- Email: [email protected]
Permeability of micro-strip patterned magnetic thin films
Journal of Magnetism and Magnetic Materials 148 (1995) 88-89
~14 Journal of magnetism and magnetic matellais
ELSEVIER
Permeability of micro-strip patterned magnetic thin films Shingo Takahashi *, Kiyoshi Yamakawa, Naoki Honda, Kazuhiro Ouchi Akita Research Institute of Advanced Technology, 4-21 Sanuki, Araya, Akita O10-16, Japan
Abstract A high permeable magnetic thin strip film is strongly required for a main pole o f an ultra high density perpendicular recording head. Permeability for single- and double-layered NiFe strips o f thickness less than 60 nm as a function o f strip width was investigated. The permeability reduced monotonically with decreasing the strip width for single-layered strips and, however, exhibited a large peak value at a certain strip width for double-layered strips with a Ti interlayer. Ely adjusting the Ti interlayer thickness, a permeability over 1500 could be obtained even for narrow strips o f 2 i~m width.
1. Introduction
The driving field was 5 mOe in amplitude. The domain structure was observed by the Bitter technique.
In order to realize a recording areal density o f 10 Gbits/inch 2, the size o f a main pole film for a perpendicular recording head should he as small as 1 Ixm width and 60 nm thick. Although multi-layered strip films show a high permeability due to exclusion o f a closure domain structure [1], a high permeability over 1000 has not yet been obtained for a strip film as narrow as 1 lxm. Furthermore, there were few reports on permeability for NiFe films thinner than 60 rim. NiFe films o f ultra-thin and narrow strip shapes were studied in terms of their permeability and domain structures.
2. Experiments NiFe films were prepared by the same procedure as has been reported [2]. Single-layered NiFe films o f thickness range from 20 to 60 nm were deposited on 50 nm thick Ti underlayers prepared on glass sheets. Double-layered NiFe films as thick as 20 nm each × 2 were also deposited on the Ti underlayers with various Ti interlayer thicknesses. Strip arrays were formed by photolithography using ionmilling. The strip width W was varied from 1 to 200 ixm. The strip interval was formed to be equal to each value o f IV, fixing the strip length at 8 mm. The hard-axis of magnetization in an original sheet film corresponded to the strip length direction. The permeability /~' in the strip length direction was measured by an 8-shaped coil me,.hod.
3. Results and discussions For single-layered films, /~' monotonically decreased with decreasing W, but the thinner the film, the larger/~' became, as shown in Fig. 1. A regular closure domain structure (Fig. 3(a)) having a cross-tie type 180 ~ wall was observed for strip films thicker than 40 nm. For a 20 nm thick strip film, the observed 180 ° walls seemed to be o f N~el type. The domain structure became irregular with decreasing W, but still retained a closure domain structure. The change became irregular with decreasing IV, but still retained a closure domain structure. The change o f / z ' was calculated by a model where magnetic flux is assumed to be conducted only by magnetization rotation within the central part other than the area o f the closure domain [3].
10000 2~nm q"'-m
1000
J W
~:~. D = 60 n m I '-"
100
f = 10 MI-Iz~, 40 nm
10 10
100
1000
10000
Strip Width W ( g / m ) * Corresponding author. Fax: [email protected].
+81-188-66-5803; c-mail:
Fig. 1. Dependence of p0rmeability on strip width for various thickness of NiFo single layered film. Solid lines indicate calculated permeability.
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. A~i rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 1 6 1 - 1
S. Takahashi etal./Journal o f Magnetism and Magnetic Materials 148 (1995) 88-89
disappearancej of walls 2500 t~[ 2000
---
I
1500 N 1000 ,J' 500
2500 2000 ~ l'3nm ~1500
..~_~
. . ,,. 4nmll
"~
1
/
I
f = 10MHz
.,~l
I
I
10
I00
I~)99
Strip
llrll I IIll'
I
1
10000
Width W q u m )
Fig. 2. Dependence of permeability on strip width for 20 n m × 2 thick double-layered film with 5 nm thick Ti interlayer (solid line) and 40 nm thick single-layered film (dashed line). A l t h o u g h the calculated /~' does not quantitatively coincide with that for the experiment, a qualitative agreement was obtained. It s e e m s hard for a single-layered film to obtain a high permeable strip film with a W less than 10 ~,m e v e n for a 10 nm thickness. O n the other hand, / x ' for a double-layered strip film, for instance, with a 5 nm thick Ti interlayer, first decreased with decreasing W from 8 m m down to around 100 ~ m as like the single-layered film, however, for further decrease of W, ~ ' increased, showing a peak, then decreased again, as shown in Fig. 2. M a n y walls nucleated from the strip edges were observed for strips wider than 50 Izm, as shown in Fig. 3(b). The edge wall length became almost equal to the closure d o m a i n width for 40 nm thick single-layered film strips. It is suggested that the region o f
Ca)
Cb)
(e)
89
tlilll
!1 I1|
10 100 1000 Strip Width W (/l m)
Fig. 4. Dependence of permeability on strip width as a function of Ti interlayer thickness for double-layered film. Arrows indicate each critical width for wall disappearance. the edge wall dose not contribute to /.d for a W o v e r 100 l~m, similarly to the closure d o m a i n o f the single-layered strips. However, ~' for a 50 Ixm strip film w a s higher than that for a 100 p~m one in spite o f the presence o f the edge walls. The edge walls could not be o b s e r v e d for strips narrower than 20 Ixm as shown in Fig. 3(c), where g ' considerably increased. The m a x i m u m /.d w a s 2250 at a W o f 10 ixm. The flux seems to flow through the entire strip width o f 10 ~ m , since /~' is equal to that o f the original sheet film o f 8 m m width. W h e n the interlayer thickness was decreased, the W for the m a x i m u m /~' b e c a m e small, and the critical width W~ at which walls disappeared also b e c a m e small as indicated by the arrows in Fig. 4. No walls could b e observed b y the Bitter technique for a W b e l o w We, implying a single domain state. E v e n for such a single domain case, an edge curling wall might occur [4]. Hence, it is supposed that the decrease o f ~ ' from the peak by narrowing the strips is caused b y an increase in the ratio o f the edge curling wall to W.
4. Conclusions A single domain like state in strips consisted o f ultrathin N i F e films exhibits a high permeability even for a narrow strip width around 2 to 10 Itm. Permeability over 1500 could be obtained for the strips o f 2 p,m width and 40 nm thickness. The strip dimension corresponds to that o f the head main pole for achieving a density o f 5 G b i t s / i n e h 2.
References Fig. 3. Domain structures (Bitter patterns) of strip patterned films, (a) domain structure of 40 nm thick film with W ~- 50 Ixm, (b) and (c) 40 nm thick double-layered film with W = 50 and 20 Itm, respectively.
[1] [2] [3] [4]
B.C. Webb et al., J. Appl. Phys. 69 (1991) 5611. S, Takahashi et al., J. Magn. Soe. Jpn. 18 Suppl. (1994) 385. B.C. Webb et al., IEEE Trans. Magn. 28 (1992) 2955. J.C. Slonezewski et al., [EEE Trans. Magn. 24 (1988) 2045.
~14 Journal of magnetism and magnetic matellais
ELSEVIER
Permeability of micro-strip patterned magnetic thin films Shingo Takahashi *, Kiyoshi Yamakawa, Naoki Honda, Kazuhiro Ouchi Akita Research Institute of Advanced Technology, 4-21 Sanuki, Araya, Akita O10-16, Japan
Abstract A high permeable magnetic thin strip film is strongly required for a main pole o f an ultra high density perpendicular recording head. Permeability for single- and double-layered NiFe strips o f thickness less than 60 nm as a function o f strip width was investigated. The permeability reduced monotonically with decreasing the strip width for single-layered strips and, however, exhibited a large peak value at a certain strip width for double-layered strips with a Ti interlayer. Ely adjusting the Ti interlayer thickness, a permeability over 1500 could be obtained even for narrow strips o f 2 i~m width.
1. Introduction
The driving field was 5 mOe in amplitude. The domain structure was observed by the Bitter technique.
In order to realize a recording areal density o f 10 Gbits/inch 2, the size o f a main pole film for a perpendicular recording head should he as small as 1 Ixm width and 60 nm thick. Although multi-layered strip films show a high permeability due to exclusion o f a closure domain structure [1], a high permeability over 1000 has not yet been obtained for a strip film as narrow as 1 lxm. Furthermore, there were few reports on permeability for NiFe films thinner than 60 rim. NiFe films o f ultra-thin and narrow strip shapes were studied in terms of their permeability and domain structures.
2. Experiments NiFe films were prepared by the same procedure as has been reported [2]. Single-layered NiFe films o f thickness range from 20 to 60 nm were deposited on 50 nm thick Ti underlayers prepared on glass sheets. Double-layered NiFe films as thick as 20 nm each × 2 were also deposited on the Ti underlayers with various Ti interlayer thicknesses. Strip arrays were formed by photolithography using ionmilling. The strip width W was varied from 1 to 200 ixm. The strip interval was formed to be equal to each value o f IV, fixing the strip length at 8 mm. The hard-axis of magnetization in an original sheet film corresponded to the strip length direction. The permeability /~' in the strip length direction was measured by an 8-shaped coil me,.hod.
3. Results and discussions For single-layered films, /~' monotonically decreased with decreasing W, but the thinner the film, the larger/~' became, as shown in Fig. 1. A regular closure domain structure (Fig. 3(a)) having a cross-tie type 180 ~ wall was observed for strip films thicker than 40 nm. For a 20 nm thick strip film, the observed 180 ° walls seemed to be o f N~el type. The domain structure became irregular with decreasing W, but still retained a closure domain structure. The change became irregular with decreasing IV, but still retained a closure domain structure. The change o f / z ' was calculated by a model where magnetic flux is assumed to be conducted only by magnetization rotation within the central part other than the area o f the closure domain [3].
10000 2~nm q"'-m
1000
J W
~:~. D = 60 n m I '-"
100
f = 10 MI-Iz~, 40 nm
10 10
100
1000
10000
Strip Width W ( g / m ) * Corresponding author. Fax: [email protected].
+81-188-66-5803; c-mail:
Fig. 1. Dependence of p0rmeability on strip width for various thickness of NiFo single layered film. Solid lines indicate calculated permeability.
0304-8853/95/$09.50 © 1995 Elsevier Science B.V. A~i rights reserved SSDI 0 3 0 4 - 8 8 5 3 ( 9 5 ) 0 0 1 6 1 - 1
S. Takahashi etal./Journal o f Magnetism and Magnetic Materials 148 (1995) 88-89
disappearancej of walls 2500 t~[ 2000
---
I
1500 N 1000 ,J' 500
2500 2000 ~ l'3nm ~1500
..~_~
. . ,,. 4nmll
"~
1
/
I
f = 10MHz
.,~l
I
I
10
I00
I~)99
Strip
llrll I IIll'
I
1
10000
Width W q u m )
Fig. 2. Dependence of permeability on strip width for 20 n m × 2 thick double-layered film with 5 nm thick Ti interlayer (solid line) and 40 nm thick single-layered film (dashed line). A l t h o u g h the calculated /~' does not quantitatively coincide with that for the experiment, a qualitative agreement was obtained. It s e e m s hard for a single-layered film to obtain a high permeable strip film with a W less than 10 ~,m e v e n for a 10 nm thickness. O n the other hand, / x ' for a double-layered strip film, for instance, with a 5 nm thick Ti interlayer, first decreased with decreasing W from 8 m m down to around 100 ~ m as like the single-layered film, however, for further decrease of W, ~ ' increased, showing a peak, then decreased again, as shown in Fig. 2. M a n y walls nucleated from the strip edges were observed for strips wider than 50 Izm, as shown in Fig. 3(b). The edge wall length became almost equal to the closure d o m a i n width for 40 nm thick single-layered film strips. It is suggested that the region o f
Ca)
Cb)
(e)
89
tlilll
!1 I1|
10 100 1000 Strip Width W (/l m)
Fig. 4. Dependence of permeability on strip width as a function of Ti interlayer thickness for double-layered film. Arrows indicate each critical width for wall disappearance. the edge wall dose not contribute to /.d for a W o v e r 100 l~m, similarly to the closure d o m a i n o f the single-layered strips. However, ~' for a 50 Ixm strip film w a s higher than that for a 100 p~m one in spite o f the presence o f the edge walls. The edge walls could not be o b s e r v e d for strips narrower than 20 Ixm as shown in Fig. 3(c), where g ' considerably increased. The m a x i m u m /.d w a s 2250 at a W o f 10 ixm. The flux seems to flow through the entire strip width o f 10 ~ m , since /~' is equal to that o f the original sheet film o f 8 m m width. W h e n the interlayer thickness was decreased, the W for the m a x i m u m /~' b e c a m e small, and the critical width W~ at which walls disappeared also b e c a m e small as indicated by the arrows in Fig. 4. No walls could b e observed b y the Bitter technique for a W b e l o w We, implying a single domain state. E v e n for such a single domain case, an edge curling wall might occur [4]. Hence, it is supposed that the decrease o f ~ ' from the peak by narrowing the strips is caused b y an increase in the ratio o f the edge curling wall to W.
4. Conclusions A single domain like state in strips consisted o f ultrathin N i F e films exhibits a high permeability even for a narrow strip width around 2 to 10 Itm. Permeability over 1500 could be obtained for the strips o f 2 p,m width and 40 nm thickness. The strip dimension corresponds to that o f the head main pole for achieving a density o f 5 G b i t s / i n e h 2.
References Fig. 3. Domain structures (Bitter patterns) of strip patterned films, (a) domain structure of 40 nm thick film with W ~- 50 Ixm, (b) and (c) 40 nm thick double-layered film with W = 50 and 20 Itm, respectively.
[1] [2] [3] [4]
B.C. Webb et al., J. Appl. Phys. 69 (1991) 5611. S, Takahashi et al., J. Magn. Soe. Jpn. 18 Suppl. (1994) 385. B.C. Webb et al., IEEE Trans. Magn. 28 (1992) 2955. J.C. Slonezewski et al., [EEE Trans. Magn. 24 (1988) 2045.