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Perpendicular magnetic anisotropy in sputtered amorphous TbFeCo films
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 310 (2007) 1744–1746 www.elsevier.com/locate/jmmm
Perpendicular magnetic anisotropy in sputtered amorphous TbFeCo films X. Liua,, A. Morisakoa, H. Sakuraib, Y. Sakuraic, M. Itouc, A. Koizumid a
Center for spin device technology, Shinshu University, Wakasato 4-17-1, 380-8553, Japan Department of Electronic Engineering, Gunma University, 1-5-1 Tenjin-cho, Kiryu 376-8515, Japan c Japan Synchrotron Radiation Research Institute (JASRI), 1-1-1 Kouto Sayo-cho Sayo-gun, Hyogo 679-5198 Japan d Department of Material Science, University of Hyogo, 1-3-3 Higashikawasaki-cho, Chuo-ku, Kobe-shi, Japan b
Available online 13 November 2006
Abstract Local chemical bonding and magnetic Compton profiles were used to study the origination of perpendicular magnetic anisotropy (PMA) in amorphous TbFeCo films. Experimental results suggested oxygen atoms can change the local chemical bonding. Anisotropic chemical bonding of Fe3d and Tb4f electrons are found for films with suitable oxygen composition. It is concluded that the origination of PMA in amorphous TbFeCo films is related to the composition of oxygen atoms in the films. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50.Bb; 75.50.Ss; 75.50.Vv Keywords: Magnetic anisotropy; Amorphous TbFeCo film
1. Introduction Sputtered TbFeCo films have potential applications in magneto–optical recording media, thermal assistant magnetic recording media and perpendicular magnetic recording media [1,2]. The films are normally amorphous with excellent perpendicular magnetic anisotropy (PMA). It is well known that in crystalline films the magnetic anisotropy is originated from magnetocrystalline anisotropy [3]. There are other mechanism of magnetic anisotropy such as stress-induced magnetic anisotropy [4], shape anisotropy [5]. However, none of the above mechanisms can explain the PMA in amorphous TbFeCo films just because of the uniformity of the amorphous nature. From both practical and theoretical points of view, it is very important to understand the origin of PMA in amorphous TbFeCo film. Recent development of magnetic Compton profiles has provided a powerful tool to study the anisotropy of spin density in the momentum space [6]. In this experiment, two kinds of TbFeCo amorphous films, one with perpendicular Corresponding author. Tel.: +81 26 269 5480; fax: +81 26 269 5485.
E-mail address: [email protected] (X. Liu). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.576
anisotropy and one with random orientation are prepared by DC magnetron sputtering. The magnetic Compton profiles and the binding energy of the films were evaluated. It is found that the PMA in amorphous TbFeCo films originated from anisotropic chemical bonding of Fe3d and Tb4f electrons. 2. Experimental procedure TbFeCo films were deposited by a DC-magnetron sputtering system in Ar atmosphere without substrate heating. The film thickness was kept constant at 1000 nm. Base pressure was below 2 10 6 Torr and sputtering pressure was 5 and 30 mTorr. A composite target consisting of Fe plate overlaid by Tb and Co chips were used to deposit TbFeCo layer. Magnetic properties were measured by using vibrating sample magnetometer (VSM) with a maximum field of 24 kOe and the film composition was determined by electron probe micro-analyzer (EPMA). Magnetic Compton profiles of the samples are taken at BL08W of SPring-8. The incident X-ray beam energy is 180 keV. The profiles were taken with incident beam
ARTICLE IN PRESS X. Liu et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 1744–1746
perpendicular and parallel to the film plane to detect the anisotropy of spin density in the momentum space. The binding energy of the Fe, Tb, Co and O in the films was detected by X-ray photoelectron spectroscopy. All the experiments and measurements were carried out at room temperature.
100 50
a
Magnetization , Ms (emu/cm3)
0 -50 -100 -24
-16
0
-8
8
16
24
16
24
400 200
b
0 -200 -400 -24
-16
8 -8 0 Applied field (kOe)
Fig. 1. Hysteresis loop of film: (a) deposited at 5 mTorr, and film (b) deposited at 30 mTorr.
3. Results and discussion Both X-ray diffraction and transmission electron microscopy results show that the all the films are amorphous at room temperature. Fig. 1 shows the hysteresis loop of film: (a) deposited at 5 mTorr, and film (b) deposited at 30 mTorr. Electron probe microanalysis results show the two films have almost the same composition of Tb27Fe63Co10. However, the magnetic properties for VSM are quite different. The film deposited at 5 mTorr shows PMA with a square hysteresis loop in the solid line in the perpendicular direction. The perpendicular coercivity is 15.5 kOe, while in-plane coercivity is almost zero in the dashed line. Film deposited at 30 mTorr shows an isotropic magnetic property with shearing hysteresis loops in both perpendicular and in-plane directions. Fig. 2 shows the magnetic Compton profile of film prepared at 5 mTorr for (a), (b) and 30 mTorr, for (c) and (d). According to the profiles, ferrimagnetic coupling between Fe3d and Tb4f was observed in the films with PMA. Moreover, the profiles (a) and (c) are clearly different, which implies the anisotropic spin density. Although the samples are prepared at base pressure below 2 10 6 Torr, oxygen has been found in all the samples. Fig. 3 shows the O1 s binding energy of the film: (a) deposited at Ar gas pressure of 5 mTorr, and film (b) deposited at Ar gas pressure of 30 mTorr. There are two peaks found for films deposited at 5 mTorr while only one peak were found for films deposited at 30 mTorr. The binding energy at 530 eV for O1 s is corresponding to the bonding of Tb–O. The energy at 532 eV for O1 s is corresponding to the bonding of Fe–O. The results show
isotropic magnetization film
perpendicular anisotropy film 0.5
0.2
c
0.4
0
Jmag(Pz)
Jmag(Pz)
a out of plane in-plane
-0.2
1745
out of plane in-plane
0.3 0.2 0.1 0
-0.4
0
4
6
8
b
0
2
d Jmag(Pz)
out of plane - in plane
0
-0.1
-0.1
10
4
6
8
10
0.1
0.1
Jmag(Pz)
2
0
-0.1
0
2
4
6
Pz [a.u.]
8
10
out of plane - in plane
0
2
4
6
8
10
Pz [a.u.]
Fig. 2. Magnetic Compton profiles of perpendicular anisotropy film and isotropic magnetization film with out-of-plane configuration and in-plane configuration.
ARTICLE IN PRESS X. Liu et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 1744–1746
1746
4. Conclusion
3000 30 mTorr
2800
O1s
5 mTorr
Magnetic Compton profiles and binding energy were used to evaluate the origination of PMA in amorphous TbFeCo films. Experimental results suggested oxygen atoms can change the local chemical bonding. Anisotropic chemical bonding of Fe3d and Tb4f electrons are found for films with suitable oxygen composition. It is concluded that the origination of PMA in amorphous TbFeCo films is related to the composition of oxygen atoms in the films.
2600
Counts
2400 2200 2000 1800 1600 1400 540
538
536
534 532 530 Binding Energy (ev)
528
526
524
Fig. 3. O1 s binding energy of the film: (a) deposited at Ar gas pressure of 5 mTorr and film, (b) deposited at Ar gas pressure of 30 mTorr.
that the local chemical ordering of the samples are different. Considering the results of magnetic compoton profiles, it is suggested there are strong relations between local chemical ordering and spin density distribution.
References [1] H. Saga, H. Nemoto, H. Sukeda, M. Takahashi, J. Magn. Soc. Jpn. 23 (1999) 225. [2] H. Katayama, S. Sawamura, Y. Ogimoto, J. Nakajima, K. Kojima, K. Ohta, J. Magn. Soc. Jpn. 23 (1999) 233. [3] S. Chikazumi, S.H. Charap, Physics of Magnetism, Wiley, New York, 1964. [4] G.S. Cargill, T. Mizoguchi, J. Appl. Phys. 49 (1978) 1753. [5] R. Naik, et al., Phys. Rev. B 51 (1995) 3549. [6] A. Koizumi, et al., Phys. Rev. Lett. 86 (2001) 5589.
Journal of Magnetism and Magnetic Materials 310 (2007) 1744–1746 www.elsevier.com/locate/jmmm
Perpendicular magnetic anisotropy in sputtered amorphous TbFeCo films X. Liua,, A. Morisakoa, H. Sakuraib, Y. Sakuraic, M. Itouc, A. Koizumid a
Center for spin device technology, Shinshu University, Wakasato 4-17-1, 380-8553, Japan Department of Electronic Engineering, Gunma University, 1-5-1 Tenjin-cho, Kiryu 376-8515, Japan c Japan Synchrotron Radiation Research Institute (JASRI), 1-1-1 Kouto Sayo-cho Sayo-gun, Hyogo 679-5198 Japan d Department of Material Science, University of Hyogo, 1-3-3 Higashikawasaki-cho, Chuo-ku, Kobe-shi, Japan b
Available online 13 November 2006
Abstract Local chemical bonding and magnetic Compton profiles were used to study the origination of perpendicular magnetic anisotropy (PMA) in amorphous TbFeCo films. Experimental results suggested oxygen atoms can change the local chemical bonding. Anisotropic chemical bonding of Fe3d and Tb4f electrons are found for films with suitable oxygen composition. It is concluded that the origination of PMA in amorphous TbFeCo films is related to the composition of oxygen atoms in the films. r 2006 Elsevier B.V. All rights reserved. PACS: 75.50.Bb; 75.50.Ss; 75.50.Vv Keywords: Magnetic anisotropy; Amorphous TbFeCo film
1. Introduction Sputtered TbFeCo films have potential applications in magneto–optical recording media, thermal assistant magnetic recording media and perpendicular magnetic recording media [1,2]. The films are normally amorphous with excellent perpendicular magnetic anisotropy (PMA). It is well known that in crystalline films the magnetic anisotropy is originated from magnetocrystalline anisotropy [3]. There are other mechanism of magnetic anisotropy such as stress-induced magnetic anisotropy [4], shape anisotropy [5]. However, none of the above mechanisms can explain the PMA in amorphous TbFeCo films just because of the uniformity of the amorphous nature. From both practical and theoretical points of view, it is very important to understand the origin of PMA in amorphous TbFeCo film. Recent development of magnetic Compton profiles has provided a powerful tool to study the anisotropy of spin density in the momentum space [6]. In this experiment, two kinds of TbFeCo amorphous films, one with perpendicular Corresponding author. Tel.: +81 26 269 5480; fax: +81 26 269 5485.
E-mail address: [email protected] (X. Liu). 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.576
anisotropy and one with random orientation are prepared by DC magnetron sputtering. The magnetic Compton profiles and the binding energy of the films were evaluated. It is found that the PMA in amorphous TbFeCo films originated from anisotropic chemical bonding of Fe3d and Tb4f electrons. 2. Experimental procedure TbFeCo films were deposited by a DC-magnetron sputtering system in Ar atmosphere without substrate heating. The film thickness was kept constant at 1000 nm. Base pressure was below 2 10 6 Torr and sputtering pressure was 5 and 30 mTorr. A composite target consisting of Fe plate overlaid by Tb and Co chips were used to deposit TbFeCo layer. Magnetic properties were measured by using vibrating sample magnetometer (VSM) with a maximum field of 24 kOe and the film composition was determined by electron probe micro-analyzer (EPMA). Magnetic Compton profiles of the samples are taken at BL08W of SPring-8. The incident X-ray beam energy is 180 keV. The profiles were taken with incident beam
ARTICLE IN PRESS X. Liu et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 1744–1746
perpendicular and parallel to the film plane to detect the anisotropy of spin density in the momentum space. The binding energy of the Fe, Tb, Co and O in the films was detected by X-ray photoelectron spectroscopy. All the experiments and measurements were carried out at room temperature.
100 50
a
Magnetization , Ms (emu/cm3)
0 -50 -100 -24
-16
0
-8
8
16
24
16
24
400 200
b
0 -200 -400 -24
-16
8 -8 0 Applied field (kOe)
Fig. 1. Hysteresis loop of film: (a) deposited at 5 mTorr, and film (b) deposited at 30 mTorr.
3. Results and discussion Both X-ray diffraction and transmission electron microscopy results show that the all the films are amorphous at room temperature. Fig. 1 shows the hysteresis loop of film: (a) deposited at 5 mTorr, and film (b) deposited at 30 mTorr. Electron probe microanalysis results show the two films have almost the same composition of Tb27Fe63Co10. However, the magnetic properties for VSM are quite different. The film deposited at 5 mTorr shows PMA with a square hysteresis loop in the solid line in the perpendicular direction. The perpendicular coercivity is 15.5 kOe, while in-plane coercivity is almost zero in the dashed line. Film deposited at 30 mTorr shows an isotropic magnetic property with shearing hysteresis loops in both perpendicular and in-plane directions. Fig. 2 shows the magnetic Compton profile of film prepared at 5 mTorr for (a), (b) and 30 mTorr, for (c) and (d). According to the profiles, ferrimagnetic coupling between Fe3d and Tb4f was observed in the films with PMA. Moreover, the profiles (a) and (c) are clearly different, which implies the anisotropic spin density. Although the samples are prepared at base pressure below 2 10 6 Torr, oxygen has been found in all the samples. Fig. 3 shows the O1 s binding energy of the film: (a) deposited at Ar gas pressure of 5 mTorr, and film (b) deposited at Ar gas pressure of 30 mTorr. There are two peaks found for films deposited at 5 mTorr while only one peak were found for films deposited at 30 mTorr. The binding energy at 530 eV for O1 s is corresponding to the bonding of Tb–O. The energy at 532 eV for O1 s is corresponding to the bonding of Fe–O. The results show
isotropic magnetization film
perpendicular anisotropy film 0.5
0.2
c
0.4
0
Jmag(Pz)
Jmag(Pz)
a out of plane in-plane
-0.2
1745
out of plane in-plane
0.3 0.2 0.1 0
-0.4
0
4
6
8
b
0
2
d Jmag(Pz)
out of plane - in plane
0
-0.1
-0.1
10
4
6
8
10
0.1
0.1
Jmag(Pz)
2
0
-0.1
0
2
4
6
Pz [a.u.]
8
10
out of plane - in plane
0
2
4
6
8
10
Pz [a.u.]
Fig. 2. Magnetic Compton profiles of perpendicular anisotropy film and isotropic magnetization film with out-of-plane configuration and in-plane configuration.
ARTICLE IN PRESS X. Liu et al. / Journal of Magnetism and Magnetic Materials 310 (2007) 1744–1746
1746
4. Conclusion
3000 30 mTorr
2800
O1s
5 mTorr
Magnetic Compton profiles and binding energy were used to evaluate the origination of PMA in amorphous TbFeCo films. Experimental results suggested oxygen atoms can change the local chemical bonding. Anisotropic chemical bonding of Fe3d and Tb4f electrons are found for films with suitable oxygen composition. It is concluded that the origination of PMA in amorphous TbFeCo films is related to the composition of oxygen atoms in the films.
2600
Counts
2400 2200 2000 1800 1600 1400 540
538
536
534 532 530 Binding Energy (ev)
528
526
524
Fig. 3. O1 s binding energy of the film: (a) deposited at Ar gas pressure of 5 mTorr and film, (b) deposited at Ar gas pressure of 30 mTorr.
that the local chemical ordering of the samples are different. Considering the results of magnetic compoton profiles, it is suggested there are strong relations between local chemical ordering and spin density distribution.
References [1] H. Saga, H. Nemoto, H. Sukeda, M. Takahashi, J. Magn. Soc. Jpn. 23 (1999) 225. [2] H. Katayama, S. Sawamura, Y. Ogimoto, J. Nakajima, K. Kojima, K. Ohta, J. Magn. Soc. Jpn. 23 (1999) 233. [3] S. Chikazumi, S.H. Charap, Physics of Magnetism, Wiley, New York, 1964. [4] G.S. Cargill, T. Mizoguchi, J. Appl. Phys. 49 (1978) 1753. [5] R. Naik, et al., Phys. Rev. B 51 (1995) 3549. [6] A. Koizumi, et al., Phys. Rev. Lett. 86 (2001) 5589.