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Magnetic properties of quasi-three-dimensional Fe thin film studied by spin-polarized secondary electron microscopy
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 272–276 (2004) 2165–2166
Magnetic properties of quasi-three-dimensional Fe thin film studied by spin-polarized secondary electron microscopy S. Uedaa,*, Y. Iwasakia,b, S. Ushiodaa,c a
Photodynamics Research Center, RIKEN (The Institute of Physical and Chemical Research), Aoba-ku, Sendai 980-0845, Japan b Micro Systems Network Company, Sony Corporation, Tagajo, Miyagi 985-0842, Japan c Research Institute of Electrical Communication, Tohoku University, Aoba-ku, Sendai 980-8577, Japan
Abstract The magnetic domain structures of Fe thin films on two dimensionally arranged hexagonal land-and-groove structures have been studied by spin-polarized secondary electron microscopy. Characteristic magnetization reversal was observed for the hexagonal land-and-groove structure in a remanent state. The coercivity difference between the land and groove was partly due to the difference in surface roughness between the land and groove. Shape-induced anisotropy also contributed to the magnetization reversal process. r 2003 Elsevier B.V. All rights reserved. PACS: 75.60.Ch; 75.70.Kw Keywords: SP-SEM; Fe; Magnetic domain; Hexagonal land-and-groove structure; Shape anisotropy
1. Introduction Roughness-modulated magnetic films show a characteristic magnetization reversal [1,2]. The magnetic domain wall pinning effects were reported for Fe films on striped structures [1] and rectangular land-and-groove structures [2] in the magnetization reversal process. In this work we have investigated the shape dependence of Fe films on land-and-groove structures by using spinpolarized secondary electron microscopy (SP-SEM). SPSEM is a non-magnetic probe that does not influence the magnetization of the sample with low-coercive force. Thus, this technique allows observation of the true magnetic state of the sample independent of the probe.
2. Experimental A land-and-groove patterned substrate with 150 nm depth was prepared ex situ by a DC magnetron sputter*Corresponding author. Tel.: +81-22-228-2124; fax: +8122-228-2128. E-mail address: [email protected] (S. Ueda).
deposition of Au through a mask on a Au (10 nm)/Cr (5 nm)/Si substrate. The patterned area of this substrate was 2 mm in diameter. We deposited layers of 3 nm thick Ta and 10 nm thick Fe on both land and groove areas in this sequence by using e-beam evaporators in the preparation chamber attached to the SP-SEM. The pressure of the preparation chamber was better than 4:0 1010 Torr during the evaporation. Each land had a hexagonal area with sides 17 mm long and height 150 nm, separated by a groove 7 mm wide. The area between the land and groove had a 2 mm wide sloped region. This film geometry formed a quasi-three-dimensional array of magnetic material. The SP-SEM system [3] has a field-emission gun (15 kV) and a medium energy (25 kV) Mott spin detector of retarding potential type. The yoke of an electromagnet was placed in the SP-SEM chamber to apply a magnetic field to the sample. Magnetic imaging of the sample in the remanent state after application of a magnetic field was performed under ultra-high vacuum better than 4:0 1010 Torr at room temperature. The surface morphology and roughness were measured by atomic force microscope (AFM) with a contact mode.
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.401
ARTICLE IN PRESS 2166
S. Ueda et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 2165–2166
Fig. 1. The magnetic domain structure of the 10 nm thick Fe film on the hexagonal land-and-groove substrate measured in the remanent state after application of 36 Oe magnetic field.
3. Results and discussion The 10 nm thick Fe film on the hexagonal land-andgroove substrate showed the characteristic magnetization reversal. Magnetic domain structures were observed by SP-SEM in the remanent state after application of a magnetic field. Nucleation of the reversed magnetic domains mainly occurred at the corners in the groove area after application of 32 Oe magnetic field. Fig. 1 shows the magnetic domain structure of the sample measured in the remanent state after application of 36 Oe magnetic field. The border area between the land and groove is indicated by a gray hexagonal frame in the magnetic image to guide the eye. The magnetic field was applied along the þy direction. The reversed magnetic domains (bright areas) on the fringes of the lands were mainly found around the corners in the groove area. On the other hand the magnetization directions of the land areas were almost parallel to the y direction (dark areas). The x component of the magnetization was negligible in the magnetization reversal process. The hysteresis loop was obtained by spin-polarization analysis of both the land and groove areas in the
remanent states (not shown in figure). The coercive forces on the land and groove were 38.8 and 37.9 Oe, respectively. Higher coercivity on the land area was previously observed in a roughness modulated Fe film [1] and an Fe film on a rectangular land-and-groove substrate [2]. Rougher surface on the land compared with the groove led to the high-coercivity on the land area. AFM results showed that the rms surface roughness on the land and groove areas were 1.69 and 0.28 nm, respectively. On the other hand nucleation of the reversed domain on the land occurred before full reversal of magnetization on the groove as shown in Fig. 1. This behavior differs from that of the rectangular land-and-groove structure [2]. This difference is partly due to the small difference in coercivity between the hexagonal land and groove areas. The hexagonal land-and-groove structure is, in other words, a combination of hexagonal dot and antidot magnetic arrays. Shape-induced anisotropy plays an important role in the coercivity and the magnetization reversal as well as magnetic domain structure for the magnetic dot and antidot arrays [4,5]. The difference in coercivity between the land and groove was small with the result that the shape-induced anisotropy contributed to the magnetization reversal process.
Acknowledgements The authors would like to acknowledge valuable advice from Prof. J. Nishizawa.
References [1] J. Swerts, K. Temst, M.J. Van Beal, C. Van Hasendonck, Y. Bruynseraede, Appl. Phys. Lett. 82 (2003) 1239. [2] S. Ueda, Y. Iwasaki, S. Ushioda, Jpn. J. Appl. Phys. 42 (2003) 6417. [3] Y. Iwasaki, K. Bessho, J. Kondis, H. Ohmori, H. Hopster, Appl. Surf. Sci. 113–114 (1997) 155. [4] E. Seynaeve, G. Rens, A.V. Volodin, K. Temst, C. Van Haesendonck, J. Appl. Phys. 89 (2001) 531. [5] I. Guedes, M. Grimsditch, V. Metlsushko, P. Vavassori, R. Camley, B. Ilic, P. Neuzil, R. Kumar, Phys. Rev. B 66 (2002) 14434.
Journal of Magnetism and Magnetic Materials 272–276 (2004) 2165–2166
Magnetic properties of quasi-three-dimensional Fe thin film studied by spin-polarized secondary electron microscopy S. Uedaa,*, Y. Iwasakia,b, S. Ushiodaa,c a
Photodynamics Research Center, RIKEN (The Institute of Physical and Chemical Research), Aoba-ku, Sendai 980-0845, Japan b Micro Systems Network Company, Sony Corporation, Tagajo, Miyagi 985-0842, Japan c Research Institute of Electrical Communication, Tohoku University, Aoba-ku, Sendai 980-8577, Japan
Abstract The magnetic domain structures of Fe thin films on two dimensionally arranged hexagonal land-and-groove structures have been studied by spin-polarized secondary electron microscopy. Characteristic magnetization reversal was observed for the hexagonal land-and-groove structure in a remanent state. The coercivity difference between the land and groove was partly due to the difference in surface roughness between the land and groove. Shape-induced anisotropy also contributed to the magnetization reversal process. r 2003 Elsevier B.V. All rights reserved. PACS: 75.60.Ch; 75.70.Kw Keywords: SP-SEM; Fe; Magnetic domain; Hexagonal land-and-groove structure; Shape anisotropy
1. Introduction Roughness-modulated magnetic films show a characteristic magnetization reversal [1,2]. The magnetic domain wall pinning effects were reported for Fe films on striped structures [1] and rectangular land-and-groove structures [2] in the magnetization reversal process. In this work we have investigated the shape dependence of Fe films on land-and-groove structures by using spinpolarized secondary electron microscopy (SP-SEM). SPSEM is a non-magnetic probe that does not influence the magnetization of the sample with low-coercive force. Thus, this technique allows observation of the true magnetic state of the sample independent of the probe.
2. Experimental A land-and-groove patterned substrate with 150 nm depth was prepared ex situ by a DC magnetron sputter*Corresponding author. Tel.: +81-22-228-2124; fax: +8122-228-2128. E-mail address: [email protected] (S. Ueda).
deposition of Au through a mask on a Au (10 nm)/Cr (5 nm)/Si substrate. The patterned area of this substrate was 2 mm in diameter. We deposited layers of 3 nm thick Ta and 10 nm thick Fe on both land and groove areas in this sequence by using e-beam evaporators in the preparation chamber attached to the SP-SEM. The pressure of the preparation chamber was better than 4:0 1010 Torr during the evaporation. Each land had a hexagonal area with sides 17 mm long and height 150 nm, separated by a groove 7 mm wide. The area between the land and groove had a 2 mm wide sloped region. This film geometry formed a quasi-three-dimensional array of magnetic material. The SP-SEM system [3] has a field-emission gun (15 kV) and a medium energy (25 kV) Mott spin detector of retarding potential type. The yoke of an electromagnet was placed in the SP-SEM chamber to apply a magnetic field to the sample. Magnetic imaging of the sample in the remanent state after application of a magnetic field was performed under ultra-high vacuum better than 4:0 1010 Torr at room temperature. The surface morphology and roughness were measured by atomic force microscope (AFM) with a contact mode.
0304-8853/$ - see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.401
ARTICLE IN PRESS 2166
S. Ueda et al. / Journal of Magnetism and Magnetic Materials 272–276 (2004) 2165–2166
Fig. 1. The magnetic domain structure of the 10 nm thick Fe film on the hexagonal land-and-groove substrate measured in the remanent state after application of 36 Oe magnetic field.
3. Results and discussion The 10 nm thick Fe film on the hexagonal land-andgroove substrate showed the characteristic magnetization reversal. Magnetic domain structures were observed by SP-SEM in the remanent state after application of a magnetic field. Nucleation of the reversed magnetic domains mainly occurred at the corners in the groove area after application of 32 Oe magnetic field. Fig. 1 shows the magnetic domain structure of the sample measured in the remanent state after application of 36 Oe magnetic field. The border area between the land and groove is indicated by a gray hexagonal frame in the magnetic image to guide the eye. The magnetic field was applied along the þy direction. The reversed magnetic domains (bright areas) on the fringes of the lands were mainly found around the corners in the groove area. On the other hand the magnetization directions of the land areas were almost parallel to the y direction (dark areas). The x component of the magnetization was negligible in the magnetization reversal process. The hysteresis loop was obtained by spin-polarization analysis of both the land and groove areas in the
remanent states (not shown in figure). The coercive forces on the land and groove were 38.8 and 37.9 Oe, respectively. Higher coercivity on the land area was previously observed in a roughness modulated Fe film [1] and an Fe film on a rectangular land-and-groove substrate [2]. Rougher surface on the land compared with the groove led to the high-coercivity on the land area. AFM results showed that the rms surface roughness on the land and groove areas were 1.69 and 0.28 nm, respectively. On the other hand nucleation of the reversed domain on the land occurred before full reversal of magnetization on the groove as shown in Fig. 1. This behavior differs from that of the rectangular land-and-groove structure [2]. This difference is partly due to the small difference in coercivity between the hexagonal land and groove areas. The hexagonal land-and-groove structure is, in other words, a combination of hexagonal dot and antidot magnetic arrays. Shape-induced anisotropy plays an important role in the coercivity and the magnetization reversal as well as magnetic domain structure for the magnetic dot and antidot arrays [4,5]. The difference in coercivity between the land and groove was small with the result that the shape-induced anisotropy contributed to the magnetization reversal process.
Acknowledgements The authors would like to acknowledge valuable advice from Prof. J. Nishizawa.
References [1] J. Swerts, K. Temst, M.J. Van Beal, C. Van Hasendonck, Y. Bruynseraede, Appl. Phys. Lett. 82 (2003) 1239. [2] S. Ueda, Y. Iwasaki, S. Ushioda, Jpn. J. Appl. Phys. 42 (2003) 6417. [3] Y. Iwasaki, K. Bessho, J. Kondis, H. Ohmori, H. Hopster, Appl. Surf. Sci. 113–114 (1997) 155. [4] E. Seynaeve, G. Rens, A.V. Volodin, K. Temst, C. Van Haesendonck, J. Appl. Phys. 89 (2001) 531. [5] I. Guedes, M. Grimsditch, V. Metlsushko, P. Vavassori, R. Camley, B. Ilic, P. Neuzil, R. Kumar, Phys. Rev. B 66 (2002) 14434.