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Size dependence of magnetic properties of permalloy microstripe arrays
Journal of Magnetism and Magnetic Materials 239 (2002) 252–254
Size dependence of magnetic properties of permalloy microstripe arrays E.E. Shalyguinaa,*, Kyung-Ho Shinb, N.M. Abrosimovaa b
a Physical Faculty, Moscow State University, 119899 Moscow, Russia Korea Institute of Science & Technology, P.O. Box 131, Cheongryang, Seoul, South Korea
Abstract Local magnetic properties of permalloy microstripes with variable aspect ratio from 2 to 15 were investigated by scanning Kerr microscopy. The local magnetic properties of the microstripes were discovered to depend on their sizes, the location of the test parts in the microstripe and also on the design of the examined samples. It was found that the inhomogeneous dipolar fields mainly determine the magnetic field behavior of the examined microstripe arrays. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Kerr microscopy; Magnetic properties; Permalloy microstripe; Micromagnetism
Local magnetic properties and magnetization reversal mechanisms of ferromagnetic microstructures have attracted considerable interest of researchers for the last few years. The control of magnetic behavior of these structures is becoming especially important as novel magnetic devices are being miniaturized. Various experimental methods are used for realizing this task (see, for example, [1–9]). In this paper, we report the results on the magnetooptical investigation of local magnetic properties of permalloy microstripes with the different aspect ratio. The influence of microstripe sizes on their local magnetic characteristics and the question as to when the microstripes begin to interact are examined. The microstripe arrays were fabricated from continuous Ni80Fe20 films using high-resolution electron-beam lithography. The total number of the microstripes in each array was about 1 106. Continuous Ni80Fe20 films were deposited by a DC magnetron sputtering system under a base pressure of o108 Torr and an argon gas pressure of 1 104 Torr. The as-prepared continuous Ni80Fe20 films exhibited an in-plane uniaxial anisotropy. In the case of the easy axis reversal of the Ni80Fe20 films *Corresponding author. Tel.: +7-95-939-4043; fax: +7-95932-8820. E-mail addresses: [email protected] (E.E. Shalyguina), [email protected] (K.-H. Shin).
with a thickness of 30 nm, the saturation field, HS ; and the coercivity, HC ; were equal to 8.2 and 1.2 Oe, respectively. The microstripe length, l, was parallel to the easy axis of the continuous films. The stripe width w was equal to 2 mm. The aspect ratio l=w varied from 2 to 15. The space between the microstripes in the row ðS1 Þ was varied from 0.25 to 4 mm. The space between the rows was S2 ¼ 2 mm. The micrographs of the examined samples showed that a good edge structure of microstripes had been achieved. Scanning Kerr microscopy was used to investigate the local magnetic properties of the examined samples. The experimental setup has a polarised microscope with 1200 magnification and a linear resolution up to 0.2 mm. The studied near-surface part of the sample is determined by the size of the slot that is located in the image plane of the microscope before the light detector. The low-frequency magnetic field, H; was applied along the microstripe length. By scanning the 0.5 2 mm2 slot along l; the distributions of in-plane magnetization components ðMt Þ; parallel to H; and magnetization components, perpendicular to the microstripe surface ðMn Þ; were measured by means of the transverse and polar Kerr effects, respectively. The local hysteresis loop was measured by means of the transverse Kerr effect. Actually, we have found the dependencies of dðL; HÞ=dS pMðL; HÞ=MS : Here d ¼ ðI2I0 Þ=I0 (I and I0 are the intensities of the reflected light from the magnetized and nonmagnetized sample,
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 5 4 7 - 9
E.E. Shalyguina et al. / Journal of Magnetism and Magnetic Materials 239 (2002) 252–254
S
Mn -components exist only near the microstripe edges. This fact can be explained by the presence of stray fields in these areas. Moreover, from Fig. 2, one can see that the local magnetization components near the microstripe edges increase with decrease in S1 ; whereas those of the central parts change insignificantly. Fig. 3 displays the dependence of the in-plane local magnetization component near the microstripe edge ðMtedge Þ as a function of S1 -magnitudes. Mtedge is the local initial (or end) magnetization magnitude that is averaged over the 0.25 2 mm2 microstripe area. From Fig. 3, one can see that at S1 o1.25 mm, Mtedge magnitudes increase with decrease in S1 ; and at large S1 -spaces, the magnetization asymptotically approaches the Mtedge magnitude at S1 ¼ N: These data can be explained as follows. According to a micromagnetic calculation [10,11], a local demagnetizing factor of a test point in an array of closely packed magnetic microstripes depends on both the location of this point in the microstripe and the microstripe
1.0
2
Reduced magnetization Mτ /M
Reduced magnetisation Mτ /M S
respectively), dS is the magnitude of d at M ¼ MS and MS is the saturation magnetization. All measurements were performed in the central part of a microstripe array. We found that the local magnitudes of HS of the examined microstripes were larger than HS of the continuous films. The local magnetic characteristics of the microstripes were discovered to depend both on their sizes and the location of the measured micropart in the microstripe. In particular, the local magnitudes of HS increase with decreasing l and with increasing distance from the microstripe center (see Fig. 1). From Fig. 1, one can see that the magnetization linearly changes with increase in the magnetic field. This fact indicates that the microstripe magnetization reversal is dominated by rotation of local magnetization vectors. Fig. 2 shows the magnetization distributions, obtained for the arrays with identical microstripe sizes but with various S1 -spaces. We have revealed that the
253
1
0.5 0.0
HS1 HS2
-0.5 2 1
-1.0 -40
-20
0
20
40
Applied field H (Oe)
(a)
1.0 0.5 0.0
HS1 HS2
-0.5
2 1
-1.0 -30 -20 -10
0
10
20
30
Applied field H (Oe)
(b)
(a)
1.0 0.9 0.8 0.7
1
0.6
S1
S1
l
0.5
W S2
0.4 0
1
2
3
4
5
Microstripe length l (µm)
2 3 6
Reduced magnetisation Mn/MS
Reduced magnetization M τ /MS
Fig. 1. Local hysteresis loops obtained for the central (loops 1) and edge (loops 2) parts of the 2 4 0.015 mm3 and 2 6 0.015 mm3 microstripes (the left panel and right panel, respectively). The measurements were carried out by means of the transverse Kerr effect. The low-frequency magnetic field was applied along the microstripe length. S1 ¼ 0:5 mm for both the microstripe arrays. The left panel inset displays microparts, where the loops were measured.
(b)
0.08 0.06 0.04 0.02 0.00 -0,02
the curve 3 the curve 2 the curve 1
-0.04 -0.06 -0.08 0
1
2
3
4
5
6
Microstripe length l (mm)
Fig. 2. Distributions of the in-plane (a) and perpendicular to the microstripe surface (b) magnetization components along the microstripe length l; obtained by means of transverse and polar Kerr effects, respectively. The curves 1, 2 and 3 were measured for the arrays with the 2 6 0.015 mm3 microstripes, but with S1 ¼ 0:25; 0.5 and 1 mm, respectively. The low-frequency magnetic field H ¼ 13 Oe was applied along l: The inset shows the sketch of a microstripe array.
254
Reduced magnetization Mτ
enge
/MS
E.E. Shalyguina et al. / Journal of Magnetism and Magnetic Materials 239 (2002) 252–254
location of the test parts in the microstripe. It was discovered that the inhomogeneous dipolar fields mainly determined the magnetic field behavior of the examined microstripe arrays. It was established that the microstripes begin to interact significantly as the S1 -spaces are below 1.25 mm.
0.60 0.55 0.50 0.45 0.40 0.35
S1 =
0.30 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Spacing S1 (µm) Fig. 3. In-plane magnetization component at the 2 6 0.015 mm3 microstripe edge as a function of S1 : The dash line is the end magnetization of an isolated microstripe ðS1 ¼ NÞ:
separating spaces. This is caused by inhomogeneous dipolar fields resulting from the accumulation of magnetic poles on the microstripe edges. This field rapidly decreases ðB1=r2 Þ as one moves from the magnetic poles. As a result, the local demagnetizing factor of the microstripe decreases rapidly as one moves away from its end toward the center, which means that a rather small rapidly decaying field can be effective in modifying a local central magnetization. As a consequence, the local magnetization components of the central parts change insignificantly and those near the microstripe edges are most strongly affected. They increase with decreasing S1 due to an enlargement of magnetostatic interaction between microstripes. In conclusion, local magnetic characteristics were found to depend on the size of microstripes and the
This investigation has been partly sponsored by Korean Institute of Science & Technology Policy and Technology Evaluation and Planning and also the Russian Fund of Fundamental Investigation (Grant No. 99-02-16595).
References [1] J.F. Smyth, S. Schultz, D.R. Fredkin, D.P. Kern, S.A. Rishton, H. Schmid, M. Cali, T.R. Koehler, J. Appl. Phys. 69 (1991) 5262. [2] A. Maeda, M. Kume, T. Ogura, K. Kuroki, T. Yamada, M. Nishikawa, Y. Harada, J. Appl. Phys. 76 (1994) 6667. [3] K.J. Kirk, J.N. Chapman, C.D. Wilkinson, J. Appl. Phys. 71 (1997) 539. [4] A. Ercole, A.O. Adeyeye, C. Daboo, J.A.C. Bland, D.G. Hasko, J. Appl. Phys. 81 (1997) 5452. [5] A. Stankiewicz, W.K. Hiebert, G.E. Ballentine, K.W. Marsh, M.R. Freeman, IEEE Trans. Magn. 34 (1998) 1003. [6] J. Shi, T. Zhu, M. Durlam, E. Chen, S. Tehrani, Y.F. Zheng, J.G. Zhu, IEEE Trans. Magn. 34 (1998) 997. [7] R.D. Gomez, T.V. Luu, A.O. Pak, K.J. Kirk, J.N. Chapman, J. Appl. Phys. 85 (1999) 6163. [8] F. Marty, A. Vaterlaus, V. Weich, C. Stamm, U. Maier, D. Pescia, J. Appl. Phys. 85 (1999) 6166. [9] T. Schrefl, J. Magn. Magn. Mater. 207 (1999) 66. [10] P.K. George, A.J. Hughes, IEEE Trans. Magn. 12 (1976) 137. [11] M. Pardavi-Horvath, IEEE Trans. Magn. 32 (Part II) (1996) 4458.
Size dependence of magnetic properties of permalloy microstripe arrays E.E. Shalyguinaa,*, Kyung-Ho Shinb, N.M. Abrosimovaa b
a Physical Faculty, Moscow State University, 119899 Moscow, Russia Korea Institute of Science & Technology, P.O. Box 131, Cheongryang, Seoul, South Korea
Abstract Local magnetic properties of permalloy microstripes with variable aspect ratio from 2 to 15 were investigated by scanning Kerr microscopy. The local magnetic properties of the microstripes were discovered to depend on their sizes, the location of the test parts in the microstripe and also on the design of the examined samples. It was found that the inhomogeneous dipolar fields mainly determine the magnetic field behavior of the examined microstripe arrays. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Kerr microscopy; Magnetic properties; Permalloy microstripe; Micromagnetism
Local magnetic properties and magnetization reversal mechanisms of ferromagnetic microstructures have attracted considerable interest of researchers for the last few years. The control of magnetic behavior of these structures is becoming especially important as novel magnetic devices are being miniaturized. Various experimental methods are used for realizing this task (see, for example, [1–9]). In this paper, we report the results on the magnetooptical investigation of local magnetic properties of permalloy microstripes with the different aspect ratio. The influence of microstripe sizes on their local magnetic characteristics and the question as to when the microstripes begin to interact are examined. The microstripe arrays were fabricated from continuous Ni80Fe20 films using high-resolution electron-beam lithography. The total number of the microstripes in each array was about 1 106. Continuous Ni80Fe20 films were deposited by a DC magnetron sputtering system under a base pressure of o108 Torr and an argon gas pressure of 1 104 Torr. The as-prepared continuous Ni80Fe20 films exhibited an in-plane uniaxial anisotropy. In the case of the easy axis reversal of the Ni80Fe20 films *Corresponding author. Tel.: +7-95-939-4043; fax: +7-95932-8820. E-mail addresses: [email protected] (E.E. Shalyguina), [email protected] (K.-H. Shin).
with a thickness of 30 nm, the saturation field, HS ; and the coercivity, HC ; were equal to 8.2 and 1.2 Oe, respectively. The microstripe length, l, was parallel to the easy axis of the continuous films. The stripe width w was equal to 2 mm. The aspect ratio l=w varied from 2 to 15. The space between the microstripes in the row ðS1 Þ was varied from 0.25 to 4 mm. The space between the rows was S2 ¼ 2 mm. The micrographs of the examined samples showed that a good edge structure of microstripes had been achieved. Scanning Kerr microscopy was used to investigate the local magnetic properties of the examined samples. The experimental setup has a polarised microscope with 1200 magnification and a linear resolution up to 0.2 mm. The studied near-surface part of the sample is determined by the size of the slot that is located in the image plane of the microscope before the light detector. The low-frequency magnetic field, H; was applied along the microstripe length. By scanning the 0.5 2 mm2 slot along l; the distributions of in-plane magnetization components ðMt Þ; parallel to H; and magnetization components, perpendicular to the microstripe surface ðMn Þ; were measured by means of the transverse and polar Kerr effects, respectively. The local hysteresis loop was measured by means of the transverse Kerr effect. Actually, we have found the dependencies of dðL; HÞ=dS pMðL; HÞ=MS : Here d ¼ ðI2I0 Þ=I0 (I and I0 are the intensities of the reflected light from the magnetized and nonmagnetized sample,
0304-8853/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 0 4 - 8 8 5 3 ( 0 1 ) 0 0 5 4 7 - 9
E.E. Shalyguina et al. / Journal of Magnetism and Magnetic Materials 239 (2002) 252–254
S
Mn -components exist only near the microstripe edges. This fact can be explained by the presence of stray fields in these areas. Moreover, from Fig. 2, one can see that the local magnetization components near the microstripe edges increase with decrease in S1 ; whereas those of the central parts change insignificantly. Fig. 3 displays the dependence of the in-plane local magnetization component near the microstripe edge ðMtedge Þ as a function of S1 -magnitudes. Mtedge is the local initial (or end) magnetization magnitude that is averaged over the 0.25 2 mm2 microstripe area. From Fig. 3, one can see that at S1 o1.25 mm, Mtedge magnitudes increase with decrease in S1 ; and at large S1 -spaces, the magnetization asymptotically approaches the Mtedge magnitude at S1 ¼ N: These data can be explained as follows. According to a micromagnetic calculation [10,11], a local demagnetizing factor of a test point in an array of closely packed magnetic microstripes depends on both the location of this point in the microstripe and the microstripe
1.0
2
Reduced magnetization Mτ /M
Reduced magnetisation Mτ /M S
respectively), dS is the magnitude of d at M ¼ MS and MS is the saturation magnetization. All measurements were performed in the central part of a microstripe array. We found that the local magnitudes of HS of the examined microstripes were larger than HS of the continuous films. The local magnetic characteristics of the microstripes were discovered to depend both on their sizes and the location of the measured micropart in the microstripe. In particular, the local magnitudes of HS increase with decreasing l and with increasing distance from the microstripe center (see Fig. 1). From Fig. 1, one can see that the magnetization linearly changes with increase in the magnetic field. This fact indicates that the microstripe magnetization reversal is dominated by rotation of local magnetization vectors. Fig. 2 shows the magnetization distributions, obtained for the arrays with identical microstripe sizes but with various S1 -spaces. We have revealed that the
253
1
0.5 0.0
HS1 HS2
-0.5 2 1
-1.0 -40
-20
0
20
40
Applied field H (Oe)
(a)
1.0 0.5 0.0
HS1 HS2
-0.5
2 1
-1.0 -30 -20 -10
0
10
20
30
Applied field H (Oe)
(b)
(a)
1.0 0.9 0.8 0.7
1
0.6
S1
S1
l
0.5
W S2
0.4 0
1
2
3
4
5
Microstripe length l (µm)
2 3 6
Reduced magnetisation Mn/MS
Reduced magnetization M τ /MS
Fig. 1. Local hysteresis loops obtained for the central (loops 1) and edge (loops 2) parts of the 2 4 0.015 mm3 and 2 6 0.015 mm3 microstripes (the left panel and right panel, respectively). The measurements were carried out by means of the transverse Kerr effect. The low-frequency magnetic field was applied along the microstripe length. S1 ¼ 0:5 mm for both the microstripe arrays. The left panel inset displays microparts, where the loops were measured.
(b)
0.08 0.06 0.04 0.02 0.00 -0,02
the curve 3 the curve 2 the curve 1
-0.04 -0.06 -0.08 0
1
2
3
4
5
6
Microstripe length l (mm)
Fig. 2. Distributions of the in-plane (a) and perpendicular to the microstripe surface (b) magnetization components along the microstripe length l; obtained by means of transverse and polar Kerr effects, respectively. The curves 1, 2 and 3 were measured for the arrays with the 2 6 0.015 mm3 microstripes, but with S1 ¼ 0:25; 0.5 and 1 mm, respectively. The low-frequency magnetic field H ¼ 13 Oe was applied along l: The inset shows the sketch of a microstripe array.
254
Reduced magnetization Mτ
enge
/MS
E.E. Shalyguina et al. / Journal of Magnetism and Magnetic Materials 239 (2002) 252–254
location of the test parts in the microstripe. It was discovered that the inhomogeneous dipolar fields mainly determined the magnetic field behavior of the examined microstripe arrays. It was established that the microstripes begin to interact significantly as the S1 -spaces are below 1.25 mm.
0.60 0.55 0.50 0.45 0.40 0.35
S1 =
0.30 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Spacing S1 (µm) Fig. 3. In-plane magnetization component at the 2 6 0.015 mm3 microstripe edge as a function of S1 : The dash line is the end magnetization of an isolated microstripe ðS1 ¼ NÞ:
separating spaces. This is caused by inhomogeneous dipolar fields resulting from the accumulation of magnetic poles on the microstripe edges. This field rapidly decreases ðB1=r2 Þ as one moves from the magnetic poles. As a result, the local demagnetizing factor of the microstripe decreases rapidly as one moves away from its end toward the center, which means that a rather small rapidly decaying field can be effective in modifying a local central magnetization. As a consequence, the local magnetization components of the central parts change insignificantly and those near the microstripe edges are most strongly affected. They increase with decreasing S1 due to an enlargement of magnetostatic interaction between microstripes. In conclusion, local magnetic characteristics were found to depend on the size of microstripes and the
This investigation has been partly sponsored by Korean Institute of Science & Technology Policy and Technology Evaluation and Planning and also the Russian Fund of Fundamental Investigation (Grant No. 99-02-16595).
References [1] J.F. Smyth, S. Schultz, D.R. Fredkin, D.P. Kern, S.A. Rishton, H. Schmid, M. Cali, T.R. Koehler, J. Appl. Phys. 69 (1991) 5262. [2] A. Maeda, M. Kume, T. Ogura, K. Kuroki, T. Yamada, M. Nishikawa, Y. Harada, J. Appl. Phys. 76 (1994) 6667. [3] K.J. Kirk, J.N. Chapman, C.D. Wilkinson, J. Appl. Phys. 71 (1997) 539. [4] A. Ercole, A.O. Adeyeye, C. Daboo, J.A.C. Bland, D.G. Hasko, J. Appl. Phys. 81 (1997) 5452. [5] A. Stankiewicz, W.K. Hiebert, G.E. Ballentine, K.W. Marsh, M.R. Freeman, IEEE Trans. Magn. 34 (1998) 1003. [6] J. Shi, T. Zhu, M. Durlam, E. Chen, S. Tehrani, Y.F. Zheng, J.G. Zhu, IEEE Trans. Magn. 34 (1998) 997. [7] R.D. Gomez, T.V. Luu, A.O. Pak, K.J. Kirk, J.N. Chapman, J. Appl. Phys. 85 (1999) 6163. [8] F. Marty, A. Vaterlaus, V. Weich, C. Stamm, U. Maier, D. Pescia, J. Appl. Phys. 85 (1999) 6166. [9] T. Schrefl, J. Magn. Magn. Mater. 207 (1999) 66. [10] P.K. George, A.J. Hughes, IEEE Trans. Magn. 12 (1976) 137. [11] M. Pardavi-Horvath, IEEE Trans. Magn. 32 (Part II) (1996) 4458.