

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3.2. Structural properties

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t ccp ( nrn ) Fig. 2. Variation of magnetic properties of CoCrPt/CoCrPtSi dual-magnetic layer media.

Figs. 5(a) and (b) show the plan-view microstructures of a dual-magnetic layer medium observed by T E M and SEM, respectively. The thickness ratio of the two magnetic layers is 1 • 1 for a total magnetic layer thickness of 30 nm. Magnetic crystal grains, whose size ranges between 20-30 nm, form chain-like clusters consisting of several crystals. Neighboring clusters are separated physically by an average distance of 3 nm. Magnetic separation among magnetic crystals a n d / o r clusters is effective in reducing the medium noise related to magnetostatic and ex-

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M. Futamoto et al. /Journal of Magnetism and Magnetic Materials 134 (1994) 298-303

boundary was also noted in the analysis. The segregation of nonmagnetic Si is a possible reason for the decrease in the magnetic coupling between crystalline columns as well as for that between the two stacked magnetic layers in the dual-layered recording media. 3.3. Recording characteristics t--

Typical rolloff performance, measured with a 1 ~ m trackwidth dual-element head, is shown in Fig. 8. The low-frequency output of 404p_p ~V and a - 6 dB rolloff linear density of 80 kFCI were observed at a head-to-medium magnetic

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Fig. 4. ~M curves measured for the dual-magnetic layered media. change coupling between crystals. The crystalline easy axes of the m e d i u m are randomly oriented in the film plane. The high-resolution cross-sectional T E M micrograph shown in Fig. 6 shows epitaxial growth of the two magnetic layers on the Cr underlayer, which has been suggested in a previous study [2]. The crystal lattice image is continuous across the Cr, CoCrPtSi and CoCrPt layers. T h e r e is no other phase between the two magnetic layers. Local compositions of a dual-magnetic layer medium with 5 nm thick CoCrPtSi stacked with a 25 nm CoCrPt layer were examined cross-sectionally using a 2 nm diameter probe in the transmission electron microscope [Hitachi HF-2000]. Compositional variations measured along a crystalline column are shown in Fig. 7. Pt atoms have diffused from the CoCrPt side to the CoCrPtSi layer. Si intensity is increased at the C o C r P t S i / CoCrPt boundary and some Si atoms even seem to have diffused into the CoCrPt layer. T h e increase in Si intensity around the columnar

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Fig. 5. Plan-view TEM (a) and SEM (b) micrographs of the dual-layered medium. The Cr underlayer is removed for TEM observation.

302

M. Futamoto et al. /Journal of Magnetism and Magnetic Materials 134 (1994) 298-303

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spacing of 0.08 Ixm. An overwrite of 30 dB, defined as the attenuation of a 7.5 M H z signal overwritten by a high-frequency 30 M H z signal, was confirmed. An M F M image of the magnetization pattern recorded with the 1 ~ m trackwidth head is shown in Fig. 9. The linear recording density is 50 kFCI, which corresponds to a bit length of 0.5 ~m. T h e neighboring tracks are separated with a guard band of 0.5 ~ m width. The magnetization irregularities of the recorded bit edges are interpreted to be reflections of the presence of chain-like clusters which tend to behave as one unit when magnetized [12]. Decoupiing the clusters should be effective to allow recording of sharp magnetic transitions at the track edges together with an improvement in the magnetic field distribution of the recording head. W h e n the h e a d / m e d i u m system is combined with extended class-IV partial response signalling and Viterbi detection, a linear density of 120 kBPI has been proved possible at a bit error rate of 10 -8. Offtrack m e a s u r e m e n t s have shown the

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Fig, 7. Variation of microscopic compositions along a columnar crystal of CoCrPt(25 nm)/CoCrPtSi(5 nm) medium. possibility of 17 kTPI track density resolution, taking into account the 0.1 ~ m positioning error of a dual-stage actuator [10]. It has been thus

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Fig. 8. Rolloff performance of dual-layered medium and dual-element composite head combination.

M. Futamoto et al. /Journal of Magnetism and Magnetic Materials 134 (1994) 298-303

303

with other new technologies, an areal density of 2 G b / i n 2 has been proved possible at a bit error rate better than 10 -6 .

Acknowledgments The authors would like to thank Dr. F. Kugiya, Dr. Y. Tsunoda and Dr. Y. Sugita of Hitachi Central Research Laboratory for their encouragement and useful discussions. They also thank Dr. H. Kakibayashi and Mr. H. Murakoshi for the electron microprobe analyses.

References Fig. 9. Magnetic force microscope image of magnetization pattern recorded with a 1 ixm trackwidth head. The arrows show magnetization directions of recorded bits.

proven that an areal density of 2 G b / i n 2 can be achieved with an error rate of better than 10 -6

[6]. 4. Summary A high-S/N thin film medium has been developed for ultra-high density magnetic recording. The medium noise can be reduced by employing a dual-layered medium structure of CoCrPt (15 nm)/CoCrPtSi(15 nm)/Cr(150 nm) with an isolated crystal grain structure. The crystal lattices of these materials are continuous along a crystalline column. The segregation of nonmagnetic Si at the magnetic layer interface and around the crystal grain boundaries plays an additional effect in reducing the medium noise. In combination

[1] T. Yogi, C. Tsang, T. Nguyen, K. Ju, G. Gorman and T. Castillo, IEEE Trans. Magn. 26 (1990) 2271. [2] M. Futamoto, F. Kugiya, M. Suzuki, H. Takano, Y. Matsuda, N. Inaba, Y. Miyamura, K. Akagi, T. Nakano, H. Sawaguchi, H. Fukuoka, T. Munemoto and T. Takagaki, IEEE Trans. Magn. 27 (1991) 5280. [3] H. Wakamatsu, K. Kiuchi, H. Shinohara, and Y. Miura, J. Magn. Soc. Jpn. 15 (1991) 875. [4] Y. Nakamura, J. Magn. Soc. Jpn. 15 (1991) 497. [5] T. Chen and T. Yamashita, IEEE Trans. Magn. 24 (1988) 2700. [6] M. Suzuki, H. Sawaguchi, H. Takano, Y. Matsuda, F. Kugiya and M. Futamoto, J. Magn. Soc. Jpn. 15 (1991) 869. [7] Y. Matsuda, N. Inaba, M. Suzuki, H. Takano and M. Futamoto, J. Magn. Soc. Jpn. 15 (1991) 1001. [8] H. Takano, H. Fukuoka, M. Suzuki, K. Shiiki and K. Kitada, IEEE Trans. Magn. 27 (1991) 4678. [9] K. Akagi, T. Nakao, T. Munemoto, Y. Miyamura and K. Mori, IEEE Trans. Magn. 27 (1991) 5301. [10] K. Mori, T. Munemoto, H. Otsuki, Y. Yamaguchi and K. Akagi, IEEE Trans. Magn. 27 (1991) 5298. [11] P.E. Kelly, K. O'Grady, P.I. Mayo and R.W. Chantrell, IEEE Trans. Magn. 25 (1989) 3881. [12] Y. Honda, N. Inaba, M. Suzuki, A. Kikugawa, Y. Matsuda and M. Futamoto, IEEE Trans. Magn. 29 (1993) 3721.