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Microstructure and intergranular coupling in CoCrTa thin-film media fabricated using an ultraclean sputtering process
Journal of Magnetism and Magnetic Materials 155 (1996) 234-237
N ELSEVIER
~ a journalof magnetism and magnetic ~ i materials
Microstructure and intergranular coupling in CoCrTa thin-film media fabricated using an ultraclean sputtering process J. Nakai
a, A.
Kikuchi b, K. Nakatani c, M. Hirasaka ~, T. Shimatsu d,* M. Takahashi d
Kobe Steel Ltd, Kobe, 651-22, Japan b Fujitsu Ltd, Kita-owaribe, Nagano, 381, Japan c Teijin Ltd, Hino, Tokyo, 191, Japan d Department of Electronic Engineering, Tohoku University, Sendai, 980-77, Japan
Abstract The microstructure and intergranular coupling in Coss.sCrm.sTa 4 and Co78Cr~7Ta 5 media fabricated using an ultraclean (UC) process are discussed in relation to the thickness of the Cr underlayer. The purification of the deposition atmosphere by applying the UC process is found to be most effective for deriving high H~ in both media. In UC-Co78Cr~7Ta5, a high H c of about 2.1 kOe remains even at a Cr thickness of 2.5 nm. With the UC process the Cr segregated grain boundary structure is enhanced even in media with thin Cr underlayers, which causes a remarkably low intergranular exchange coupling. The increase in the strength of intergranular magnetostatic coupling due to the grain size reduction strongly influences the reduction in Hc/Hkgra'n with decreasing Cr thickness. In UC-Co78Cr17Ta 5 with extremely thin Cr underlayers, the sufficiently segregated grain boundary structure and the intrinsically low intergranular magnetostatic coupling are responsible for the realization of very high H J H f ai".
1. Introduction High coercive force and low noise are essential requirements for thin-film media to achieve high-density recording. These properties are closely related to the microstructure of the media through the degree of intergranular magnetic coupling. We propose that an ultraclean (UC) sputtering process is a key technology for improving the magnetic properties through the control of thin-film growth [1,21. In this study we have examined the magnetic properties and microstructure in C%5.sCrm.sTa 4 and Co78Cr~7Ta s media fabricated using the UC process. The correlation between the microstructure and the degree of intergranular magnetic coupling of these media is discussed in connection with the grain boundary structure and grain size.
2. Experimental procedure In order to realize the ultraclean atmosphere during the film deposition, two physical factors were taken into account. One was to use a process chamber with very low outgassing. The inside surfaces of the process chamber
Corresponding author. Tel/fax: + 81-22-263-9402.
used (ILC 3013, Anelva) were polished by electrochemical buffing to reduce the outgassing. The base pressure was less than 3 × 10 - 9 Tort and the build-up rate was less than 5 × 10 -7 Torr I/s:, these values are about two orders lower than those of normally used sputtering chambers. Another key technology was the use of ultraclean Ar gas (UC-Ar). The impurity level of UC-Ar is about 1 ppb (H20 level) at the point of use, which is two or three orders lower than that of the high-grade Ar gas normally used. The Co85.5Cr10.sTa4/Cr and C07sCrl7Tas/Cr media were deposited on extremely smooth non-textured NiP/AI substrates with surface roughnesses R, < 1 nm. The CoCrTa layer thickness was 16 nm for Coss.sCrlo.sTa ~ media and 28 nm for Co78Cr17Ta 5 media, corresponding to a remanence-thickness product of about 100 G Ixm. During film deposition the substrate temperature was kept at 250°C, and the substrate bias voltages were fixed at -200/0 for Co85.sCrmsTa4/Cr and 0 / 0 V for Co78CrjvTas/Cr media. The substrate surface was cleaned by dry etching using UC-Ar in the process chamber, just before film deposition (about 0.2-0.3 nm etched depth). For comparison, Co85.sCrm.sTa 4 media were fabricated using the normal sputtering process (1 × 10 - 7 Torr base pressure, with the same cathode types) using normal Ar gas (n-Coss.sCrmsTa4).
0304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0304-8853(95)00745-8
235
J. Nakai et a l . / Journal of Magnetism and Magnetic Materials 155 (1996) 234 237 .
The magnetic properties were measured with a vibrating sample magnetometer (VSM). The magnetocrystalline anisotropy field and the degree of intergranular magnetic coupling were evaluated by rotational hysteresis loss analysis [3,4]. The microstructures of the media were examined by transmission electron microscopy (TEM). 3.
Results
a n d
.
.
,
.
3
,.F2S2
1~ ~ T a 4
1
0.5
1 0
'
C°855Crm 5Ta4 : 16nmt Co78 Cr17 Ta5:28nm ~
-
i
i
i
10
20
30
.
i
40
.
.
50
.
|
60
Cr thickness (nm) Fig. 1, Coercive force H, in UC-Co~55CrlosTa ~ and UCCov~Cr~TTaS media versus the Cr underlayer thickness. The results lor n-Co85.sCrm.sTa, fabricated under normal sputtering conditions are also shown for comparison.
.
,
UC-Co78Cr17Ta5
5 4 n-C085
::1::~ 3 In the media examined, i.e. isotropic media, macroscopic uniaxial magnetic anisotropy was not observed in the magnetization curve. Fig. 1 shows the dependence of the coercive force H~ on the Cr underlayer thickness in the Co85.sCr~o.sTa 4 and C07sCr~TTa 5 media fabricated using the UC process (UCCos55CrmsTa 4 and UC-Co78CrI7Tas). The results for n-Cos.~.sCrao.sTa 4 are also shown for comparison. The values of the saturation magnetization M~ of UCCos5 5CrmsTa 4 and UC-Co78CrwTa 5 were about 700 and 470 e m u / c m 3, respectively. By applying the UC process to the fabrication of Cos~.sCrmsTa 4 media, H~ values more than twice as high as those in n-Co85.sCrmsTa 4 were realized at all Cr thicknesses. This result means that the purification of the atmosphere during film deposition is most effective for deriving excellent high H~ in CoCrTa media. In both kinds of UC-processed media, H~ shows an almost constant value of about 2.3 kOe for Cr thicknesses up to 10 nm. With further reductions in the Cr thickness to
,
6
=_ 3. I. Magnetic properties
.
7
~" O
d i s c u s s i o n
,
5Cr10
5Ta
4
2
1
C0855Crlo5Ta4 : 16nm COTs Cry7 Tas :28rim i
0
10
,
i
20
,
i
30
~
i
40
,
i
50
.
60
Cr thickness (nm) Fig. 2. Values of the magnetocrystalline anisotropy field of grains, Hkgrain, Of UC-Cos55Cr105Ta4, UC-CoT~CrlTTa ~ media and nCo~5 5Crt05Ta4 versus the Cr underlayer thickness. H~'~"n is obtained as the magnetic field at which the rotational hysteresis loss just vanishes.
2.5 nm, the H~ values of UC-Co85 5Cr ~~sTa4 fall dramatically to about 1.5 kOe. In contrast, in UC-CoT~CrwTa 5, a high H~. value of about 2.1 kOe remains even at a Cr thickness of 2.5 nm. In order to clarify the physical origin of the appearance of high H c, two physical factors should be taken into account. One is the magnetocrystalline anisotropy field of the CoCrTa grains, and the other is the degree of intergranular magnetic coupling. In Fig. 2, the values of the magnetocrystalline anisotropy field of grains, H kgrain, are plotted against the Cr thickness. Here, H gr~m is obtained as the magnetic field at which the rotational hysteresis loss just vanishes [4]. In UC-Coss. .5Crm . . . sTa 4. H kgrain shows remarkably higher values than in n-Coa5.sCrmsTa 4, which implies that the realization of large magnetocrystalline anisotropy field of the grains plays an important role in the appearance of high H~ [4]. It should be noted that the values of Hgramk in the UC-processed media are almost the same. at about 6 kOe, independent of the Cr and Ta contents, and that Hgrain remains high even at 2.5 nm Cr. k In Fig. 3 the values of normalized coercive force H c ///"4 gkr a i n are plotted against the Cr thickness. The value of H /c H / kgr~" is related to the degree of intergranular magnetic coupling and reaches a maximum value of about 0.5 in the case of isotropic media without intergranular magnetic coupling [3]. The values of ,/ 4' c l/ /,4'gk ra in in UCCos5.sCrm.sTa 4 are much higher than those in nCoss.sCrmsTa ~, which means that extremely low inter-
236
J. Nakai et al. / Journal of Magnetism and Magnetic Materials 155 (1996) 234-237 0,5
.
,
.
,
-
UC-Co78Cr17Ta5 0.4
[3 '- 0.3
~UC-Co85.5Cr 1o.5Ta4
O3
-r'° 0.2
n-Co85.5Crlo.5Ta4
U C - C o 855 C r l 0 5 T a 4
0.1 C085.5Cr~o.sTa4 : 16nm C078 Cr17 Ta5:28nm 0
i
i
i
i
i
10
20
30
40
50
60
Cr thickness (nm) Fig. 3. Values of the normalized coercive force, H,,/Hk~'~i", of UC-Coss.~Cr10.~Ta4 and UC-Co7sCrt7Ta 5 media and nCo85 sCrl0,sTa4 versus the Cr underlayer thickness.
granular magnetic coupling is realized by applying the UC process. In UC-C°85.sCrlo.sTa4, --c/--k/4//4grainreaches high values of about 0.37 for Cr thicknesses up to l0 nm, but it falls dramatically to about 0.25 with further reductions in Cr thickness to 2.5 rim. In UC-C°78CrlTTas, "'c/"k/4//4grain maintains a high value of about 0.35 even at 2.5 nm Cr. This result means that the UC process enables the realization of low intergranular magnetic coupling even in Co85.sCrjo.sTa 4 with low Cr and Ta contents relative to those in Co78CrtTTa 5. Furthermore, the increase in Cr and Ta contents is found to result in further low intergranular coupling in media with extremely thin Cr underlayers.
UC-Co 78Cr17Ta 5 Fig. 4. TEM images of UC-Co~ 5Crlo 5Ta~ and UC-Co78Cr17Ta5 with 50 nm Cr underlayers. higher Cr and Ta contents, further enhancement of the Cr segregated boundary structure takes place by applying the UC process. Fig. 5 shows TEM images of the UC-Co85.sCr10.sTa4 and UC-Co78Cr~TTa 5 media with extremely thin Cr under-
3.2. Microstructure
The low intergranular coupling realized by applying the UC process, and its dependence on Cr underlayer thickness mentioned above, is strongly correlated with changes in the microstructure of the films. Fig. 4 shows TEM images of UC-Coss.sCrlo.sTa4 and UC-Co78Cr17Ta 5 with 50 nm Cr underlayers. In UC-Co85.sCrto.sTa 4, the CoCrTa grains are clearly separated by grain boundaries due to the enhancement of the Cr segregated boundary structure by applying the UC process [4]. This boundary phase is suggested to consist of a non-magnetic phase with an extremely high Cr concentration [5], from which follows the extremely low intergranular exchange coupling in this medium. In UC-CoTsCr~TTa5, the CoCrTa grains are further separated by much thicker Cr segregated grain boundaries even though the film was deposited without a substrate bias voltage. This means that in the media with
UC-C0853 Crl03Ta4
UC-Co 78Cr17Ta 5 Fig. 5. TEM images of UC-Cos~ ~Crlo ~Ta4 and UC-CoTsCrrTTa 5 with extremely thin (2.5 nm) Cr underlayers.
J. Nakai et al. / Journal of Magnetism and Magnetic Materials 155 (1996) 234 237 layers of 2.5 nm. With the decrease in the Cr thickness to 2.5 nm, the CoCrTa grain size is found to decrease to less than 10 nm in both UC-processed media. It has been revealed that in both kinds of media the grain size is strongly correlated with the Cr underlayer thickness and decreases dramatically from about 25-30 nm to about 7 - 1 0 nm with decreasing Cr thickness from 50 to 2.5 nm. It should be noted that in UC-C%5.sCrmsTa4 even with extremely thin Cr underlayers, each CoCrTa grain is clearly separated by a segregated grain boundary, which explains the remarkable reduction in the intergranular exchange coupling in this medium. The further clear Cr segregated boundary structure realized in UC-CovsCrwTa 5 explains the further low intergranular exchange coupling. In order to clarify the Cr thickness dependence of the intergranular magnetic coupling in connection with the microstructure, both the separation of grains at grain boundaries and the grain size should be taken into account. The grain size reduction induces the increase in intergranular magnetostatic coupling even in media with widely separated grains segregated by a grain boundary. Furthermore, in the media where intergranular magnetostatic coupling is predominant, the ratio of 4arM~ to Hkgrain means the strength of the effect of intergranular magnetostatic coupling on the coercive force (e.g. [6]). In this study, the value of 4 w M ~ / H gn'in in UC-Co7sCrwTa s is about 70% of that in UC-Coss..~CrmsTa 4, which means intrinsically low intergranular magnetostatic coupling in UCCowCrlTTa 5. Therefore, it is suggested that the increase in the strength of intergranular magnetostatic coupling due to the grain size reduction strongly influences the reduction in H . / H ~ ~i" with decreasing Cr thickness shown in Fig. 3. In UC-Cov~Crl7Tas, the more sufficiently segregated grain boundary structure and the intrinsically low intergranular magnetostatic coupling (low 4~rM~/H~ ~i") are responsible for the realization of the very high /,4, ~//M, ,g krain even in media with extremely thin Cr underlayers.
4, Conclusions The microstructure and intergranular coupling in C%5.sCr]o.sTa 4 and C078CrwTa 5 media fabricated using the UC process are discussed in connection with the magnetic properties. The main results are as follows:
237
(I) In the UC process, the purification of the atmosphere during film deposition is found to be most effective for deriving excellent high H~ in both media. In UCCo7sCrlTTa 5, a high H~ value of about 2.1 kOe remains even at a Cr thickness of 2.5 nm. (2) In both media H g'~in shows almost the same high value of about 6 kOe, independent of the Cr and Ta contents, and this remains even with thin Cr layers of 2.5 rim.
(3) The UC process enables the enhancement of the formation of Cr segregated grain boundary structures even in media with thin Cr underlayers, which cause the remarkably low intergranular exchange coupling. (4) The increase in the strength of intergranular magnetostatic coupling due to the grain size reduction strongly influences the reduction in H~/H~ r`'i'~ with decreasing Cr thickness. In UC-CovsCrwTa 5 with extremely thin Cr underlayers, the more sufficiently segregated grain boundary structure and the intrinsically low intergranular magnetostatic coupling (low 4"rtM~/H~ ''i'') are responsible for the realization of the very high H J H ~ '~'m.
Acknowledgements This study was conducted by Tohoku University in cooperation with 13 companies (Kobe Steel, Fujitsu, KAO, Teijin, Asahi Glass, Fuji Electric Co. R & D , Nippon Sheet Glass, Anelva, Japan Energy, Mitsubishi Materials, Hoya, Hitachi Metals, ALPS Electric).
References [1] M. lmakawa, S. Ishibashi, A. Hatashita, T. Shimatsu and M. Takahashi, Proc. 14th ICMFS and EMRS Joint Colloq., Germany (1994) p. 247. [2] T. Shimatsu, M. Imakawa, S. Ishibashi and M. Takahashi, Proc. 14th ICMFS and EMRS Joint Colloq.. Germany (1994) p. 213. [3] M. Takahashi, T. Shimatsu. M. Suekane, M. Miyamura, K. Yamaguchi and H. Yamasaki, IEEE Trans. Magn. 28 (1992) 3285. [4] M. Imakawa. S. lshibashi, M. Kuwabara. T. Shimatsu and M. Takahashi, J. Magn. Magn. Mate]'., to be published. [5] J. Nakai, E. Kusumoto, T. Miyamoto. K. Yoshikawa and K, Itayama. IEEE Trans. Magn. 30 (1994) 3696. [6] J. Zhu and N. Bertram, J. Appl. Phys. 63 (1988) 3248.
N ELSEVIER
~ a journalof magnetism and magnetic ~ i materials
Microstructure and intergranular coupling in CoCrTa thin-film media fabricated using an ultraclean sputtering process J. Nakai
a, A.
Kikuchi b, K. Nakatani c, M. Hirasaka ~, T. Shimatsu d,* M. Takahashi d
Kobe Steel Ltd, Kobe, 651-22, Japan b Fujitsu Ltd, Kita-owaribe, Nagano, 381, Japan c Teijin Ltd, Hino, Tokyo, 191, Japan d Department of Electronic Engineering, Tohoku University, Sendai, 980-77, Japan
Abstract The microstructure and intergranular coupling in Coss.sCrm.sTa 4 and Co78Cr~7Ta 5 media fabricated using an ultraclean (UC) process are discussed in relation to the thickness of the Cr underlayer. The purification of the deposition atmosphere by applying the UC process is found to be most effective for deriving high H~ in both media. In UC-Co78Cr~7Ta5, a high H c of about 2.1 kOe remains even at a Cr thickness of 2.5 nm. With the UC process the Cr segregated grain boundary structure is enhanced even in media with thin Cr underlayers, which causes a remarkably low intergranular exchange coupling. The increase in the strength of intergranular magnetostatic coupling due to the grain size reduction strongly influences the reduction in Hc/Hkgra'n with decreasing Cr thickness. In UC-Co78Cr17Ta 5 with extremely thin Cr underlayers, the sufficiently segregated grain boundary structure and the intrinsically low intergranular magnetostatic coupling are responsible for the realization of very high H J H f ai".
1. Introduction High coercive force and low noise are essential requirements for thin-film media to achieve high-density recording. These properties are closely related to the microstructure of the media through the degree of intergranular magnetic coupling. We propose that an ultraclean (UC) sputtering process is a key technology for improving the magnetic properties through the control of thin-film growth [1,21. In this study we have examined the magnetic properties and microstructure in C%5.sCrm.sTa 4 and Co78Cr~7Ta s media fabricated using the UC process. The correlation between the microstructure and the degree of intergranular magnetic coupling of these media is discussed in connection with the grain boundary structure and grain size.
2. Experimental procedure In order to realize the ultraclean atmosphere during the film deposition, two physical factors were taken into account. One was to use a process chamber with very low outgassing. The inside surfaces of the process chamber
Corresponding author. Tel/fax: + 81-22-263-9402.
used (ILC 3013, Anelva) were polished by electrochemical buffing to reduce the outgassing. The base pressure was less than 3 × 10 - 9 Tort and the build-up rate was less than 5 × 10 -7 Torr I/s:, these values are about two orders lower than those of normally used sputtering chambers. Another key technology was the use of ultraclean Ar gas (UC-Ar). The impurity level of UC-Ar is about 1 ppb (H20 level) at the point of use, which is two or three orders lower than that of the high-grade Ar gas normally used. The Co85.5Cr10.sTa4/Cr and C07sCrl7Tas/Cr media were deposited on extremely smooth non-textured NiP/AI substrates with surface roughnesses R, < 1 nm. The CoCrTa layer thickness was 16 nm for Coss.sCrlo.sTa ~ media and 28 nm for Co78Cr17Ta 5 media, corresponding to a remanence-thickness product of about 100 G Ixm. During film deposition the substrate temperature was kept at 250°C, and the substrate bias voltages were fixed at -200/0 for Co85.sCrmsTa4/Cr and 0 / 0 V for Co78CrjvTas/Cr media. The substrate surface was cleaned by dry etching using UC-Ar in the process chamber, just before film deposition (about 0.2-0.3 nm etched depth). For comparison, Co85.sCrm.sTa 4 media were fabricated using the normal sputtering process (1 × 10 - 7 Torr base pressure, with the same cathode types) using normal Ar gas (n-Coss.sCrmsTa4).
0304-8853/96/$15.00 © 1996 Elsevier Science B.V. All rights reserved SSDI 0304-8853(95)00745-8
235
J. Nakai et a l . / Journal of Magnetism and Magnetic Materials 155 (1996) 234 237 .
The magnetic properties were measured with a vibrating sample magnetometer (VSM). The magnetocrystalline anisotropy field and the degree of intergranular magnetic coupling were evaluated by rotational hysteresis loss analysis [3,4]. The microstructures of the media were examined by transmission electron microscopy (TEM). 3.
Results
a n d
.
.
,
.
3
,.F2S2
1~ ~ T a 4
1
0.5
1 0
'
C°855Crm 5Ta4 : 16nmt Co78 Cr17 Ta5:28nm ~
-
i
i
i
10
20
30
.
i
40
.
.
50
.
|
60
Cr thickness (nm) Fig. 1, Coercive force H, in UC-Co~55CrlosTa ~ and UCCov~Cr~TTaS media versus the Cr underlayer thickness. The results lor n-Co85.sCrm.sTa, fabricated under normal sputtering conditions are also shown for comparison.
.
,
UC-Co78Cr17Ta5
5 4 n-C085
::1::~ 3 In the media examined, i.e. isotropic media, macroscopic uniaxial magnetic anisotropy was not observed in the magnetization curve. Fig. 1 shows the dependence of the coercive force H~ on the Cr underlayer thickness in the Co85.sCr~o.sTa 4 and C07sCr~TTa 5 media fabricated using the UC process (UCCos55CrmsTa 4 and UC-Co78CrI7Tas). The results for n-Cos.~.sCrao.sTa 4 are also shown for comparison. The values of the saturation magnetization M~ of UCCos5 5CrmsTa 4 and UC-Co78CrwTa 5 were about 700 and 470 e m u / c m 3, respectively. By applying the UC process to the fabrication of Cos~.sCrmsTa 4 media, H~ values more than twice as high as those in n-Co85.sCrmsTa 4 were realized at all Cr thicknesses. This result means that the purification of the atmosphere during film deposition is most effective for deriving excellent high H~ in CoCrTa media. In both kinds of UC-processed media, H~ shows an almost constant value of about 2.3 kOe for Cr thicknesses up to 10 nm. With further reductions in the Cr thickness to
,
6
=_ 3. I. Magnetic properties
.
7
~" O
d i s c u s s i o n
,
5Cr10
5Ta
4
2
1
C0855Crlo5Ta4 : 16nm COTs Cry7 Tas :28rim i
0
10
,
i
20
,
i
30
~
i
40
,
i
50
.
60
Cr thickness (nm) Fig. 2. Values of the magnetocrystalline anisotropy field of grains, Hkgrain, Of UC-Cos55Cr105Ta4, UC-CoT~CrlTTa ~ media and nCo~5 5Crt05Ta4 versus the Cr underlayer thickness. H~'~"n is obtained as the magnetic field at which the rotational hysteresis loss just vanishes.
2.5 nm, the H~ values of UC-Co85 5Cr ~~sTa4 fall dramatically to about 1.5 kOe. In contrast, in UC-CoT~CrwTa 5, a high H~. value of about 2.1 kOe remains even at a Cr thickness of 2.5 nm. In order to clarify the physical origin of the appearance of high H c, two physical factors should be taken into account. One is the magnetocrystalline anisotropy field of the CoCrTa grains, and the other is the degree of intergranular magnetic coupling. In Fig. 2, the values of the magnetocrystalline anisotropy field of grains, H kgrain, are plotted against the Cr thickness. Here, H gr~m is obtained as the magnetic field at which the rotational hysteresis loss just vanishes [4]. In UC-Coss. .5Crm . . . sTa 4. H kgrain shows remarkably higher values than in n-Coa5.sCrmsTa 4, which implies that the realization of large magnetocrystalline anisotropy field of the grains plays an important role in the appearance of high H~ [4]. It should be noted that the values of Hgramk in the UC-processed media are almost the same. at about 6 kOe, independent of the Cr and Ta contents, and that Hgrain remains high even at 2.5 nm Cr. k In Fig. 3 the values of normalized coercive force H c ///"4 gkr a i n are plotted against the Cr thickness. The value of H /c H / kgr~" is related to the degree of intergranular magnetic coupling and reaches a maximum value of about 0.5 in the case of isotropic media without intergranular magnetic coupling [3]. The values of ,/ 4' c l/ /,4'gk ra in in UCCos5.sCrm.sTa 4 are much higher than those in nCoss.sCrmsTa ~, which means that extremely low inter-
236
J. Nakai et al. / Journal of Magnetism and Magnetic Materials 155 (1996) 234-237 0,5
.
,
.
,
-
UC-Co78Cr17Ta5 0.4
[3 '- 0.3
~UC-Co85.5Cr 1o.5Ta4
O3
-r'° 0.2
n-Co85.5Crlo.5Ta4
U C - C o 855 C r l 0 5 T a 4
0.1 C085.5Cr~o.sTa4 : 16nm C078 Cr17 Ta5:28nm 0
i
i
i
i
i
10
20
30
40
50
60
Cr thickness (nm) Fig. 3. Values of the normalized coercive force, H,,/Hk~'~i", of UC-Coss.~Cr10.~Ta4 and UC-Co7sCrt7Ta 5 media and nCo85 sCrl0,sTa4 versus the Cr underlayer thickness.
granular magnetic coupling is realized by applying the UC process. In UC-C°85.sCrlo.sTa4, --c/--k/4//4grainreaches high values of about 0.37 for Cr thicknesses up to l0 nm, but it falls dramatically to about 0.25 with further reductions in Cr thickness to 2.5 rim. In UC-C°78CrlTTas, "'c/"k/4//4grain maintains a high value of about 0.35 even at 2.5 nm Cr. This result means that the UC process enables the realization of low intergranular magnetic coupling even in Co85.sCrjo.sTa 4 with low Cr and Ta contents relative to those in Co78CrtTTa 5. Furthermore, the increase in Cr and Ta contents is found to result in further low intergranular coupling in media with extremely thin Cr underlayers.
UC-Co 78Cr17Ta 5 Fig. 4. TEM images of UC-Co~ 5Crlo 5Ta~ and UC-Co78Cr17Ta5 with 50 nm Cr underlayers. higher Cr and Ta contents, further enhancement of the Cr segregated boundary structure takes place by applying the UC process. Fig. 5 shows TEM images of the UC-Co85.sCr10.sTa4 and UC-Co78Cr~TTa 5 media with extremely thin Cr under-
3.2. Microstructure
The low intergranular coupling realized by applying the UC process, and its dependence on Cr underlayer thickness mentioned above, is strongly correlated with changes in the microstructure of the films. Fig. 4 shows TEM images of UC-Coss.sCrlo.sTa4 and UC-Co78Cr17Ta 5 with 50 nm Cr underlayers. In UC-Co85.sCrto.sTa 4, the CoCrTa grains are clearly separated by grain boundaries due to the enhancement of the Cr segregated boundary structure by applying the UC process [4]. This boundary phase is suggested to consist of a non-magnetic phase with an extremely high Cr concentration [5], from which follows the extremely low intergranular exchange coupling in this medium. In UC-CoTsCr~TTa5, the CoCrTa grains are further separated by much thicker Cr segregated grain boundaries even though the film was deposited without a substrate bias voltage. This means that in the media with
UC-C0853 Crl03Ta4
UC-Co 78Cr17Ta 5 Fig. 5. TEM images of UC-Cos~ ~Crlo ~Ta4 and UC-CoTsCrrTTa 5 with extremely thin (2.5 nm) Cr underlayers.
J. Nakai et al. / Journal of Magnetism and Magnetic Materials 155 (1996) 234 237 layers of 2.5 nm. With the decrease in the Cr thickness to 2.5 nm, the CoCrTa grain size is found to decrease to less than 10 nm in both UC-processed media. It has been revealed that in both kinds of media the grain size is strongly correlated with the Cr underlayer thickness and decreases dramatically from about 25-30 nm to about 7 - 1 0 nm with decreasing Cr thickness from 50 to 2.5 nm. It should be noted that in UC-C%5.sCrmsTa4 even with extremely thin Cr underlayers, each CoCrTa grain is clearly separated by a segregated grain boundary, which explains the remarkable reduction in the intergranular exchange coupling in this medium. The further clear Cr segregated boundary structure realized in UC-CovsCrwTa 5 explains the further low intergranular exchange coupling. In order to clarify the Cr thickness dependence of the intergranular magnetic coupling in connection with the microstructure, both the separation of grains at grain boundaries and the grain size should be taken into account. The grain size reduction induces the increase in intergranular magnetostatic coupling even in media with widely separated grains segregated by a grain boundary. Furthermore, in the media where intergranular magnetostatic coupling is predominant, the ratio of 4arM~ to Hkgrain means the strength of the effect of intergranular magnetostatic coupling on the coercive force (e.g. [6]). In this study, the value of 4 w M ~ / H gn'in in UC-Co7sCrwTa s is about 70% of that in UC-Coss..~CrmsTa 4, which means intrinsically low intergranular magnetostatic coupling in UCCowCrlTTa 5. Therefore, it is suggested that the increase in the strength of intergranular magnetostatic coupling due to the grain size reduction strongly influences the reduction in H . / H ~ ~i" with decreasing Cr thickness shown in Fig. 3. In UC-Cov~Crl7Tas, the more sufficiently segregated grain boundary structure and the intrinsically low intergranular magnetostatic coupling (low 4~rM~/H~ ~i") are responsible for the realization of the very high /,4, ~//M, ,g krain even in media with extremely thin Cr underlayers.
4, Conclusions The microstructure and intergranular coupling in C%5.sCr]o.sTa 4 and C078CrwTa 5 media fabricated using the UC process are discussed in connection with the magnetic properties. The main results are as follows:
237
(I) In the UC process, the purification of the atmosphere during film deposition is found to be most effective for deriving excellent high H~ in both media. In UCCo7sCrlTTa 5, a high H~ value of about 2.1 kOe remains even at a Cr thickness of 2.5 nm. (2) In both media H g'~in shows almost the same high value of about 6 kOe, independent of the Cr and Ta contents, and this remains even with thin Cr layers of 2.5 rim.
(3) The UC process enables the enhancement of the formation of Cr segregated grain boundary structures even in media with thin Cr underlayers, which cause the remarkably low intergranular exchange coupling. (4) The increase in the strength of intergranular magnetostatic coupling due to the grain size reduction strongly influences the reduction in H~/H~ r`'i'~ with decreasing Cr thickness. In UC-CovsCrwTa 5 with extremely thin Cr underlayers, the more sufficiently segregated grain boundary structure and the intrinsically low intergranular magnetostatic coupling (low 4"rtM~/H~ ''i'') are responsible for the realization of the very high H J H ~ '~'m.
Acknowledgements This study was conducted by Tohoku University in cooperation with 13 companies (Kobe Steel, Fujitsu, KAO, Teijin, Asahi Glass, Fuji Electric Co. R & D , Nippon Sheet Glass, Anelva, Japan Energy, Mitsubishi Materials, Hoya, Hitachi Metals, ALPS Electric).
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