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Effects of interlayers on the soft magnetic properties of Fe-Si multilayers
Journal of Magnetism and Magnetic Materials 126 (1993) 580-582 North-Holland
Effects of interlayers on the soft magnetic properties of F e - S i multilayers H. Ohji, M. Kataoka, Y. Uehara, S. Tanabe and T. Ozeki Materials & Electronic Det:ices Laboratory, Mitsubishi Electric Corporation, Hw~go 661, Japan
In order to realize low coercivity in Fe-9.0wt%Si-based multilayer films, the correlation between coercivity and the crystalline structure has been investigated. Crystalline orientation is an important factor for the low coereivity of Fe Si-based multilayer films. Permal[oy is found to be the most suitable interlayer for achieving low coercivity. 1. Introduction High-saturation magnetization (M~) films are required for magnetic recording heads to generate enough magnetic field to write on high-coercivity media. Fe-based alloy is considered as a high M~ head material. To obtain good soft magnetic properties, both a low saturation magnctostriction constant (A~) and low magnetocrystalline anisotropy are nceded [13]. In the case of F e - S i alloy, it is easy to obtain the low )t~ by controlling the Si content, but it is difficult to obtain a small magnetocrystalline anisotropy. Reducing the grain size by separating the Fe-based alloy by interlayers [4,5] is an effective method to reduce the magnetocrystalline anisotropy. In this study, the coercivity, grain size and crystalline structure of the multilayer films were measured. 2. Experimental The multilayer films were prepared on glass substrates by a conventional rf sputtering method. A d c magnetic field of 50 Oe was applied parallel to the substrate surface during sputtering. The sputtering conditions arc shown in table 1. The multilayer films were deposited in the following sequence: first, the interlayer was deposited on the substrate, followed by an Fe-9.0wt%Si layer. This sequence was repeated until the required total thickness was obtained. The coercivity and the saturation magnetostriction constant were measured using an M - H loop tracer and an optical cantilever method, respectively. The Si content was measured by E P M A . The crystalline orientation was investigated by the X-ray diffraction method.
The grain size was estimated from the broadening of the diffraction peak using the Scherrer formula. 3. Effects ofinterlayers The saturation magnetostriction constant (a~) of F e - S i alloy is drastically changed by the Si content. The effect of the Si content was investigated to obtain low a~, and the result is shown in fig. 1. The a~ becomes zero at around 9wt%Si. Thereforc, Fe 9.0wt%Si alloy was used for the following experiments, but the magnetocrystalline anisotropy is still large [6]. To reducc the anisotropy by reducing the grain size of the F e - S i layer, the effects of six kinds of interlayers (Permalloy, Sendust, amorphous C o - Z r - N b , Cu, Zr and Cr films) were examined. The total film thickness was 1.6 l~m and the thickness ratio of the F e - S i layer to thc interlayer was (/.18:0.02 ~m. The interlayer dependence of the (110)/(211) ratio of the X-ray diffraction intensity, the F e - S i grain size and the eoercivity of the multilayer film are shown in table 2. Among the six samples, the coercivities of those with ferromagnetic interlayers arc relatively low, which seems to indicate that the magnetic interactions arc very important lk~r low coercivity. We do not havc a clear explanation of the relationship, but in general, the soft magnetic properties improve with decreasing grain size [1]. However, the coercivity is determined not only by grain size, as shown in table 2. For example, the F e - S i grain size of a sample with a C o Z r N b interlayer is smaller than that with a Permalloy interlayer, but the cocrcivity of thc former sample is higher Table 1 Target ct)mposition and sputtering conditions Target composition
Correspondence to: H. Ohji, Materials & Electronic Devices
Laboratory, Mitsubishi Electric Corporation, 8-1-1 Tsukaguchi-Honmachi, Amagasaki, Hyogo 661, Japan. Tel: + 81-6497-7506; fax: + 81-6-497-7600.
Sputtering pressure Power density Substrate-target distance
0304-8853/93/$06.00 ~ 1993 - Elsevier Science Publishers B.V. (North-Holland)
Fe-9.99wlC;- Si Ni 19.8wt'3;Fe 10 mTorr Ar 5.5 W/cm ~ 4 cm
H. Ohji et al. / Soft magnetic properties of Fe-Si multilayers
g-. 'o 4
581
._o 300 Intensity
rr
4"~ O
¢,-
~
Coercivity
2oo
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o
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Table 2 Intensity ratio, grain size and coercivity of Fe-Si multilayer films Interlayer
Intensity ratio (110)/(211)
Grain site
Coercivity (Oe)
Permalloy Sendust CoZrNb Cu Zr Cr
131 126 29 30 20 202
29 28 19 26 20 35
0.4 4.4 3.7 6.6 10.8 3.2
than that of the latter. Permalloy, Sendust and Cr interlayers induce high (110)/(211) ratios of F e - S i layers among the six samples. F e - S i / P e r m a l l o y multilayer film has the lowest coercivity.
4. Fe-9.0wt%Si / Permalloy films First, the correlation between the (110)/(211) intensity ratio and the coercivity was investigated systemati-
~annealed
at 500°C
e-.,.-o
0
e.~
[]~
0.0 0.0
m
C
D~
0.5
(5
Oa ' ~ s . d e p o s i t e d
. 0.2
.
. 0.4
as-deposited
2o
Fe-Si T h i c k n e s s (/1 m )
[]
10 0
0.8
---D
Permalloy Thickness 0.02 ff m
. 0.6
cally for the F e - S i / P e r m a l l o y multilayer films. The F e - S i thickness was altered as an experimental parameter to change the intensity ratio. The orientation of Fe-Si(110) texture is enhanced when deposited on the P e r m a l l o y ( l l l ) texture, because the textures of both lattice constants are similar [7]. Therefore, the intensity ratio depends on the F e - S i thickness.The Permalloy thickness was fixed at 0.05 txm and the total thickness of the multilayer film was 3 ixm. The (110)/(211) intensity ratio increased with decreasing F e - S i thickness, as shown in fig. 2. The orientation of F e - S i ( l l 0 ) texture is dominant for high (110)/(211) ratio films and the rest of the peaks are very weak. The coercivity increases with the increasing F e - S i thickness, that is, with the decrease in the (110)/(211) ratio. We have already reported that the permeability of the multilayer film is independent of the Permalloy interlayer thickness [8]. As a result, it can be said that the F e - S i ( l l 0 ) texture is important in obtaining a low coercivity for the F e - S i / P e r m a l l o y multilayer film.
annealed at 500°C - - / u
1.5
1.0
1.2
Fig. 2. (110)/(211) X-ray diffraction intensity ratio and coercivity for Fe-9.0wt%Si/Permal]oy films as a function of Fe0.9wt%Si layer thickness.
--- 50 E c v 40 N 30
v
"~
0 .8
(b) 60
Permalloy Thickness 0.02/~ m a
O
0.4
ge-Si T h i c k n e s s (ff m)
Fig. 1. Si content dependence of the saturation magnetostriction constant for Fe-Si single-layer films.
2.0
0
0
Si Content X (wt%
(a)
10
a) o
.0
0.0
i
i
i
0.2
0.4
0.6
J
0 .8
.0
Fe-Si T h i c k n e s s ( ffm
Fig. 3. (a) Coercivity of as-deposited and annealed Fe-Si/Permalloy films as a function of Fe-Si layer thickness. (b) Grain size of as-deposited and annealed Fe-Si/Permalloy films as a function of Fe-Si layer thickness.
582
H. Ohji et al. / Soft magnetic properties of Fe-Si multilayers
Second, the correlation between the grain size and the coercivity of F e - S i / P e r m a l l o y multilayer films was investigated. The grain size was changed by altering the F e - S i thickness and by annealing. The Permalloy thickness was kept constant at 0.02 txm and the total thickness was about 1.6 ~m. In figs. 3(a) and (b), the coercivity and grain size measured before and after annealing are plotted as functions of the F e - S i thickness, respectively. The films were annealed at 500°C for 1 h in vacuum without applying a magnetic field and cooled gradually to room temperature. Although the annealing does not change the grain size, it increases the coercivity of the samples with F e - S i layers thinner than 0.3 ~ m due to mutual diffusion between the F e - S i and Permalloy layers in [7]. In contrast, for the samples with layers thicker than 0.6 ~m, the coercivity does not change by annealing in spite of the grain growth. These results indicate that the crystalline orientation is the dominant factor determining a low coercivity of the F e - S i / P e r m a l l o y multilayer films. The M~ was 1.6 T for the Fe-Si(0.18 ~m)/Permalloy(0.02 ~m) multilayer film. 5. Conclusions
Soft magnetic properties are determined by factors such as crystalline orientation, grain size, magnetostric-
tion, stress and magnetic interactions. A m o n g these factors, thc (110)/(211) intensity ratio and the grain size of multilayer films were investigated. It became clear that a high (110)/(21 1) intensity ratio is necessary for the low coercivity. In ordcr to realize low coercivity, Permalloy is the best interlayer among the six interlayers that we investigated. The crystalline orientation is the dominant factor in the low cocrcivity of the F c 9.0wt%Si/Permalloy multilayer films. Acknowledgements': The authors would like to thank H. Sugahara for encouragement and K. Inoue for helpful discussions. References
[1] M. Takahashi and T. Shimatsu, IEEE Trans. Magn. 26 (1990) 1485. [2] T. Shimatsu, H. Shoji, M. Kyoho, M. Takahashi and T. Wakiyama, J. Mag. Soc. Jpn. 15 (1991) 63. [3] H. Hoffmann, Phys. Stat. Solidi 33 (1969) 175. [4] N. Kumasaka, N. Saito, Y. Shiroishi, K. Shiiki, H. Fujiwara and M. Kudo, J. Appl. Phys. 55 (1991) 2238. [5] T. Kobayashi, N. Nakatani, S. Otomo and N. Kumasaka, IEEE Trans. Magn. 23 (1987) 2746. [6] L.P. Tarasov, Phys. Rev. 56 (1939) 1231. [7] H. Ohji, S. Tanabe, M. Kataoka and T. Ozeki, IEEE Trans. Magn. 28 (1992) 2937. [8] H. Ohji, S. Tanabe, M. Kataoka and T. Ozeki, J. Mag. Soc. Jpn. 15 (1991) 119.
Effects of interlayers on the soft magnetic properties of F e - S i multilayers H. Ohji, M. Kataoka, Y. Uehara, S. Tanabe and T. Ozeki Materials & Electronic Det:ices Laboratory, Mitsubishi Electric Corporation, Hw~go 661, Japan
In order to realize low coercivity in Fe-9.0wt%Si-based multilayer films, the correlation between coercivity and the crystalline structure has been investigated. Crystalline orientation is an important factor for the low coereivity of Fe Si-based multilayer films. Permal[oy is found to be the most suitable interlayer for achieving low coercivity. 1. Introduction High-saturation magnetization (M~) films are required for magnetic recording heads to generate enough magnetic field to write on high-coercivity media. Fe-based alloy is considered as a high M~ head material. To obtain good soft magnetic properties, both a low saturation magnctostriction constant (A~) and low magnetocrystalline anisotropy are nceded [13]. In the case of F e - S i alloy, it is easy to obtain the low )t~ by controlling the Si content, but it is difficult to obtain a small magnetocrystalline anisotropy. Reducing the grain size by separating the Fe-based alloy by interlayers [4,5] is an effective method to reduce the magnetocrystalline anisotropy. In this study, the coercivity, grain size and crystalline structure of the multilayer films were measured. 2. Experimental The multilayer films were prepared on glass substrates by a conventional rf sputtering method. A d c magnetic field of 50 Oe was applied parallel to the substrate surface during sputtering. The sputtering conditions arc shown in table 1. The multilayer films were deposited in the following sequence: first, the interlayer was deposited on the substrate, followed by an Fe-9.0wt%Si layer. This sequence was repeated until the required total thickness was obtained. The coercivity and the saturation magnetostriction constant were measured using an M - H loop tracer and an optical cantilever method, respectively. The Si content was measured by E P M A . The crystalline orientation was investigated by the X-ray diffraction method.
The grain size was estimated from the broadening of the diffraction peak using the Scherrer formula. 3. Effects ofinterlayers The saturation magnetostriction constant (a~) of F e - S i alloy is drastically changed by the Si content. The effect of the Si content was investigated to obtain low a~, and the result is shown in fig. 1. The a~ becomes zero at around 9wt%Si. Thereforc, Fe 9.0wt%Si alloy was used for the following experiments, but the magnetocrystalline anisotropy is still large [6]. To reducc the anisotropy by reducing the grain size of the F e - S i layer, the effects of six kinds of interlayers (Permalloy, Sendust, amorphous C o - Z r - N b , Cu, Zr and Cr films) were examined. The total film thickness was 1.6 l~m and the thickness ratio of the F e - S i layer to thc interlayer was (/.18:0.02 ~m. The interlayer dependence of the (110)/(211) ratio of the X-ray diffraction intensity, the F e - S i grain size and the eoercivity of the multilayer film are shown in table 2. Among the six samples, the coercivities of those with ferromagnetic interlayers arc relatively low, which seems to indicate that the magnetic interactions arc very important lk~r low coercivity. We do not havc a clear explanation of the relationship, but in general, the soft magnetic properties improve with decreasing grain size [1]. However, the coercivity is determined not only by grain size, as shown in table 2. For example, the F e - S i grain size of a sample with a C o Z r N b interlayer is smaller than that with a Permalloy interlayer, but the cocrcivity of thc former sample is higher Table 1 Target ct)mposition and sputtering conditions Target composition
Correspondence to: H. Ohji, Materials & Electronic Devices
Laboratory, Mitsubishi Electric Corporation, 8-1-1 Tsukaguchi-Honmachi, Amagasaki, Hyogo 661, Japan. Tel: + 81-6497-7506; fax: + 81-6-497-7600.
Sputtering pressure Power density Substrate-target distance
0304-8853/93/$06.00 ~ 1993 - Elsevier Science Publishers B.V. (North-Holland)
Fe-9.99wlC;- Si Ni 19.8wt'3;Fe 10 mTorr Ar 5.5 W/cm ~ 4 cm
H. Ohji et al. / Soft magnetic properties of Fe-Si multilayers
g-. 'o 4
581
._o 300 Intensity
rr
4"~ O
¢,-
~
Coercivity
2oo
v
O
o
>
c
g 1
•
0
o
2 o "- 100 t'M
\
O "--
O)
-1
0
,
i
i
5
10
1
,r n ~ D 0.0
Table 2 Intensity ratio, grain size and coercivity of Fe-Si multilayer films Interlayer
Intensity ratio (110)/(211)
Grain site
Coercivity (Oe)
Permalloy Sendust CoZrNb Cu Zr Cr
131 126 29 30 20 202
29 28 19 26 20 35
0.4 4.4 3.7 6.6 10.8 3.2
than that of the latter. Permalloy, Sendust and Cr interlayers induce high (110)/(211) ratios of F e - S i layers among the six samples. F e - S i / P e r m a l l o y multilayer film has the lowest coercivity.
4. Fe-9.0wt%Si / Permalloy films First, the correlation between the (110)/(211) intensity ratio and the coercivity was investigated systemati-
~annealed
at 500°C
e-.,.-o
0
e.~
[]~
0.0 0.0
m
C
D~
0.5
(5
Oa ' ~ s . d e p o s i t e d
. 0.2
.
. 0.4
as-deposited
2o
Fe-Si T h i c k n e s s (/1 m )
[]
10 0
0.8
---D
Permalloy Thickness 0.02 ff m
. 0.6
cally for the F e - S i / P e r m a l l o y multilayer films. The F e - S i thickness was altered as an experimental parameter to change the intensity ratio. The orientation of Fe-Si(110) texture is enhanced when deposited on the P e r m a l l o y ( l l l ) texture, because the textures of both lattice constants are similar [7]. Therefore, the intensity ratio depends on the F e - S i thickness.The Permalloy thickness was fixed at 0.05 txm and the total thickness of the multilayer film was 3 ixm. The (110)/(211) intensity ratio increased with decreasing F e - S i thickness, as shown in fig. 2. The orientation of F e - S i ( l l 0 ) texture is dominant for high (110)/(211) ratio films and the rest of the peaks are very weak. The coercivity increases with the increasing F e - S i thickness, that is, with the decrease in the (110)/(211) ratio. We have already reported that the permeability of the multilayer film is independent of the Permalloy interlayer thickness [8]. As a result, it can be said that the F e - S i ( l l 0 ) texture is important in obtaining a low coercivity for the F e - S i / P e r m a l l o y multilayer film.
annealed at 500°C - - / u
1.5
1.0
1.2
Fig. 2. (110)/(211) X-ray diffraction intensity ratio and coercivity for Fe-9.0wt%Si/Permal]oy films as a function of Fe0.9wt%Si layer thickness.
--- 50 E c v 40 N 30
v
"~
0 .8
(b) 60
Permalloy Thickness 0.02/~ m a
O
0.4
ge-Si T h i c k n e s s (ff m)
Fig. 1. Si content dependence of the saturation magnetostriction constant for Fe-Si single-layer films.
2.0
0
0
Si Content X (wt%
(a)
10
a) o
.0
0.0
i
i
i
0.2
0.4
0.6
J
0 .8
.0
Fe-Si T h i c k n e s s ( ffm
Fig. 3. (a) Coercivity of as-deposited and annealed Fe-Si/Permalloy films as a function of Fe-Si layer thickness. (b) Grain size of as-deposited and annealed Fe-Si/Permalloy films as a function of Fe-Si layer thickness.
582
H. Ohji et al. / Soft magnetic properties of Fe-Si multilayers
Second, the correlation between the grain size and the coercivity of F e - S i / P e r m a l l o y multilayer films was investigated. The grain size was changed by altering the F e - S i thickness and by annealing. The Permalloy thickness was kept constant at 0.02 txm and the total thickness was about 1.6 ~m. In figs. 3(a) and (b), the coercivity and grain size measured before and after annealing are plotted as functions of the F e - S i thickness, respectively. The films were annealed at 500°C for 1 h in vacuum without applying a magnetic field and cooled gradually to room temperature. Although the annealing does not change the grain size, it increases the coercivity of the samples with F e - S i layers thinner than 0.3 ~ m due to mutual diffusion between the F e - S i and Permalloy layers in [7]. In contrast, for the samples with layers thicker than 0.6 ~m, the coercivity does not change by annealing in spite of the grain growth. These results indicate that the crystalline orientation is the dominant factor determining a low coercivity of the F e - S i / P e r m a l l o y multilayer films. The M~ was 1.6 T for the Fe-Si(0.18 ~m)/Permalloy(0.02 ~m) multilayer film. 5. Conclusions
Soft magnetic properties are determined by factors such as crystalline orientation, grain size, magnetostric-
tion, stress and magnetic interactions. A m o n g these factors, thc (110)/(211) intensity ratio and the grain size of multilayer films were investigated. It became clear that a high (110)/(21 1) intensity ratio is necessary for the low coercivity. In ordcr to realize low coercivity, Permalloy is the best interlayer among the six interlayers that we investigated. The crystalline orientation is the dominant factor in the low cocrcivity of the F c 9.0wt%Si/Permalloy multilayer films. Acknowledgements': The authors would like to thank H. Sugahara for encouragement and K. Inoue for helpful discussions. References
[1] M. Takahashi and T. Shimatsu, IEEE Trans. Magn. 26 (1990) 1485. [2] T. Shimatsu, H. Shoji, M. Kyoho, M. Takahashi and T. Wakiyama, J. Mag. Soc. Jpn. 15 (1991) 63. [3] H. Hoffmann, Phys. Stat. Solidi 33 (1969) 175. [4] N. Kumasaka, N. Saito, Y. Shiroishi, K. Shiiki, H. Fujiwara and M. Kudo, J. Appl. Phys. 55 (1991) 2238. [5] T. Kobayashi, N. Nakatani, S. Otomo and N. Kumasaka, IEEE Trans. Magn. 23 (1987) 2746. [6] L.P. Tarasov, Phys. Rev. 56 (1939) 1231. [7] H. Ohji, S. Tanabe, M. Kataoka and T. Ozeki, IEEE Trans. Magn. 28 (1992) 2937. [8] H. Ohji, S. Tanabe, M. Kataoka and T. Ozeki, J. Mag. Soc. Jpn. 15 (1991) 119.