Preparation of CoCrTa films for high-density magnetic recording tape

Journ~t of Magnetism and Magnetic Materials 148 (1995) 357-358

Joumalof magnetism and maunetic materials

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Preparation of Co-Cr-Ta films for high-density magnetic recording tape Hideaki Yoshimoto *, Kiyoshi Kuga, Yoshiro Yoneda, Junji Numazawa NHK Science and Technical Research Labs., 1-10-11 Kinuta, Setagaya-la6 Tokyo 157, Japan

Abstract A perpendicular magnetic recording tape for ultrahigh density recording requires a high H c, a high H k and a small A H J H c. Furthermore for use in video tape recorders (VTR), mechanical properties such as reduction in tape curling and optimization of tape stiffness are important. In this study the effect of Ta added to C o - C r film and Kr sputtering gas were examined. In addition, C o - C r - T a thin film media, suitable for VTR tapes, were prepared using a facing targets sputtering (FTS) system.

A perpendicular magnetic recording tape for ultrahigh density recording requires a high perpendicular coercivity H c, a high perpendicular magnetic anisotropy field Hk, and a small particle coercivity distribution A H c / H e [1]. Furthermore for use in video tape recorders (VTR), mechanical properties such as a reduction in tape curling due to internal stress in the film and optimization of tape stiffness are important. In this study the effect of Ta added to C o - C r film and Kr sputtering gas on crystatlinity, H k, A H c / / H c and curling were examined. In addition C o - C r Ta thin film media, suitable for VTR tapes were prepared using a facing targets sputtering (FTS) system. In this study C o - C r - T a films were prepared using the FTS system as the roll coating machine. A 10 p.m thick and 100 m m wide polyimide long sheet was used as a substrate. The preparation conditions were a substrate temperature of 135°C and input power of 1 kW. The film thickness was controlled through the adjustment of the sheet feed speed. The magnetic characteristics of Co--CrTa film such as M s, H c, H k and A H c / H c were determined on the M - H loops measured using a vibrating sample magnetometer (VSM). The crystallinity was estimated by X-ray diffraction intensity from hcp(O02) planes l(ooz). The curvature radius r e of the specimen tapes was measured by using an interference microscope and the degree of tape curling was estimated by its reciprocal, 1 / r c. The stiffness of the tape was measured by a loop stiffness-tester. In order to prepare C o - C r - T a films with a small transition area, it is essential to reduce the A H d H c [1].

Fig. 1 shows the relationship between A H e / t t c and I(002) of C o - C r films prepared under various conditions [2]. A strong correlation could be observed between A t f c / H c and I(002), so enhancing the crystalline texture o f C o - C r films proved effective in reducing A H c / H c. Fig. 2 shows the dependence of H k and A H ~ / H c on Ta content, CTa, in the C o - C r - T a films prepared by Ar gas sputtering [3]. Hk was increased by the addition of a small amount of Ta and reached its maximum value when CTa was 2.5 at%. Conversely the decrease in / t H e / H e may be a result o f the improvement in the crystalline texture of C o - C r f d m s brought about by the addition o f Ta. The Kr ion has a larger atomic mass than Co and Cr atoms, so the Co and Cr atoms sputtered by Kr ions have greater kinetic energy. Furthermore since the Kr ions do not easily recoil at the target surface, the growing C o - C r Ta film was not destroyed and its internal stress could be decreased [4]. Fig. 3 shows the dependence of the degree

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* C~rresponding author. Fax: + 81-3-3466-1699; e-marl: [email protected].

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H. Yoshimoto et al. /Journal of Magnetism and Magnetic Materials 148 (1995) 357-358

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o f cuffing 1 / r c and A H c / H ¢ of Co¢9.sCrlsTa2. 5 (at%) films on the gas pressure. The values o f 1 / r c and A H c / H c o f C o - C r - T a films deposited by Kr sputtering were smaller than those b y Ar sputtering. The decrease o f A H c / H ~ was caused by thc improvement o f the crystallinity and the optimum Kr gas pressure was 1 mTorr. The improvement o f A H c / H c by adding Ta and using Kr sputtering gas is shown in Table 1. To reduce the tape curling and optimize the stiffness, the sputtering ga~ pressure was optimized and also C o Table 1 The improvement of A H c / H c

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C r - T a film was sputtered on the back side o f the sheet. Fig. 4 shows the dependence o f the curling 1 / r c and stiffness on the thickness o f the film sputtered on the back side D n, while the thickness o f the film on the top side D T was 240 nm. When D B was 150 rim, there was no curling and the stiffness was nearly the same as for metal powder tape used for broadcasting. In summary, C c - C r - T a thin film for perpendicular magnetic recording tape suitable for ultrahigh density video tape recording has been prepared using the facing targets sputtering (FTS) system. The result obtained are as follows: (1) A strong correlation between A H ¢ / H ¢ and ~oo2) o f C o - C r films was observed, which proved effective in enhancing the texture o f C o - C r films in order to decrease A H c / H c. (2) By the addition o f a small amount of Ta, H k and A H ¢ / H o could be improved. (3) Tape curling and A H ¢ / H ¢ could be improved by Kr gas sputtering. (4) A curl-free C o - C r - T a tape which had nearly the same stiffness as metal powder tape was prepared. Referen¢es [1] I. Tagawa et al., IEEE Trans. Magn. 27 (1991) 4957. [2] K. Kuga et al., L Hagn. Soc. Jpn. 15, Suppl. $2 (1991) 507. [3] K. Kuga et al., Digest of 38th Ann. Coal of MMM, CB-12 (1992). [4] S. Nakagawa et aL, J. Magn. Soc. Jpn. 15, Suppl. $2 (1991) 27.