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Magnetic properties of nanometer-scale FeNi antidot array system
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) e792–e793 www.elsevier.com/locate/jmmm
Magnetic properties of nanometer-scale FeNi antidot array system M. Tanakaa,, K. Itoha, H. Iwamotoa, A. Yamaguchia, H. Miyajimaa, T. Yamaokab a
Department of Physics, Keio University, 3-14-1 Hiyoshi, Yokohama, Kanagawa 223-8522, Japan Application Engineering Section, SII NanoTechnology Inc., 2-15-5 Shintomi, Chuo-ku, Tokyo 104-0041, Japan
b
Available online 27 November 2006
Abstract The magnetic anisotropy and the magnetization reversal process of a nanometer-scale FeNi antidot array system of triangle lattice were investigated at room temperature by means of torque and resonating-sample magnetometries. The magnetic anisotropy of the system has a sixfold rotational symmetry refracting the structural symmetry of the array of holes. The magnetic anisotropy energy between the hard and easy axes is about 3:2 104 J=m3 . r 2006 Published by Elsevier B.V. PACS: 75.30.Gw; 75.50.Bb; 75.60.Jk Keywords: Nanostructure; Resonating sample magnetometer; Magnetic torque magnetometer; Magnetic anisotropy
Patterned magnetic structures fabricated by lithography, such as small particles with dimension on sub-micron scale, are candidates for a new generation of magnetic high storage media. Alternatively, the antidot array, which consists of a mesh of holes in a continuous magnetic film, has also been discussed as a competitor for high-density storage media with the merit of higher stability than continuous media [1–3]. In these systems, the magnetic structure depends mainly on the shape anisotropy introduced by the holes and the direction of the magnetic field. In this paper, the magnetic anisotropy and the magnetization reversal process of a FeNi antidot array system of triangle lattice were investigated by using torque and resonating-sample magnetometries. The magnetic torque curve shows that the magnetic anisotropy of the antidot system has a sixfold rotational symmetry reflecting the structural symmetry of the array. The magnetization curve indicates that no magnetic domain walls are pinned during the measurement. The sample investigated in this study is a Fe19 Ni81 antidot array system of triangle lattice. This sample was fabricated by a lift-off technique using electron-beam lithography. First, thin resist (ZEP-520A) 100 nm thick Corresponding author. Tel.: +81 45 566 1677.
E-mail address: [email protected] (M. Tanaka). 0304-8853/$ - see front matter r 2006 Published by Elsevier B.V. doi:10.1016/j.jmmm.2006.11.099
was spin-coated onto a thermally oxidized Si substrate. The resist mask was then patterned with an electron beam at 20 keV beam energy. After the development, Fe19 Ni81 was deposited by electron-beam evaporation in a vacuum of 108 Pa at the deposition rate of 0.1 nm/s. The sample was obtained after the resist mask was removed in a solvent. A scanning electron micrograph of the antidot system is shown in Fig. 1. The antidot pitch is kept at 500 nm, the diameter of antidot is 400 nm and the thickness of the film is 20 nm. The system includes about 3:6 107 holes. The inplane magnetic anisotropy of the system was investigated at room temperature by using a torque magnetometer with sensitivity up to 5 1010 N m. The magnetization process of the system was measured at room temperature as a function of y, where y denotes the in-plane direction of the magnetic field as shown in Fig. 1. Fig. 2 shows the magnetic torque curve for m0 H ¼ 0:1 T of the system. Before the measurement, the initial magnetic field of m0 H ¼ 0:1 T was applied to the direction of y ¼ 30 . The dashed and solid lines denote the experiments whose rotative directions are clockwise and counterclockwise, respectively. These lines overlap each other, which means the rotational hysteresis is almost zero. The torque curve LðyÞ, which is equal to the rate of change of energy density E with respect of angle y, i.e. LðyÞ ¼ qE=qy [4], clearly shows a sixfold rotational symmetry as
ARTICLE IN PRESS M. Tanaka et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e792–e793
a
e793
1
H
500 nm θ
b
-1 1
M/Ms
0
0
c
-1 1
θ=0 deg.
θ=15 deg.
Fig. 1. Scanning-electron micrograph of the Fe19 Ni81 antidot array system of a square mesh. The antidot pitch is kept at 500 nm, the diameter of antidot is 400 nm and the thickness of the film is 20 nm.
0
3 θ=30 deg.
-1
L(10 5 J/m3)
-0.1
0 μ 0H (T)
0.1
Fig. 3. The magnetization curves of the antidot system at room temperature as a function of y; (a) y ¼ 0 , (b) y ¼ 15 , (c) y ¼ 30 .
0
μ0 H=0.1 T -3 0
60
120
180 θ (degree)
240
300
360
Fig. 2. A magnetic-torque curve for m0 H ¼ 0:1 T at room temperature. The dashed and solid lines denote the clockwise and the counterclockwise rotation measurements, respectively.
LðyÞ ¼ sinð6yÞ. These torque curves indicates that the directions of easy magnetization are y ¼ 0 , 60 , 120 , 180 , 240 , and 300 . The array which has a sixfold rotational symmetry induces this magnetic anisotropy, because the crystalline anisotropy of a Fe19 Ni81 alloy can be neglected at room temperature [5]. The roughly estimated magnetic anisotropy energy between the hard and easy axes is 3:2 104 J=m3 . Figs. 3(a)–(c) show the magnetization curves of the system at the various field direction y. Before these measurements, the initial magnetic field m0 H ¼ 0:1 T was applied so as to demagnetize the system. At y ¼ 0 , by decreasing the magnetic field from 0.1 T, when a magnetic field in the negative sense is applied to the system magnetized in the positive sense, a dropped steep change of magnetization appears
at a negative field m0 H ¼ 0:033 T, which corresponds to the magnetization reversal field in the system. Then, by applying the inverse magnetic field, the magnetization jumped at m0 H ¼ 0:033 T. In these results, the magnetization curves seem various shapes according to the direction of the magnetic fields. The reversal fields for y ¼ 0 , 15 , 30 are 0:033, 0:038, and 0:041 T, respectively. These magnetization curves show a single jump, which indicates that no magnetic domain walls are pinned at the system during the measurement. This work was supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan. References [1] P. Vavassori, G. Gubbiotti, G. Zangari, C.T. Yu, H. Yin, H. Jiang, G.J. Mankey, J. Appl. Phys. 91 (2002) 7992. [2] F.J. Castan˜o, K. Nielsch, C.A. Ross, J.W.A. Robinson, R. Krishnan, Appl. Phys. Lett. 85 (2004) 2872. [3] C.C. Wang, A.O. Adeyeye, N. Singh, Y.S. Huang, Y.H. Wu, Phys. Rev. B 72 (2005) 174426. [4] R.M. Bozorth, Ferromagnetism, IEEE Press, Piscataway, NJ, 1993, p. 555. [5] J.D. Kleis, Phys. Rev. 50 (1936) 1178.
Journal of Magnetism and Magnetic Materials 310 (2007) e792–e793 www.elsevier.com/locate/jmmm
Magnetic properties of nanometer-scale FeNi antidot array system M. Tanakaa,, K. Itoha, H. Iwamotoa, A. Yamaguchia, H. Miyajimaa, T. Yamaokab a
Department of Physics, Keio University, 3-14-1 Hiyoshi, Yokohama, Kanagawa 223-8522, Japan Application Engineering Section, SII NanoTechnology Inc., 2-15-5 Shintomi, Chuo-ku, Tokyo 104-0041, Japan
b
Available online 27 November 2006
Abstract The magnetic anisotropy and the magnetization reversal process of a nanometer-scale FeNi antidot array system of triangle lattice were investigated at room temperature by means of torque and resonating-sample magnetometries. The magnetic anisotropy of the system has a sixfold rotational symmetry refracting the structural symmetry of the array of holes. The magnetic anisotropy energy between the hard and easy axes is about 3:2 104 J=m3 . r 2006 Published by Elsevier B.V. PACS: 75.30.Gw; 75.50.Bb; 75.60.Jk Keywords: Nanostructure; Resonating sample magnetometer; Magnetic torque magnetometer; Magnetic anisotropy
Patterned magnetic structures fabricated by lithography, such as small particles with dimension on sub-micron scale, are candidates for a new generation of magnetic high storage media. Alternatively, the antidot array, which consists of a mesh of holes in a continuous magnetic film, has also been discussed as a competitor for high-density storage media with the merit of higher stability than continuous media [1–3]. In these systems, the magnetic structure depends mainly on the shape anisotropy introduced by the holes and the direction of the magnetic field. In this paper, the magnetic anisotropy and the magnetization reversal process of a FeNi antidot array system of triangle lattice were investigated by using torque and resonating-sample magnetometries. The magnetic torque curve shows that the magnetic anisotropy of the antidot system has a sixfold rotational symmetry reflecting the structural symmetry of the array. The magnetization curve indicates that no magnetic domain walls are pinned during the measurement. The sample investigated in this study is a Fe19 Ni81 antidot array system of triangle lattice. This sample was fabricated by a lift-off technique using electron-beam lithography. First, thin resist (ZEP-520A) 100 nm thick Corresponding author. Tel.: +81 45 566 1677.
E-mail address: [email protected] (M. Tanaka). 0304-8853/$ - see front matter r 2006 Published by Elsevier B.V. doi:10.1016/j.jmmm.2006.11.099
was spin-coated onto a thermally oxidized Si substrate. The resist mask was then patterned with an electron beam at 20 keV beam energy. After the development, Fe19 Ni81 was deposited by electron-beam evaporation in a vacuum of 108 Pa at the deposition rate of 0.1 nm/s. The sample was obtained after the resist mask was removed in a solvent. A scanning electron micrograph of the antidot system is shown in Fig. 1. The antidot pitch is kept at 500 nm, the diameter of antidot is 400 nm and the thickness of the film is 20 nm. The system includes about 3:6 107 holes. The inplane magnetic anisotropy of the system was investigated at room temperature by using a torque magnetometer with sensitivity up to 5 1010 N m. The magnetization process of the system was measured at room temperature as a function of y, where y denotes the in-plane direction of the magnetic field as shown in Fig. 1. Fig. 2 shows the magnetic torque curve for m0 H ¼ 0:1 T of the system. Before the measurement, the initial magnetic field of m0 H ¼ 0:1 T was applied to the direction of y ¼ 30 . The dashed and solid lines denote the experiments whose rotative directions are clockwise and counterclockwise, respectively. These lines overlap each other, which means the rotational hysteresis is almost zero. The torque curve LðyÞ, which is equal to the rate of change of energy density E with respect of angle y, i.e. LðyÞ ¼ qE=qy [4], clearly shows a sixfold rotational symmetry as
ARTICLE IN PRESS M. Tanaka et al. / Journal of Magnetism and Magnetic Materials 310 (2007) e792–e793
a
e793
1
H
500 nm θ
b
-1 1
M/Ms
0
0
c
-1 1
θ=0 deg.
θ=15 deg.
Fig. 1. Scanning-electron micrograph of the Fe19 Ni81 antidot array system of a square mesh. The antidot pitch is kept at 500 nm, the diameter of antidot is 400 nm and the thickness of the film is 20 nm.
0
3 θ=30 deg.
-1
L(10 5 J/m3)
-0.1
0 μ 0H (T)
0.1
Fig. 3. The magnetization curves of the antidot system at room temperature as a function of y; (a) y ¼ 0 , (b) y ¼ 15 , (c) y ¼ 30 .
0
μ0 H=0.1 T -3 0
60
120
180 θ (degree)
240
300
360
Fig. 2. A magnetic-torque curve for m0 H ¼ 0:1 T at room temperature. The dashed and solid lines denote the clockwise and the counterclockwise rotation measurements, respectively.
LðyÞ ¼ sinð6yÞ. These torque curves indicates that the directions of easy magnetization are y ¼ 0 , 60 , 120 , 180 , 240 , and 300 . The array which has a sixfold rotational symmetry induces this magnetic anisotropy, because the crystalline anisotropy of a Fe19 Ni81 alloy can be neglected at room temperature [5]. The roughly estimated magnetic anisotropy energy between the hard and easy axes is 3:2 104 J=m3 . Figs. 3(a)–(c) show the magnetization curves of the system at the various field direction y. Before these measurements, the initial magnetic field m0 H ¼ 0:1 T was applied so as to demagnetize the system. At y ¼ 0 , by decreasing the magnetic field from 0.1 T, when a magnetic field in the negative sense is applied to the system magnetized in the positive sense, a dropped steep change of magnetization appears
at a negative field m0 H ¼ 0:033 T, which corresponds to the magnetization reversal field in the system. Then, by applying the inverse magnetic field, the magnetization jumped at m0 H ¼ 0:033 T. In these results, the magnetization curves seem various shapes according to the direction of the magnetic fields. The reversal fields for y ¼ 0 , 15 , 30 are 0:033, 0:038, and 0:041 T, respectively. These magnetization curves show a single jump, which indicates that no magnetic domain walls are pinned at the system during the measurement. This work was supported by Grants-in-Aid for Scientific Research from the Ministry of Education, Culture, Sports, Science and Technology, Japan. References [1] P. Vavassori, G. Gubbiotti, G. Zangari, C.T. Yu, H. Yin, H. Jiang, G.J. Mankey, J. Appl. Phys. 91 (2002) 7992. [2] F.J. Castan˜o, K. Nielsch, C.A. Ross, J.W.A. Robinson, R. Krishnan, Appl. Phys. Lett. 85 (2004) 2872. [3] C.C. Wang, A.O. Adeyeye, N. Singh, Y.S. Huang, Y.H. Wu, Phys. Rev. B 72 (2005) 174426. [4] R.M. Bozorth, Ferromagnetism, IEEE Press, Piscataway, NJ, 1993, p. 555. [5] J.D. Kleis, Phys. Rev. 50 (1936) 1178.