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Underlayer diffusion-induced enhancement of coercivity in high anisotropy FePt thin films
ARTICLE IN PRESS Journal of Magnetism and Magnetic Materials 320 (2008) 3068–3070
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Underlayer diffusion-induced enhancement of coercivity in high anisotropy FePt thin films J.F. Hu a,, J.S. Chen b, B.C. Lim a, B. Liu a a b
Data Storage Institute, DSI building, 5 Engineering Drive 1 (off Kent Ridge Crescent, NUS), Singapore 117608, Singapore Department of Materials Science and Engineering, National University of Singapore, Singapore 117576, Singapore
a r t i c l e in f o
a b s t r a c t
Available online 9 August 2008
The L10 ordered FePt films have been prepared at 300 1C with a basic structure of CrRu/MgO/FePt, followed by a post-annealing process at temperatures from 200 to 350 1C. The magnetic properties and the microstructure of the films were investigated. It is found that coercivity of FePt films increases greatly from 3.57 to 9.1 kOe with the increasing annealing temperature from 200 to 350 1C. The loop slope of the M–H curves decreases with the increasing annealing temperature, which is due to the grain isolation induced by MgO underlayer diffusion during the annealing process. The underlayer diffusion could be a useful approach to prepare the FePt-based composite films for high-density recording media. & 2008 Elsevier B.V. All rights reserved.
PACS: 75.50.Ss 75.50.Vv 75.70.Cn Keywords: FePt MgO Underlayer diffusion Post-annealing
1. Introduction The L10 ordered FePt thin film is one of the most promising candidates of the perpendicular recording media, due to its high magnetocrystalline anisotropy. The formation of the high anisotropy FePt film requires either post-deposition annealing to transform the fcc phase to fct phase or in-situ substrate heating during film deposition. From application viewpoint, small FePt grain sizes and magnetically decoupled FePt grains in the recording layer are required to increase the areal density and to reduce the intergranular interactions. The most common approach to reduce the intergranular exchange coupling is to fabricate the FePt-based composite films by element doping [1–8]. As mentioned before, the formation of the high anisotropy FePt film needs a high-temperature process, which is not favorable to form the small grain sizes. Efforts have been made to reduce the deposition or post-annealing temperature. It was reported that the ordering temperature of the FePt could be reduced to about 300 1C by incorporating Cu into FePt films [9]. However, the formation of the FePtCu ternary alloy demonstrated a crystallographic orientation of (111) instead of the desirable (0 0 1). The reduction of the process temperature may cause the deterioration of the FePt (0 0 1) texture and the magnetic properties accordingly. It was reported that the strain from the lattice mismatch favored the chemical ordering of the L10 FePt films [10]. However, Corresponding author. Tel.: +65 6874 8622; fax: +65 6777 2406.
E-mail address: [email protected] (J.F. Hu). 0304-8853/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.08.037
continuous FePt films prepared at lower processing temperature normally showed a small coercivity. In present work, we try to develop an approach to improve the magnetic properties of the FePt films prepared at a substrate temperature of 300 1C. A postannealing process at temperatures ranging from 200 to 350 1C was used. The pure FePt films were used in present study to eliminate the microstructure variation caused by the element doping during the annealing process.
2. Experimental details Film with structure of corning glass/CrRu 30 nm/MgO 2 nm/ FePt 15 nm was prepared by an ultrahigh vacuum magnetron sputtering system. The FePt (0 0 1) texture could be formed via hetero-epitaxial growth following the orientation relationship of FePt (0 0 1) [1 0 0]JMgO (1 0 0) [1 0 0]JCrRu (0 0 2) [11 0] [2,11,12]. The substrate temperature for deposition of the CrRu and FePt was 300 1C and the MgO underlayer was deposited at 80 1C. The substrate was heated by holding for 15 min at the setting temperature before the magnetron sputtering. The prepared samples were annealed in an argon atmosphere at temperatures ranging from 200 to 350 1C for 1.5 h. The texture of the composite films was investigated by X-ray diffraction (XRD) with Cu Ka radiation. Magnetic properties were measured by a vibrating sample magnetometer (VSM). The X-ray photoelectron spectroscopy (XPS) with depth profiling capability was used to analyze the chemical composition of the films.
ARTICLE IN PRESS J.F. Hu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 3068–3070
Transmission electron microscopy (TEM) was used to observe the microstructure of the FePt films.
3. Results and discussions Fig. 1 shows the XRD patterns of the FePt thin films before and after heat treatment. It could be seen from the XRD pattern of the as-deposited FePt film that the FePt (0 0 1) texture was formed. For samples annealed at a temperature lower than 300 1C, the peak position of FePt (0 0 1) remained, while for 350 1C annealed sample, the (0 0 1) diffraction peak shifted to lower angle direction, implied an increase of the lattice constant in the direction of film normal. According to the growth mechanism of the FePt on CrRu underlayer [13], the shift of the (0 0 1) diffraction peak was caused by a stress release in the FePt thin film. The peak position of the FePt (2 0 0)+(0 0 2) shifted to lower angle direction when annealing temperature reached or over 300 1C, which suggested the increase of the fcc phase in the films. To evaluate the effect of the post-annealing on the chemical ordering of the L10 phase in a quantitative way, the ordering parameter S was calculated using following equation: S ¼ 0:72½I001 =I002 1=2
3069
as-deposited sample. Results shown in Fig. 2 indicated a decrease of the ordering parameter with the increasing annealing temperature, implying a deterioration of the FePt (0 0 1) texture. Fig. 3 illustrates the M–H loops and the coercivity variation as a function of post-annealing temperature. It was found that the coercivity of the FePt films enhanced greatly with the increasing annealing temperature. The coercivity of the as-deposited FePt film was 3.57 kOe and it increased to 6.86 and 9.1 kOe for sample annealed at 300 and 350 1C, respectively. It was noted that a kink was observed in the M–H loops of the samples annealed at 300 and 350 1C, which was due to the fcc phase [14]. Besides the enhancement of the coercivity, a decrease of the loop slope of the M–H curves with the increasing annealing
(1)
where I001 and I002 are the integrated intensities of the FePt (0 0 1) and (0 0 2) diffraction peaks from the XRD patterns. The longrange order parameter S of L10 ordered FePt films annealed at different temperature was calculated and compared to that of the
Fig. 1. The XRD patterns of the CrRu/MgO/FePt films before and after annealing.
Fig. 2. The long-range order S of L10 ordered FePt film as a function of the annealing temperature.
Fig. 3. M–H loops of and the coercivity variation of FePt films before and after annealing.
Fig. 4. Angular dependence of coercivity of CrRu/MgO/FePt films before and after annealing.
ARTICLE IN PRESS 3070
J.F. Hu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 3068–3070
Fig. 5. Mg 1s XPS narrow scan of CrRu/MgO/FePt films before and after annealing, (a) As-deposited, (b) 300 1C, (c) 350 1C.
temperature was observed, which implied the reduction of the intergranular exchange coupling. Considering the XRD results shown above, the enhancement of the coercivity was not due to the improvement of the FePt (0 0 1) texture. If the reduction of the intergranular exchange coupling was the reason, the magnetization-reversal mechanism of the film should be changed. In order to investigate the magnetization-reversal mechanism of the post-annealed FePt films, the angular dependence of the coercivity of FePt films was done, as shown in Fig. 4. The profile of the coercivity angular dependence demonstrated a tendency of the magnetization-reversal mechanism varying from typical domain wall motion behavior for as-deposited sample to S–W rotational mode for 350 1C annealed samples. The decrease of the loop slope of the M–H curves and the variation of the reversal mechanism of FePt indicated a decrease of the intergranular exchange coupling after annealing. It may be due to the isolation of the FePt grains. Since the samples investigated in our study are pure FePt films, the grain isolation is most probably resulting from the diffusion of the MgO underlayer during the annealing process. The XPS was used to check whether the diffusion of the MgO occurred by analyzing the surface chemical status of the films. Fig. 5 shows the Mg 1s XPS narrow scan at the surface of the FePt films before and after annealing. It was noted that no Mg 1s peak was observed for as-deposited sample, while for samples annealed at temperature over 300 1C, the Mg 1s peak was presented in the XPS spectra. Results confirm the diffusion of the MgO underlayer during the annealing process when treatment temperature is over 300 1C. The depth profile of the 350 1C annealed sample (data not shown) revealed that the MgO was distributed throughout the whole FePt layer. Fig. 6 shows the cross-sectional TEM images of (a) as-deposited FePt films and (b) FePt film after annealing at 350 1C. TEM observations revealed that the as-deposited sample illustrated a continuous layer structure, while the sample annealed at 350 1C demonstrated a granular type of structure. The white-contrasted grain boundaries observed in Fig. 6 (b) might be originated from the MgO diffused from the underlayer, which was consistent with the XPS measurement results.
Fig. 6. Cross-sectional TEM images of CrRu/MgO/FePt films, (a) as-deposited and (b) after annealing at 350 1C.
4. Summary In summary, the microstructure and the magnetic properties of the FePt films post-annealed at different temperatures were investigated. The diffusion of MgO from underlayer through FePt grain boundaries causes the decrease of the ordering parameter S and the enhancement of the film coercivity. The magnetizationreversal mechanism of the FePt film shows a trend from a typical domain wall motion behavior to S–W rotational mode with the increasing annealing temperature. The enhancement of coercivity and the variation of the magnetization-reversal mechanism are attributed to the FePt grain isolation. References [1] M.L. Yuan, H. Zeng, N. Powers, D.J. Sellmyer, J. Appl. Phys. 91 (2002) 8471. [2] J.S. chen, B.C. Lim, J.F. Hu, B. Liu, G.M. Chow, G. Ju, Appl. Phys. Lett. 91 (2007) 132506. [3] C.P. Luo, D.J. Sellmyer, Appl. Phys. Lett. 75 (1999) 3162. [4] C.P. Luo, S.H. Liou, L. Gao, Y. Liu, D.J. Sellmyer, Appl. Phys. Lett. 77 (2000) 2225. [5] C.P. Luo, S.H. Liou, D.J. Sellmyer, J. Appl. Phys. 87 (2000) 6941. [6] Y.K. Takahashi, T. Ohkubo, M. Ohnuma, K. Hono, J. Appl. Phys. 93 (2003) 7166. [7] K. Kang, T. Suzuki, Z.G. Zhang, C. Papusoi, J. Appl. Phys. 95 (2004) 7273. [8] M.L. Yan, R.F. Sabirianov, Y.F. Xu, X.Z. Li, D.J. Sellmyer, IEEE Trans. Magn. 40 (2004) 2470. [9] T. Maeda, T. Kai, A. Kikitsu, T. Nagase, J.I. Akiyama, Appl. Phys. Lett. 80 (2002) 2147. [10] Y.F. Ding, J.S. Chen, E. Liu, C.J. Sun, G.M. Chow, J. Appl. Phys. 97 (2005) 10H303. [11] J.S. Chen, B.C. Lim, Y.F. Ding, G.M. Chow, J. Magn. Magn. Mater. 303 (2006) 309. [12] Y.F. Ding, J.S. Chen, E. Liu, S.Y. Chow, J. Magn. Magn. Mater. 303 (2006) e238. [13] J.S. Chen, B.C. Lim, J.F. Hu, Y.K. Lim, B. Liu, G.M. Chow, Appl. Phys. Lett. 90 (2007) 042508. [14] J.S. Chen, J.F. Hu, B.C. Lim, Y.K. Lim, B. Liu, G.M. Chow, G. Ju, J. Appl. Phys. 103 (2008) 07F517.
Contents lists available at ScienceDirect
Journal of Magnetism and Magnetic Materials journal homepage: www.elsevier.com/locate/jmmm
Underlayer diffusion-induced enhancement of coercivity in high anisotropy FePt thin films J.F. Hu a,, J.S. Chen b, B.C. Lim a, B. Liu a a b
Data Storage Institute, DSI building, 5 Engineering Drive 1 (off Kent Ridge Crescent, NUS), Singapore 117608, Singapore Department of Materials Science and Engineering, National University of Singapore, Singapore 117576, Singapore
a r t i c l e in f o
a b s t r a c t
Available online 9 August 2008
The L10 ordered FePt films have been prepared at 300 1C with a basic structure of CrRu/MgO/FePt, followed by a post-annealing process at temperatures from 200 to 350 1C. The magnetic properties and the microstructure of the films were investigated. It is found that coercivity of FePt films increases greatly from 3.57 to 9.1 kOe with the increasing annealing temperature from 200 to 350 1C. The loop slope of the M–H curves decreases with the increasing annealing temperature, which is due to the grain isolation induced by MgO underlayer diffusion during the annealing process. The underlayer diffusion could be a useful approach to prepare the FePt-based composite films for high-density recording media. & 2008 Elsevier B.V. All rights reserved.
PACS: 75.50.Ss 75.50.Vv 75.70.Cn Keywords: FePt MgO Underlayer diffusion Post-annealing
1. Introduction The L10 ordered FePt thin film is one of the most promising candidates of the perpendicular recording media, due to its high magnetocrystalline anisotropy. The formation of the high anisotropy FePt film requires either post-deposition annealing to transform the fcc phase to fct phase or in-situ substrate heating during film deposition. From application viewpoint, small FePt grain sizes and magnetically decoupled FePt grains in the recording layer are required to increase the areal density and to reduce the intergranular interactions. The most common approach to reduce the intergranular exchange coupling is to fabricate the FePt-based composite films by element doping [1–8]. As mentioned before, the formation of the high anisotropy FePt film needs a high-temperature process, which is not favorable to form the small grain sizes. Efforts have been made to reduce the deposition or post-annealing temperature. It was reported that the ordering temperature of the FePt could be reduced to about 300 1C by incorporating Cu into FePt films [9]. However, the formation of the FePtCu ternary alloy demonstrated a crystallographic orientation of (111) instead of the desirable (0 0 1). The reduction of the process temperature may cause the deterioration of the FePt (0 0 1) texture and the magnetic properties accordingly. It was reported that the strain from the lattice mismatch favored the chemical ordering of the L10 FePt films [10]. However, Corresponding author. Tel.: +65 6874 8622; fax: +65 6777 2406.
E-mail address: [email protected] (J.F. Hu). 0304-8853/$ - see front matter & 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2008.08.037
continuous FePt films prepared at lower processing temperature normally showed a small coercivity. In present work, we try to develop an approach to improve the magnetic properties of the FePt films prepared at a substrate temperature of 300 1C. A postannealing process at temperatures ranging from 200 to 350 1C was used. The pure FePt films were used in present study to eliminate the microstructure variation caused by the element doping during the annealing process.
2. Experimental details Film with structure of corning glass/CrRu 30 nm/MgO 2 nm/ FePt 15 nm was prepared by an ultrahigh vacuum magnetron sputtering system. The FePt (0 0 1) texture could be formed via hetero-epitaxial growth following the orientation relationship of FePt (0 0 1) [1 0 0]JMgO (1 0 0) [1 0 0]JCrRu (0 0 2) [11 0] [2,11,12]. The substrate temperature for deposition of the CrRu and FePt was 300 1C and the MgO underlayer was deposited at 80 1C. The substrate was heated by holding for 15 min at the setting temperature before the magnetron sputtering. The prepared samples were annealed in an argon atmosphere at temperatures ranging from 200 to 350 1C for 1.5 h. The texture of the composite films was investigated by X-ray diffraction (XRD) with Cu Ka radiation. Magnetic properties were measured by a vibrating sample magnetometer (VSM). The X-ray photoelectron spectroscopy (XPS) with depth profiling capability was used to analyze the chemical composition of the films.
ARTICLE IN PRESS J.F. Hu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 3068–3070
Transmission electron microscopy (TEM) was used to observe the microstructure of the FePt films.
3. Results and discussions Fig. 1 shows the XRD patterns of the FePt thin films before and after heat treatment. It could be seen from the XRD pattern of the as-deposited FePt film that the FePt (0 0 1) texture was formed. For samples annealed at a temperature lower than 300 1C, the peak position of FePt (0 0 1) remained, while for 350 1C annealed sample, the (0 0 1) diffraction peak shifted to lower angle direction, implied an increase of the lattice constant in the direction of film normal. According to the growth mechanism of the FePt on CrRu underlayer [13], the shift of the (0 0 1) diffraction peak was caused by a stress release in the FePt thin film. The peak position of the FePt (2 0 0)+(0 0 2) shifted to lower angle direction when annealing temperature reached or over 300 1C, which suggested the increase of the fcc phase in the films. To evaluate the effect of the post-annealing on the chemical ordering of the L10 phase in a quantitative way, the ordering parameter S was calculated using following equation: S ¼ 0:72½I001 =I002 1=2
3069
as-deposited sample. Results shown in Fig. 2 indicated a decrease of the ordering parameter with the increasing annealing temperature, implying a deterioration of the FePt (0 0 1) texture. Fig. 3 illustrates the M–H loops and the coercivity variation as a function of post-annealing temperature. It was found that the coercivity of the FePt films enhanced greatly with the increasing annealing temperature. The coercivity of the as-deposited FePt film was 3.57 kOe and it increased to 6.86 and 9.1 kOe for sample annealed at 300 and 350 1C, respectively. It was noted that a kink was observed in the M–H loops of the samples annealed at 300 and 350 1C, which was due to the fcc phase [14]. Besides the enhancement of the coercivity, a decrease of the loop slope of the M–H curves with the increasing annealing
(1)
where I001 and I002 are the integrated intensities of the FePt (0 0 1) and (0 0 2) diffraction peaks from the XRD patterns. The longrange order parameter S of L10 ordered FePt films annealed at different temperature was calculated and compared to that of the
Fig. 1. The XRD patterns of the CrRu/MgO/FePt films before and after annealing.
Fig. 2. The long-range order S of L10 ordered FePt film as a function of the annealing temperature.
Fig. 3. M–H loops of and the coercivity variation of FePt films before and after annealing.
Fig. 4. Angular dependence of coercivity of CrRu/MgO/FePt films before and after annealing.
ARTICLE IN PRESS 3070
J.F. Hu et al. / Journal of Magnetism and Magnetic Materials 320 (2008) 3068–3070
Fig. 5. Mg 1s XPS narrow scan of CrRu/MgO/FePt films before and after annealing, (a) As-deposited, (b) 300 1C, (c) 350 1C.
temperature was observed, which implied the reduction of the intergranular exchange coupling. Considering the XRD results shown above, the enhancement of the coercivity was not due to the improvement of the FePt (0 0 1) texture. If the reduction of the intergranular exchange coupling was the reason, the magnetization-reversal mechanism of the film should be changed. In order to investigate the magnetization-reversal mechanism of the post-annealed FePt films, the angular dependence of the coercivity of FePt films was done, as shown in Fig. 4. The profile of the coercivity angular dependence demonstrated a tendency of the magnetization-reversal mechanism varying from typical domain wall motion behavior for as-deposited sample to S–W rotational mode for 350 1C annealed samples. The decrease of the loop slope of the M–H curves and the variation of the reversal mechanism of FePt indicated a decrease of the intergranular exchange coupling after annealing. It may be due to the isolation of the FePt grains. Since the samples investigated in our study are pure FePt films, the grain isolation is most probably resulting from the diffusion of the MgO underlayer during the annealing process. The XPS was used to check whether the diffusion of the MgO occurred by analyzing the surface chemical status of the films. Fig. 5 shows the Mg 1s XPS narrow scan at the surface of the FePt films before and after annealing. It was noted that no Mg 1s peak was observed for as-deposited sample, while for samples annealed at temperature over 300 1C, the Mg 1s peak was presented in the XPS spectra. Results confirm the diffusion of the MgO underlayer during the annealing process when treatment temperature is over 300 1C. The depth profile of the 350 1C annealed sample (data not shown) revealed that the MgO was distributed throughout the whole FePt layer. Fig. 6 shows the cross-sectional TEM images of (a) as-deposited FePt films and (b) FePt film after annealing at 350 1C. TEM observations revealed that the as-deposited sample illustrated a continuous layer structure, while the sample annealed at 350 1C demonstrated a granular type of structure. The white-contrasted grain boundaries observed in Fig. 6 (b) might be originated from the MgO diffused from the underlayer, which was consistent with the XPS measurement results.
Fig. 6. Cross-sectional TEM images of CrRu/MgO/FePt films, (a) as-deposited and (b) after annealing at 350 1C.
4. Summary In summary, the microstructure and the magnetic properties of the FePt films post-annealed at different temperatures were investigated. The diffusion of MgO from underlayer through FePt grain boundaries causes the decrease of the ordering parameter S and the enhancement of the film coercivity. The magnetizationreversal mechanism of the FePt film shows a trend from a typical domain wall motion behavior to S–W rotational mode with the increasing annealing temperature. The enhancement of coercivity and the variation of the magnetization-reversal mechanism are attributed to the FePt grain isolation. References [1] M.L. Yuan, H. Zeng, N. Powers, D.J. Sellmyer, J. Appl. Phys. 91 (2002) 8471. [2] J.S. chen, B.C. Lim, J.F. Hu, B. Liu, G.M. Chow, G. Ju, Appl. Phys. Lett. 91 (2007) 132506. [3] C.P. Luo, D.J. Sellmyer, Appl. Phys. Lett. 75 (1999) 3162. [4] C.P. Luo, S.H. Liou, L. Gao, Y. Liu, D.J. Sellmyer, Appl. Phys. Lett. 77 (2000) 2225. [5] C.P. Luo, S.H. Liou, D.J. Sellmyer, J. Appl. Phys. 87 (2000) 6941. [6] Y.K. Takahashi, T. Ohkubo, M. Ohnuma, K. Hono, J. Appl. Phys. 93 (2003) 7166. [7] K. Kang, T. Suzuki, Z.G. Zhang, C. Papusoi, J. Appl. Phys. 95 (2004) 7273. [8] M.L. Yan, R.F. Sabirianov, Y.F. Xu, X.Z. Li, D.J. Sellmyer, IEEE Trans. Magn. 40 (2004) 2470. [9] T. Maeda, T. Kai, A. Kikitsu, T. Nagase, J.I. Akiyama, Appl. Phys. Lett. 80 (2002) 2147. [10] Y.F. Ding, J.S. Chen, E. Liu, C.J. Sun, G.M. Chow, J. Appl. Phys. 97 (2005) 10H303. [11] J.S. Chen, B.C. Lim, Y.F. Ding, G.M. Chow, J. Magn. Magn. Mater. 303 (2006) 309. [12] Y.F. Ding, J.S. Chen, E. Liu, S.Y. Chow, J. Magn. Magn. Mater. 303 (2006) e238. [13] J.S. Chen, B.C. Lim, J.F. Hu, Y.K. Lim, B. Liu, G.M. Chow, Appl. Phys. Lett. 90 (2007) 042508. [14] J.S. Chen, J.F. Hu, B.C. Lim, Y.K. Lim, B. Liu, G.M. Chow, G. Ju, J. Appl. Phys. 103 (2008) 07F517.