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TiCr perpendicular magnetic recording medium
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Journal of Magnetism and Magnetic Materials 212 (2000) 293}299
In#uence of substrates on magnetic property and crystalline orientation of CoCrTa/TiCr perpendicular magnetic recording medium T. Asahi!, M. Ikeda", A. Takizawa", T. Onoue", T. Osaka!,",* !Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, 2-8-26 Nishi-Waseda, Shinjuku-ku, Tokyo 169-0051, Japan "Department of Applied Chemistry, School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan Received 31 March 1999; received in revised form 10 October 1999
Abstract The in#uence of substrates on magnetic property and crystalline orientation of CoCrTa/TiCr perpendicular magnetic recording medium was investigated using a glass plate and a Si(1 0 0) single crystal. Magnetic measurements revealed that the "lm sputtered on a Si(1 0 0) substrate possessed a higher perpendicular coercive force than that on glass, where the pretreatment of the Si(1 0 0) substrate by aqueous HF solution e!ectively improved the magnetic properties. X-ray di!raction analysis of the "lms indicated that the crystallinity of the TiCr layer formed on the Si(1 0 0) substrate with the HF pretreatment was higher than that on the glass substrate. It was also found that CoCrTa and TiCr layers on the Si(1 0 0) substrate with the HF pretreatment had a narrower distribution of their c-axis orientation than those on glass substrate. The results of X-ray di!raction analysis were consistent with those of the TEM observation for cross-section bright- and dark-"eld images and the corresponding THEED patterns. These results suggest that the crystalline surface of the Si(1 0 0) substrate with HF pretreatment has the e!ect of inducing preferred orientation in the TiCr underlayer, which leads to a decrease in distribution of the c-axis orientation of Co grains in the CoCrTa layer, resulting in an increase in the perpendicular coercive force. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 75.50.Ss; 75.70.i; 81.65.Cf Keywords: Thin "lms; Perpendicular magnetic recording medium; E!ect of kind of substrate; Pretreatment to surface of substrate
1. Introduction The technology of longitudinal magnetic recording has been established, but it is still being * Correspondence address. Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, 2-8-26 Nishi-Waseda, Shinjuku-ku, Tokyo 169-0051, Japan. Tel.: #81-3-52863783; fax: #81-3-32035710. E-mail address: [email protected] (T. Osaka)
investigated to achieve a higher recording density [1}5]. In the development of longitudinal recording media for ultrahigh recording density, a thin magnetic "lm with high coercive force (H ) might # be a promising longitudinal recording medium. However, a magnetic recording medium with an excessively high-H requires an excellent performance # recording head with a high saturated magnetic #ux density (B ) and a large magnetic permeability [6]. 4
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T. Asahi et al. / Journal of Magnetism and Magnetic Materials 212 (2000) 293}299
Based on the performance of presently available practical magnetic recording heads, it is clear that H of the medium must be lower than around # 3000 Oe. Furthermore, the longitudinal recording medium with an extremely thin magnetic layer (less than 10 nm) tends to be in the state of superparamagnetism, which causes erasure of recorded information [7]. Accordingly, the magnetic recording density obtainable by using longitudinal recording media will reach a maximum limit in the near future. On the other hand, perpendicular magnetic recording does not require an extremely thin magnetic "lm with high-H in principle [8]. In this # respect, the perpendicular magnetic recording has a potential of solving serious problems in the highdensity magnetic recording in the next generation [9,10]. In spite of this potential advantage, the perpendicular magnetic recording has been investigated to a much less extent than the longitudinal magnetic recording. In this study, attention was focussed on the in#uence of substrates on magnetic property and crystalline orientation of the perpendicular magnetic recording medium consisting of a magnetic layer of CoCrTa and an underlayer of TiCr. The CoCrTa "lm is widely used for both longitudinal and perpendicular magnetic recording media, whose characteristics are improved by the formation of nanostructure of Co-rich core grain surrounded by a Cr-rich shell [11,12]. On the other hand, it has been reported that the TiCr underlayer with Ti rich composition, as well as Ti underlayer, induces the preferred orientation of the c-axis of Co grain in the direction perpendicular to the "lm surface [13]. Therefore, the CoCrTa/ TiCr magnetic recording medium was selected as a typical example for this basic research. A Si crystal with surfaces of (1 0 0) plane (hereafter abbreviated as Si(1 0 0)) and a glass plate were used as crystalline and noncrystalline substrates, respectively. Magnetic properties and crystalline orientation of the "lms deposited on these substrates were compared. Moreover, the e!ect of pretreatment of the surface of Si(1 0 0) by aqueous HF solution was also examined to reveal the in#uence of surface conditions of Si(1 0 0) substrate.
2. Experimental details An underlayer of TiCr and a magnetic layer of CoCrTa were successively deposited on a Si(1 0 0) substrate and a glass substrate by using a DC magnetron sputtering system. The base pressure was 9]10~7 Torr. The substrate temperature was 2203C. Sputtering was performed at input power of 350 W under Ar gas pressure of 6 mTorr. These conditions were determined from the preliminary experiments based on the previous report [13]. The thickness of the TiCr underlayer was approximately 35 nm. The magnetic layer deposited for the comparison between Si(1 0 0) and glass substrates was 100 nm in thickness, while the magnetic layer for the investigation of the e!ect of pretreatment by aqueous 10 wt% HF solution was 60 nm. The compositions of the media were determined to be Co Cr Ta and Ti Cr by an inductively 86 20 2 97 3 coupled plasma spectroscopy, respectively. The surfaces of Si(1 0 0) and glass substrates were cleaned in a di!erent manner prior to the deposition. The Si(1 0 0) substrate was immersed in a 10 wt% HF solution for 2.5 min to remove the surface oxide [14]. The glass substrate was cleaned ultrasonically in ethanol and then in acetone for 10 min each. Magnetic properties were measured by using a vibrating sample magnetometer (VSM), and the crystalline orientation of the "lm was investigated by X-ray di!ractometer at room temperature. H and squareness S, which is de"ned as M (re# 3 manent magnetization)/M (saturated magneti4 zation) in directions parallel and vertical to the "lm (denoted as E and o, respectively), were determined from an M}H hysteresis loop measured by using VSM with magnetic "elds up to 10 kOe. A Cu K radiation (j"1.540 As ) from the rotating anode a generator (Rigaku RU200) was used with a 50 kV of acceleration voltage and a 150 mA of emission current. A monochromator of graphite was used to remove Cu K radiation. X-ray di!raction (XRD) b patterns were obtained by step scanning at a rate of 0.013/step. The same sample was used for VSM and XRD measurements. Bright- and dark-"eld images and corresponding transmission high-energy electron di!raction (THEED) patterns by using a transmission electron microscope (TEM) with
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295
a 300 kV of acceleration voltage were investigated to obtain the crystallinity and the orientation distribution of Co and Ti grains. The samples for TEM observation were prepared by ion-milling technique.
3. Results 3.1. Ewect of kind of substrate Table 1(a) shows magnetic properties of the 100 nm thick "lms on Si(1 0 0) with the HF pretreatment and glass substrates. The "lm on Si(1 0 0) substrate possessed H (o) by 200 Oe higher than # that on glass substrate. S(o) and S(E) were almost the same, although H (E) values were slightly di!er# ent by 80 Oe. Table 1(b) shows magnetic properties of the "lms on Si substrate with and without the HF pretreatment. Without the HF pretreatment, the magnetic properties became worse. Therefore, we made the HF pretreatment for Si(1 0 0) substrate whenever the "lms on Si(1 0 0) and glass substrates were compared. The detailed explanation to the e!ect of the HF pretreatment will be related in Section 3.2. Figs. 1a and b show XRD patterns of the "lms on Si and glass substrates, respectively. The peaks corresponding to Co(0 0 1) and Ti(0 0 1) planes were explicitly observed in both "lms. The di!raction intensity of Co(0 0 2) of the "lm on Si(1 0 0) was smaller than that on glass. On the other hand, the di!raction intensity of Ti(0 0 2) of
Table 1 (a) Magnetic properties of CoCrTa/TiCr "lms on Si(100) with the HF pretreatment and glass substrates Substrate Si(1 0 0) Glass
H (o) (Oe) # 960 760
H (E) (Oe) #
S(o)
S(E)
440 360
0.18 0.15
0.62 0.61
(b) Magnetic properties of CoCrTa/TiCr "lms on Si(1 0 0) substrate with and without the HF pretreatment HF treatment
H (o) (Oe) #
H (E) (Oe) #
S(o)
S(E)
L *
1040 300
540 280
0.20 0.07
0.54 0.83
Fig. 1. XRD patterns of CoCrTa/TiCr "lms on substrates of (a) Si(1 0 0) and (b) glass. Thickness of the magnetic layer was 100 nm. Unidenti"ed peaks were indicated by solid triangles.
the "lm on Si(1 0 0) was stronger than that on glass. The broad peak around 2h"483 corresponded to Co(1 0 1) plane, and the unidenti"ed broad peak indicated by solid triangles were seen around 503. Solid curves in Figs. 2a and b are rocking curves of Co(0 0 2) for the "lms on Si(1 0 0) and glass substrates, respectively. The rocking curves were obtained by measuring the intensity as a function of the sample-rotating angle h with the position of a detector "xed at 2h. The curve is sharper for the "lm on Si(1 0 0) than for the "lm on glass. The full-width at the half-maximum, which is de"ned as *h , was obtained by "tting the curves to a Gaus50 sian distribution. The broken lines show the "tted curves. Table 2(a) lists the values of *h . The value 50 of *h of Co(0 0 2) for the "lm on Si(1 0 0) was 23 50 smaller than that for the "lm on glass. Figs. 3a and b show rocking curves of Ti(0 0 2) for the "lms on Si(1 0 0) and glass substrates, respectively. A conspicuously sharp rocking curve was obtained for the "lm on Si(1 0 0) as compared with the "lm on
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T. Asahi et al. / Journal of Magnetism and Magnetic Materials 212 (2000) 293}299 Table 2 (a) *h for CoCrTa/TiCr "lms on Si(1 0 0) with the HF pret50 reatment and glass substrates Substrate
Si(1 0 0) Glass
*h (3) 50 Co(0 0 2)
Ti(0 02)
7.8 9.8
5.5 11.8
(b) *h for CoCrTa/TiCr "lms on Si(1 0 0) substrate with and 50 without the HF pretreatment HF treatment
*h (3) 50 Co(0 0 2)
L *
17.2 35.4
Ti(0 0 2) 5.3 7.6
Fig. 2. Rocking curves of Co(0 0 2) for CoCrTa/TiCr "lms on substrates of (a) Si(1 0 0) and (b) glass with 100 nm thickness of the magnetic layer. Solid and broken lines were the experimental and the "tting data, respectively.
glass. As shown in Table 2(a), the value of *h of 50 Ti(0 0 2) for the "lm on Si(1 0 0) was only one-half of that for the "lm on glass. Figs. 4a}d show TEM bright- and dark-"eld images of the "lms on Si(1 0 0) and glass substrates, respectively. Columnar grains of Co and Ti were observed, and the grain size of Co, ca. 20}30 nm, for the "lm on Si(1 0 0) was equivalent to that for the "lm on glass. It was shown that the crystallinity of TiCr layer for the "lm on Si(1 0 0) is much higher than that for the "lm on glass. On the other hand, the "lm on Si(1 0 0) had larger regions of initial growth layer in the CoCrTa layer than the "lm on glass. Figs. 4e and f show THEED patterns corresponding to the bright- and dark-"eld images for the "lms on Si(1 0 0) and glass substrates, respectively. The hexagonal sharp spots in Fig. 4e
Fig. 3. Rocking curves of Ti(0 0 2) for CoCrTa/TiCr "lms on substrates of (a) Si(1 0 0) and (b) glass with 100 nm thickness of the magnetic layer. Solid and broken lines were the experimental and the "tting data, respectively.
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297
Fig. 4. Cross-sectional TEM images of CoCrTa (100 nm thick)/TiCr/substrates. (a), (b) and (c), (d): bright- and dark-"eld images on Si(1 0 0) and glass substrates, respectively. (e), (f): THEED patterns of (a) and (b). In (e) and (f), Co(0 0 2) and Ti(0 0 2) re#ections are indicated by arrows, respectively.
originated from a diamond structure of Si substrate. The di!raction patterns indicate Co(0 0 2) and Ti(0 0 2) planes are oriented parallel to the "lm surface for both "lms. The intensity distribution along the arc of Ti(0 0 2) re#ection for the "lm on Si(1 0 0) was narrower than that for the "lm on glass, which indicates a small orientation of Ti[0 0 1]. However, the di!erence in the distribution of Co(0 0 2) re#ection was not clearly found between Figs. 4e and f due to the overlap of several re#ections. 3.2. Ewect of HF pretreatment of Si substrate It has been shown that the surface conditions of Si(1 0 0) substrate depend on the HF pretreatment performed prior to the metal deposition [14]. To examine the in#uence of surface conditions of the Si(1 0 0) substrate, the 60 nm thick "lms were deposited on Si(1 0 0) substrates with and without the HF pretreatment, respectively. Figs. 5a and b show
the M}H hysteresis loops for both "lms, which di!ered in the shape of the loop. As shown in Table 1(b), the HF pretreatment was found to increase coercive force, in particular H (o). # The XRD patterns of the "lm on Si(1 0 0) substrate with and without the HF pretreatment are shown in Figs. 6a and b, respectively. Co(0 0 2) and Ti(0 0 2) re#ections were observed in both "lms, and an unidenti"ed broad peak was seen around 2h"493 only for the "lm without the HF pretreatment. It was found that the intensity of Co(0 0 2) and Ti(0 0 2) were increased by the HF pretreatment. The values of *h were obtained by inves50 tigating the rocking curves of Co(0 0 2) and Ti(0 0 2) observed in Fig. 6, respectively. Table 2(b) shows the value of *h for Co(0 0 2) and Ti(0 0 2) 50 peaks. The values of *h of Co(0 0 2) and Ti(0 0 2) 50 were both smaller with the HF pretreatment than without it. The grain size of Co was estimated less than 20 nm for both "lms with and without the HF
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4. Discussion
Fig. 5. M}H hysteresis loops of CoCrTa/TiCr "lms on Si(1 0 0) substrate (a) with and (b) without the HF pretreatment, where the ordinate is normalized by saturated magnetization M . 4 Thickness of the magnetic layer was 60 nm.
Fig. 6. XRD patterns of CoCrTa/TiCr "lms on Si(1 0 0) (a) with and (b) without the HF pretreatment. Thickness of the magnetic layer was 60 nm. An unidenti"ed peak was indicated by a solid triangle.
pretreatment from plane-view TEM images though these "gures are not indicated here. Furthermore, the plane-view THEED patterns indicated that the Debye}Scherrer ring of Co(0 0 2) for the "lm without the HF pretreatment was broad and weak, which means low crystallinity of the CoCrTa layer and was consistent with the result of XRD.
It was revealed that the "lm on Si substrate possessed a higher perpendicular coercive force, H (o), than the "lm on glass substrate. Measure# ments of *h of Co(0 0 2) in XRD patterns showed 50 that the distribution of Co[0 0 1] is smaller for the "lm on Si(1 0 0) than for the "lm on glass. On the other hand, the decrease in *h of Ti(0 0 2) for 50 the TiCr underlayer deposited on Si(1 0 0) substrate suggests that the Si(1 0 0) substrate has an e!ect to orient the c-axis of Ti grains in the TiCr underlayer vertically to the substrate. This e!ect was also con"rmed by the results of THEED patterns that the intensity distribution of Ti(0 0 2) was narrow for the "lm on Si(1 0 0) as compared with the "lm on glass. Therefore, it is considered that the decrease in the distribution of the c-axis orientation of Co grains in the magnetic layer for the "lm on Si(1 0 0) substrate was caused by the improvement of preferred orientation of Ti grains in the TiCr underlayer, resulting in the increase in H (o) of the "lm on # Si(1 0 0) substrate. From observations of XRD patterns and crosssection bright- and dark-"eld images of TEM, it was found that the Si(1 0 0) substrate increases the crystallinity of the TiCr underlayer. Therefore, it can be said that the crystalline surface of Si(1 0 0) promotes the crystal growth of Ti grains in the TiCr underlayer. On the other hand, from the decrease in Co(0 0 2) peak intensity and the increase in the intensity of the unidenti"ed peak which might correspond to (2 0 0) plane of FCC Co structure, as seen in Figs. 1a and b, it does not seem that the crystallinity of the magnetic layer was not necessarily improved by the use of Si(1 0 0) substrate. This is possibly due to an increase in the degree of mismatch between the TiCr underlayer and the CoCrTa magnetic layer caused by an improvement in the crystallinity of the underlayer. Actually, bright- and dark-"eld images of TEM observation indicate that regions of the initial growth layer in the magnetic layer of the "lm on Si(1 0 0) are larger than those of the "lm on glass. As for the surface treatment of HF solution of Si(1 0 0), the surface of Si(1 0 0) substrate depended largely on the treatment performed prior to the metal deposition. Without the HF pretreatment,
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H (o) and the re#ection intensity of Co(0 0 2) and # Ti(0 0 2) in XRD patterns were considerably small, which were caused by low crystallinity of the "lm. This e!ect is attributed to the deterioration of surface conditions of the substrate, that is, the surface oxide remained on the Si(1 0 0) substrate without the HF pretreatment. On the other hand, the crystalline surface of Si freshly exposed by the pretreatment is likely to promote the crystal growth and the preferred orientation of Ti grains in the TiCr underlayer, which, in turn, in#uences directly the crystalline structure of the CoCrTa magnetic layer. Thus, it was revealed that the fresh single-crystalline surface of Si(1 0 0) substrate contributes to the improvement of magnetic properties of a perpendicular magnetic recording medium, CoCrTa/TiCr.
299
the substrate of Si(1 0 0) with the HF pretreatment, while the crystallinity of the CoCrTa layer was rather lower for the "lm on Si(1 0 0) with the HF pretreatment than for the "lm on glass.
Acknowledgements This work was supported by the &Research for the Future' program of the Japan Society for the Promotion of Science. The authors thank Dr. J. Hokkyo and Dr. T. Momma for their contribution to the discussion, and Dr. Y. Okinaka for critically reading the manuscript.
References 5. Conclusion CoCrTa/TiCr magnetic "lms were deposited on substrates of glass and Si(1 0 0) with and without the HF pretreatment to investigate the in#uence of substrates on magnetic properties and crystalline orientation of the "lms. Magnetic measurements revealed that the "lm on Si(1 0 0) substrate with the HF pretreatment possessed H (o) by 200 Oe high# er than that on glass. Furthermore, magnetic properties of CoCrTa/TiCr as a perpendicular magnetic recording medium were found to be improved conspicuously by the pretreatment with HF to the surface of Si(1 0 0) substrate. Results of X-ray and transmission electron microscopy studies on the crystalline orientation of the TiCr underlayer and the CoCrTa magnetic layer indicated that both distributions of c-axis orientation of Co and Ti grains in the direction vertical to the substrate decreased for the "lm on Si(1 0 0) with the HF pretreatment as compared with the "lm on glass. It was found from these experiments that the crystallinity of the TiCr underlayer was improved using
[1] H. Yoda, H. Iwasaki, T. Kobayashi, A. Tsukai, M. Sahashi, IEEE Trans. Magn. MAG-32 (1996) 3363. [2] H. Mutoh, H. Kanai, I. Okamoto, Y. Ohtsuka, T. Sugawara, T. Koshikawa, J. Toda, Y. Uematsu, M. Shinohara, Y. Mizoshita, IEEE Trans. Magn. MAG-32 (1996) 3914. [3] M.H. Kryder, W. Messner, L.R. Carley, J. Appl. Phys. 79 (1996) 4485. [4] D.N. Lambeth, E.M.T. Velu, G. Bellesis, L.L. Lee, D.E. Langhlin, J. Appl. Phys. 79 (1996) 4496. [5] T. Yogi, J. Magn. Soc. Jpn. 15 (S2) (1991) 475. [6] T. Osaka, M. Takai, K. Hayashi, K. Ohashi, M. Saito, K. Yamada, Nature 392 (1998) 796. [7] P.L. Lu, S.H. Charap, IEEE Trans. Magn. MAG-30 (1994) 4230. [8] S. Iwasaki, Y. Nakamura, IEEE Trans. Magn. MAG-13 (1977) 1272. [9] S. Iwasaki, N. Honda, J. Magn. Soc. Jpn. 21 (S2) (1997) 1. [10] D.A. Thompson, J. Magn. Soc. Jpn. 21 (S2) (1997) 9. [11] K. Kimoto, Y. Hirayama, M. Futamoto, J. Magn. Magn. Mater. 159 (1996) 401. [12] M. Futamoto, N. Inaba, Y. Hirayama, K. Ito, Y. Honda, J. Magn. Magn. Mater. 193 (1999) 36. [13] Y. Matsuda, Y. Shiroishi, T. Shimotsu, K. Takagi, J. Magn. Soc. Jpn. 13 (S1) (1989) 391. [14] T. Osaka, T. Homma, Y. Kurokawa, T. Taguchi, A. Takizawa, J. Magn. Soc. Jpn. 21 (1997) 213.
Journal of Magnetism and Magnetic Materials 212 (2000) 293}299
In#uence of substrates on magnetic property and crystalline orientation of CoCrTa/TiCr perpendicular magnetic recording medium T. Asahi!, M. Ikeda", A. Takizawa", T. Onoue", T. Osaka!,",* !Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, 2-8-26 Nishi-Waseda, Shinjuku-ku, Tokyo 169-0051, Japan "Department of Applied Chemistry, School of Science and Engineering, Waseda University, 3-4-1 Okubo, Shinjuku-ku, Tokyo 169-8555, Japan Received 31 March 1999; received in revised form 10 October 1999
Abstract The in#uence of substrates on magnetic property and crystalline orientation of CoCrTa/TiCr perpendicular magnetic recording medium was investigated using a glass plate and a Si(1 0 0) single crystal. Magnetic measurements revealed that the "lm sputtered on a Si(1 0 0) substrate possessed a higher perpendicular coercive force than that on glass, where the pretreatment of the Si(1 0 0) substrate by aqueous HF solution e!ectively improved the magnetic properties. X-ray di!raction analysis of the "lms indicated that the crystallinity of the TiCr layer formed on the Si(1 0 0) substrate with the HF pretreatment was higher than that on the glass substrate. It was also found that CoCrTa and TiCr layers on the Si(1 0 0) substrate with the HF pretreatment had a narrower distribution of their c-axis orientation than those on glass substrate. The results of X-ray di!raction analysis were consistent with those of the TEM observation for cross-section bright- and dark-"eld images and the corresponding THEED patterns. These results suggest that the crystalline surface of the Si(1 0 0) substrate with HF pretreatment has the e!ect of inducing preferred orientation in the TiCr underlayer, which leads to a decrease in distribution of the c-axis orientation of Co grains in the CoCrTa layer, resulting in an increase in the perpendicular coercive force. ( 2000 Elsevier Science B.V. All rights reserved. PACS: 75.50.Ss; 75.70.i; 81.65.Cf Keywords: Thin "lms; Perpendicular magnetic recording medium; E!ect of kind of substrate; Pretreatment to surface of substrate
1. Introduction The technology of longitudinal magnetic recording has been established, but it is still being * Correspondence address. Kagami Memorial Laboratory for Materials Science and Technology, Waseda University, 2-8-26 Nishi-Waseda, Shinjuku-ku, Tokyo 169-0051, Japan. Tel.: #81-3-52863783; fax: #81-3-32035710. E-mail address: [email protected] (T. Osaka)
investigated to achieve a higher recording density [1}5]. In the development of longitudinal recording media for ultrahigh recording density, a thin magnetic "lm with high coercive force (H ) might # be a promising longitudinal recording medium. However, a magnetic recording medium with an excessively high-H requires an excellent performance # recording head with a high saturated magnetic #ux density (B ) and a large magnetic permeability [6]. 4
0304-8853/00/$ - see front matter ( 2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 9 9 ) 0 0 7 7 4 - X
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Based on the performance of presently available practical magnetic recording heads, it is clear that H of the medium must be lower than around # 3000 Oe. Furthermore, the longitudinal recording medium with an extremely thin magnetic layer (less than 10 nm) tends to be in the state of superparamagnetism, which causes erasure of recorded information [7]. Accordingly, the magnetic recording density obtainable by using longitudinal recording media will reach a maximum limit in the near future. On the other hand, perpendicular magnetic recording does not require an extremely thin magnetic "lm with high-H in principle [8]. In this # respect, the perpendicular magnetic recording has a potential of solving serious problems in the highdensity magnetic recording in the next generation [9,10]. In spite of this potential advantage, the perpendicular magnetic recording has been investigated to a much less extent than the longitudinal magnetic recording. In this study, attention was focussed on the in#uence of substrates on magnetic property and crystalline orientation of the perpendicular magnetic recording medium consisting of a magnetic layer of CoCrTa and an underlayer of TiCr. The CoCrTa "lm is widely used for both longitudinal and perpendicular magnetic recording media, whose characteristics are improved by the formation of nanostructure of Co-rich core grain surrounded by a Cr-rich shell [11,12]. On the other hand, it has been reported that the TiCr underlayer with Ti rich composition, as well as Ti underlayer, induces the preferred orientation of the c-axis of Co grain in the direction perpendicular to the "lm surface [13]. Therefore, the CoCrTa/ TiCr magnetic recording medium was selected as a typical example for this basic research. A Si crystal with surfaces of (1 0 0) plane (hereafter abbreviated as Si(1 0 0)) and a glass plate were used as crystalline and noncrystalline substrates, respectively. Magnetic properties and crystalline orientation of the "lms deposited on these substrates were compared. Moreover, the e!ect of pretreatment of the surface of Si(1 0 0) by aqueous HF solution was also examined to reveal the in#uence of surface conditions of Si(1 0 0) substrate.
2. Experimental details An underlayer of TiCr and a magnetic layer of CoCrTa were successively deposited on a Si(1 0 0) substrate and a glass substrate by using a DC magnetron sputtering system. The base pressure was 9]10~7 Torr. The substrate temperature was 2203C. Sputtering was performed at input power of 350 W under Ar gas pressure of 6 mTorr. These conditions were determined from the preliminary experiments based on the previous report [13]. The thickness of the TiCr underlayer was approximately 35 nm. The magnetic layer deposited for the comparison between Si(1 0 0) and glass substrates was 100 nm in thickness, while the magnetic layer for the investigation of the e!ect of pretreatment by aqueous 10 wt% HF solution was 60 nm. The compositions of the media were determined to be Co Cr Ta and Ti Cr by an inductively 86 20 2 97 3 coupled plasma spectroscopy, respectively. The surfaces of Si(1 0 0) and glass substrates were cleaned in a di!erent manner prior to the deposition. The Si(1 0 0) substrate was immersed in a 10 wt% HF solution for 2.5 min to remove the surface oxide [14]. The glass substrate was cleaned ultrasonically in ethanol and then in acetone for 10 min each. Magnetic properties were measured by using a vibrating sample magnetometer (VSM), and the crystalline orientation of the "lm was investigated by X-ray di!ractometer at room temperature. H and squareness S, which is de"ned as M (re# 3 manent magnetization)/M (saturated magneti4 zation) in directions parallel and vertical to the "lm (denoted as E and o, respectively), were determined from an M}H hysteresis loop measured by using VSM with magnetic "elds up to 10 kOe. A Cu K radiation (j"1.540 As ) from the rotating anode a generator (Rigaku RU200) was used with a 50 kV of acceleration voltage and a 150 mA of emission current. A monochromator of graphite was used to remove Cu K radiation. X-ray di!raction (XRD) b patterns were obtained by step scanning at a rate of 0.013/step. The same sample was used for VSM and XRD measurements. Bright- and dark-"eld images and corresponding transmission high-energy electron di!raction (THEED) patterns by using a transmission electron microscope (TEM) with
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295
a 300 kV of acceleration voltage were investigated to obtain the crystallinity and the orientation distribution of Co and Ti grains. The samples for TEM observation were prepared by ion-milling technique.
3. Results 3.1. Ewect of kind of substrate Table 1(a) shows magnetic properties of the 100 nm thick "lms on Si(1 0 0) with the HF pretreatment and glass substrates. The "lm on Si(1 0 0) substrate possessed H (o) by 200 Oe higher than # that on glass substrate. S(o) and S(E) were almost the same, although H (E) values were slightly di!er# ent by 80 Oe. Table 1(b) shows magnetic properties of the "lms on Si substrate with and without the HF pretreatment. Without the HF pretreatment, the magnetic properties became worse. Therefore, we made the HF pretreatment for Si(1 0 0) substrate whenever the "lms on Si(1 0 0) and glass substrates were compared. The detailed explanation to the e!ect of the HF pretreatment will be related in Section 3.2. Figs. 1a and b show XRD patterns of the "lms on Si and glass substrates, respectively. The peaks corresponding to Co(0 0 1) and Ti(0 0 1) planes were explicitly observed in both "lms. The di!raction intensity of Co(0 0 2) of the "lm on Si(1 0 0) was smaller than that on glass. On the other hand, the di!raction intensity of Ti(0 0 2) of
Table 1 (a) Magnetic properties of CoCrTa/TiCr "lms on Si(100) with the HF pretreatment and glass substrates Substrate Si(1 0 0) Glass
H (o) (Oe) # 960 760
H (E) (Oe) #
S(o)
S(E)
440 360
0.18 0.15
0.62 0.61
(b) Magnetic properties of CoCrTa/TiCr "lms on Si(1 0 0) substrate with and without the HF pretreatment HF treatment
H (o) (Oe) #
H (E) (Oe) #
S(o)
S(E)
L *
1040 300
540 280
0.20 0.07
0.54 0.83
Fig. 1. XRD patterns of CoCrTa/TiCr "lms on substrates of (a) Si(1 0 0) and (b) glass. Thickness of the magnetic layer was 100 nm. Unidenti"ed peaks were indicated by solid triangles.
the "lm on Si(1 0 0) was stronger than that on glass. The broad peak around 2h"483 corresponded to Co(1 0 1) plane, and the unidenti"ed broad peak indicated by solid triangles were seen around 503. Solid curves in Figs. 2a and b are rocking curves of Co(0 0 2) for the "lms on Si(1 0 0) and glass substrates, respectively. The rocking curves were obtained by measuring the intensity as a function of the sample-rotating angle h with the position of a detector "xed at 2h. The curve is sharper for the "lm on Si(1 0 0) than for the "lm on glass. The full-width at the half-maximum, which is de"ned as *h , was obtained by "tting the curves to a Gaus50 sian distribution. The broken lines show the "tted curves. Table 2(a) lists the values of *h . The value 50 of *h of Co(0 0 2) for the "lm on Si(1 0 0) was 23 50 smaller than that for the "lm on glass. Figs. 3a and b show rocking curves of Ti(0 0 2) for the "lms on Si(1 0 0) and glass substrates, respectively. A conspicuously sharp rocking curve was obtained for the "lm on Si(1 0 0) as compared with the "lm on
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T. Asahi et al. / Journal of Magnetism and Magnetic Materials 212 (2000) 293}299 Table 2 (a) *h for CoCrTa/TiCr "lms on Si(1 0 0) with the HF pret50 reatment and glass substrates Substrate
Si(1 0 0) Glass
*h (3) 50 Co(0 0 2)
Ti(0 02)
7.8 9.8
5.5 11.8
(b) *h for CoCrTa/TiCr "lms on Si(1 0 0) substrate with and 50 without the HF pretreatment HF treatment
*h (3) 50 Co(0 0 2)
L *
17.2 35.4
Ti(0 0 2) 5.3 7.6
Fig. 2. Rocking curves of Co(0 0 2) for CoCrTa/TiCr "lms on substrates of (a) Si(1 0 0) and (b) glass with 100 nm thickness of the magnetic layer. Solid and broken lines were the experimental and the "tting data, respectively.
glass. As shown in Table 2(a), the value of *h of 50 Ti(0 0 2) for the "lm on Si(1 0 0) was only one-half of that for the "lm on glass. Figs. 4a}d show TEM bright- and dark-"eld images of the "lms on Si(1 0 0) and glass substrates, respectively. Columnar grains of Co and Ti were observed, and the grain size of Co, ca. 20}30 nm, for the "lm on Si(1 0 0) was equivalent to that for the "lm on glass. It was shown that the crystallinity of TiCr layer for the "lm on Si(1 0 0) is much higher than that for the "lm on glass. On the other hand, the "lm on Si(1 0 0) had larger regions of initial growth layer in the CoCrTa layer than the "lm on glass. Figs. 4e and f show THEED patterns corresponding to the bright- and dark-"eld images for the "lms on Si(1 0 0) and glass substrates, respectively. The hexagonal sharp spots in Fig. 4e
Fig. 3. Rocking curves of Ti(0 0 2) for CoCrTa/TiCr "lms on substrates of (a) Si(1 0 0) and (b) glass with 100 nm thickness of the magnetic layer. Solid and broken lines were the experimental and the "tting data, respectively.
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297
Fig. 4. Cross-sectional TEM images of CoCrTa (100 nm thick)/TiCr/substrates. (a), (b) and (c), (d): bright- and dark-"eld images on Si(1 0 0) and glass substrates, respectively. (e), (f): THEED patterns of (a) and (b). In (e) and (f), Co(0 0 2) and Ti(0 0 2) re#ections are indicated by arrows, respectively.
originated from a diamond structure of Si substrate. The di!raction patterns indicate Co(0 0 2) and Ti(0 0 2) planes are oriented parallel to the "lm surface for both "lms. The intensity distribution along the arc of Ti(0 0 2) re#ection for the "lm on Si(1 0 0) was narrower than that for the "lm on glass, which indicates a small orientation of Ti[0 0 1]. However, the di!erence in the distribution of Co(0 0 2) re#ection was not clearly found between Figs. 4e and f due to the overlap of several re#ections. 3.2. Ewect of HF pretreatment of Si substrate It has been shown that the surface conditions of Si(1 0 0) substrate depend on the HF pretreatment performed prior to the metal deposition [14]. To examine the in#uence of surface conditions of the Si(1 0 0) substrate, the 60 nm thick "lms were deposited on Si(1 0 0) substrates with and without the HF pretreatment, respectively. Figs. 5a and b show
the M}H hysteresis loops for both "lms, which di!ered in the shape of the loop. As shown in Table 1(b), the HF pretreatment was found to increase coercive force, in particular H (o). # The XRD patterns of the "lm on Si(1 0 0) substrate with and without the HF pretreatment are shown in Figs. 6a and b, respectively. Co(0 0 2) and Ti(0 0 2) re#ections were observed in both "lms, and an unidenti"ed broad peak was seen around 2h"493 only for the "lm without the HF pretreatment. It was found that the intensity of Co(0 0 2) and Ti(0 0 2) were increased by the HF pretreatment. The values of *h were obtained by inves50 tigating the rocking curves of Co(0 0 2) and Ti(0 0 2) observed in Fig. 6, respectively. Table 2(b) shows the value of *h for Co(0 0 2) and Ti(0 0 2) 50 peaks. The values of *h of Co(0 0 2) and Ti(0 0 2) 50 were both smaller with the HF pretreatment than without it. The grain size of Co was estimated less than 20 nm for both "lms with and without the HF
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4. Discussion
Fig. 5. M}H hysteresis loops of CoCrTa/TiCr "lms on Si(1 0 0) substrate (a) with and (b) without the HF pretreatment, where the ordinate is normalized by saturated magnetization M . 4 Thickness of the magnetic layer was 60 nm.
Fig. 6. XRD patterns of CoCrTa/TiCr "lms on Si(1 0 0) (a) with and (b) without the HF pretreatment. Thickness of the magnetic layer was 60 nm. An unidenti"ed peak was indicated by a solid triangle.
pretreatment from plane-view TEM images though these "gures are not indicated here. Furthermore, the plane-view THEED patterns indicated that the Debye}Scherrer ring of Co(0 0 2) for the "lm without the HF pretreatment was broad and weak, which means low crystallinity of the CoCrTa layer and was consistent with the result of XRD.
It was revealed that the "lm on Si substrate possessed a higher perpendicular coercive force, H (o), than the "lm on glass substrate. Measure# ments of *h of Co(0 0 2) in XRD patterns showed 50 that the distribution of Co[0 0 1] is smaller for the "lm on Si(1 0 0) than for the "lm on glass. On the other hand, the decrease in *h of Ti(0 0 2) for 50 the TiCr underlayer deposited on Si(1 0 0) substrate suggests that the Si(1 0 0) substrate has an e!ect to orient the c-axis of Ti grains in the TiCr underlayer vertically to the substrate. This e!ect was also con"rmed by the results of THEED patterns that the intensity distribution of Ti(0 0 2) was narrow for the "lm on Si(1 0 0) as compared with the "lm on glass. Therefore, it is considered that the decrease in the distribution of the c-axis orientation of Co grains in the magnetic layer for the "lm on Si(1 0 0) substrate was caused by the improvement of preferred orientation of Ti grains in the TiCr underlayer, resulting in the increase in H (o) of the "lm on # Si(1 0 0) substrate. From observations of XRD patterns and crosssection bright- and dark-"eld images of TEM, it was found that the Si(1 0 0) substrate increases the crystallinity of the TiCr underlayer. Therefore, it can be said that the crystalline surface of Si(1 0 0) promotes the crystal growth of Ti grains in the TiCr underlayer. On the other hand, from the decrease in Co(0 0 2) peak intensity and the increase in the intensity of the unidenti"ed peak which might correspond to (2 0 0) plane of FCC Co structure, as seen in Figs. 1a and b, it does not seem that the crystallinity of the magnetic layer was not necessarily improved by the use of Si(1 0 0) substrate. This is possibly due to an increase in the degree of mismatch between the TiCr underlayer and the CoCrTa magnetic layer caused by an improvement in the crystallinity of the underlayer. Actually, bright- and dark-"eld images of TEM observation indicate that regions of the initial growth layer in the magnetic layer of the "lm on Si(1 0 0) are larger than those of the "lm on glass. As for the surface treatment of HF solution of Si(1 0 0), the surface of Si(1 0 0) substrate depended largely on the treatment performed prior to the metal deposition. Without the HF pretreatment,
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H (o) and the re#ection intensity of Co(0 0 2) and # Ti(0 0 2) in XRD patterns were considerably small, which were caused by low crystallinity of the "lm. This e!ect is attributed to the deterioration of surface conditions of the substrate, that is, the surface oxide remained on the Si(1 0 0) substrate without the HF pretreatment. On the other hand, the crystalline surface of Si freshly exposed by the pretreatment is likely to promote the crystal growth and the preferred orientation of Ti grains in the TiCr underlayer, which, in turn, in#uences directly the crystalline structure of the CoCrTa magnetic layer. Thus, it was revealed that the fresh single-crystalline surface of Si(1 0 0) substrate contributes to the improvement of magnetic properties of a perpendicular magnetic recording medium, CoCrTa/TiCr.
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the substrate of Si(1 0 0) with the HF pretreatment, while the crystallinity of the CoCrTa layer was rather lower for the "lm on Si(1 0 0) with the HF pretreatment than for the "lm on glass.
Acknowledgements This work was supported by the &Research for the Future' program of the Japan Society for the Promotion of Science. The authors thank Dr. J. Hokkyo and Dr. T. Momma for their contribution to the discussion, and Dr. Y. Okinaka for critically reading the manuscript.
References 5. Conclusion CoCrTa/TiCr magnetic "lms were deposited on substrates of glass and Si(1 0 0) with and without the HF pretreatment to investigate the in#uence of substrates on magnetic properties and crystalline orientation of the "lms. Magnetic measurements revealed that the "lm on Si(1 0 0) substrate with the HF pretreatment possessed H (o) by 200 Oe high# er than that on glass. Furthermore, magnetic properties of CoCrTa/TiCr as a perpendicular magnetic recording medium were found to be improved conspicuously by the pretreatment with HF to the surface of Si(1 0 0) substrate. Results of X-ray and transmission electron microscopy studies on the crystalline orientation of the TiCr underlayer and the CoCrTa magnetic layer indicated that both distributions of c-axis orientation of Co and Ti grains in the direction vertical to the substrate decreased for the "lm on Si(1 0 0) with the HF pretreatment as compared with the "lm on glass. It was found from these experiments that the crystallinity of the TiCr underlayer was improved using
[1] H. Yoda, H. Iwasaki, T. Kobayashi, A. Tsukai, M. Sahashi, IEEE Trans. Magn. MAG-32 (1996) 3363. [2] H. Mutoh, H. Kanai, I. Okamoto, Y. Ohtsuka, T. Sugawara, T. Koshikawa, J. Toda, Y. Uematsu, M. Shinohara, Y. Mizoshita, IEEE Trans. Magn. MAG-32 (1996) 3914. [3] M.H. Kryder, W. Messner, L.R. Carley, J. Appl. Phys. 79 (1996) 4485. [4] D.N. Lambeth, E.M.T. Velu, G. Bellesis, L.L. Lee, D.E. Langhlin, J. Appl. Phys. 79 (1996) 4496. [5] T. Yogi, J. Magn. Soc. Jpn. 15 (S2) (1991) 475. [6] T. Osaka, M. Takai, K. Hayashi, K. Ohashi, M. Saito, K. Yamada, Nature 392 (1998) 796. [7] P.L. Lu, S.H. Charap, IEEE Trans. Magn. MAG-30 (1994) 4230. [8] S. Iwasaki, Y. Nakamura, IEEE Trans. Magn. MAG-13 (1977) 1272. [9] S. Iwasaki, N. Honda, J. Magn. Soc. Jpn. 21 (S2) (1997) 1. [10] D.A. Thompson, J. Magn. Soc. Jpn. 21 (S2) (1997) 9. [11] K. Kimoto, Y. Hirayama, M. Futamoto, J. Magn. Magn. Mater. 159 (1996) 401. [12] M. Futamoto, N. Inaba, Y. Hirayama, K. Ito, Y. Honda, J. Magn. Magn. Mater. 193 (1999) 36. [13] Y. Matsuda, Y. Shiroishi, T. Shimotsu, K. Takagi, J. Magn. Soc. Jpn. 13 (S1) (1989) 391. [14] T. Osaka, T. Homma, Y. Kurokawa, T. Taguchi, A. Takizawa, J. Magn. Soc. Jpn. 21 (1997) 213.