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Pt multilayer films
Thin Solid Films 515 (2007) 8009 – 8012 www.elsevier.com/locate/tsf
Effect of the underlayer (Ag, Ti or Bi) on the magnetic properties of Fe/Pt multilayer films C. Feng a , B.H. Li a,b , G. Han a , J. Teng a , Y. Jiang a , T. Yang a , G.H. Yu a,⁎ a
Department of Materials Physics and Chemistry, University of Science and Technology Beijing, Beijing 100083, China b Department of Mathematics and Physics, Beijing Technology and Business University, Beijing 100037, China Received 18 September 2006; received in revised form 11 January 2007; accepted 30 March 2007 Available online 11 April 2007
Abstract Fe/Pt multilayer films with an Ag, Ti or Bi underlayer were prepared by magnetron sputtering and effect of the underlayer on the magnetic properties of annealed Fe/Pt multilayer films was also investigated. Our results demonstrate that Ag underlayer has little effect on the in-plane coercivity Hc of the Fe/Pt multilayer film due to the formation of Ag and L10-FePt phases in the films. The Hc of Ti/[Fe/Pt]13 film keeps on rising with the increasing annealing temperature Ta up to 500 °C and then decreases with the further increasing Ta. The reason should be attributed to the formation of some intermetallic compounds between Ti and Fe; Pt at a high Ta which is harmful to the structure of L10-FePt. The Hc of [Fe/Pt]13 films is enhanced by the Bi underlayer, which can be understood by considering the promotion of the L10-FePt phase by Bi diffusion. © 2007 Elsevier B.V. All rights reserved. Keywords: L10-FePt thin films; Ordering temperature; Underlayer; Multilayer structure; Coercivity
1. Introduction Recently, due to the large uniaxial magnetocrystalline anisotropy (7 × 106 J/m3) [1,2] of the ordered L10-FePt alloy, FePt thin films have drawn considerable attention as a potential candidate for ultrahigh density recording media. The FePt thin film deposited at the room temperature has a disordered and face-centered cubic (fcc) structure which shows a soft magnetic behavior. Thus, it is necessary to deposit the film on a heated substrate or anneal it after deposition usually above 500 °C [3] to obtain an ordered and face-centered-tetragonal (fct) structure showing hard magnetic performance. But this high temperature process results in the undesirable large grain size and surface roughness which are drawbacks for improving the areal density. So how to reduce the ordering temperature of the FePt film and achieve a high coercivity are urgent problems [4]. Recently, several attempts have been made to lower the ordering temperature with the introduction of underlayers, such as Ag, Ti, Ta and AuCu [5–9]. Endo et al. [5] achieved the lowtemperature ordering of FePt film at 300 °C with the ⁎ Corresponding author. Tel.: +86 10 62332342; fax: +86 10 62334950. E-mail address: [email protected] (G.H. Yu). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.03.181
introduction of Fe/Pt multilayer structure, but the coercivity of films was not high enough. Hsu et al. [6] found that an Ag underlayer was beneficial for reducing the ordering temperature of FePt films, and Chen et al. [7] reported that a Ti underlayer enhanced the coercivity of FePt films and obtained fine FePt grains. Our previous work [10] showed that Bi underlayer can accelerate ordering process of FePt films. But until now, the systematic research works on effect of Ag, Ti or Bi underlayer on Fe/Pt multilayer films are scarcely reported. In this paper, the L10-FePt films with Fe/Pt multilayer structure and an Ag, Ti or Bi underlayer simultaneously were prepared. Effect of the underlayer on the magnetic properties of annealed Fe/Pt multilayer films was investigated. 2. Experimental details All films were prepared by a magnetron sputtering system with a base pressure of 5 × 10− 5 Pa. Endo et al. [5] reported that the Fe/Pt multilayers films showed the highest coercivity when Fe and Pt layer have equal thickness. So, in this paper, the films with the structure of Ag, Ti or Bi (40 nm)/[Fe(1.5 nm)/Pt (1.5 nm)]13 and [Fe(1.5 nm)/Pt(1.5 nm)]13 were deposited on glass substrates by magnetron sputtering Ag, Ti, Bi, Fe and Pt
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targets with high purity (99.99%). The composition of FePt in all films was measured to be about Fe48Pt52 by inductively coupled plasma atomic emission spectrometer with the error less than 1%. To conveniently compare with above Fe/Pt multilayer films, FePt (40 nm) films with composition of Fe48Pt52 were deposited on glass at the same condition. All films were grown on 250 °C heated clean glass substrates. Ar pressure was kept at 0.45 Pa during sputtering. The as-deposited films were annealed at the temperature of 300–550 °C in a vacuum of 3 × 10− 5 Pa for 20 min. Magnetic properties were measured using an alternating gradient field magnetometer (AGM Micromag2900) with an applied in-plane field up to 1432 kA/m. The crystal structures of all films were identified by a MXT21VAHF X-ray diffractometer (XRD) using Cu–Kα radiation (40 kV × 200 mA) and a graphite monochromator using a continuous scanning mode with rate of 8° (2θ)/min. The chemical status of surface elements was studied using X-ray photoelectron spectroscopy (XPS). An Mg Kα radiation was used with the X-ray source running at 14.5 kV. The energy analyzer was operated at constant pass energy of 50 eV. The XPS detectable depth d = 3λ sin α, where λ and α are respectively inelastic mean-free paths (IMFPs) for photoelectrons and a take off angle for photoelectrons with respect to the samples surface plane [11]. For an Mg Kα radiation source, the IMFPs for Bi 4f in Bi are about 2.15 nm. So the detectable depth d of Bi atoms is 6.45 nm when α = 90°. 3. Results and discussion Fig. 1 shows the dependence of the in-plane coercivity Hc for FePt (40 nm) and [Fe (1.5 nm)/Pt (1.5 nm)]13 films on annealing temperature Ta. Ta ranges from 300 to 550 °C, and the annealing time is 20 min. It has been reported that the Hc of a FePt film is mainly affected with its ordering degree [12]. From Fig. 1, a Ta around 500 °C is required for realizing a high Hc = 437.8 kA/m for the FePt film. On the other hand, Ta = 350 °C is high enough for the [Fe/Pt]13 film to remarkably increase Hc to 421.9 kA/m. Moreover, the Hc of [Fe/Pt]13 films
Fig. 1. The dependence of the in-plane coercivity Hc for FePt (40 nm) and [Fe (1.5 nm)/Pt (1.5 nm)]13 films on annealing temperature Ta for 20 min. Ta ranges from 300 to 550 °C.
Fig. 2. The variation of Hc for Ag, Ti or Bi (40 nm)/[Fe (1.5 nm)/Pt (1.5 nm)]13 and [Fe (1.5 nm)/Pt (1.5 nm)]13 films with Ta for 20 min. Ta ranges from 300 to 550 °C.
is far larger than that of the FePt film at the same Ta. In other words, the low-temperature ordering L10-FePt films with enhanced coercivity have been achieved by introduction of Fe/Pt multilayer structure. As reported by Endo et al. [5], the structure of Fe/Pt multilayer structure offers an extra driven force—interfacial energy. So the appreciable reduction of ordering temperature is correlated with rapid diffusion at the interface of Fe/Pt, and subsequently the faster corruption of the multilayer structure which leads to form the better ordered L10FePt phase and the enhanced Hc of films. So it was based on [Fe/Pt]13 film that effect of underlayer on magnetic performance was studied below. Fig. 2 shows the variation of Hc for Ag, Ti or Bi (40 nm)/[Fe (1.5 nm)/Pt (1.5 nm)]13 and [Fe (1.5 nm)/Pt (1.5 nm)]13 films with Ta. Ta ranges from 300 to 550 °C, and the annealing time is 20 min. As shown in the figure, the Hc of Ag, Ti or Bi/[Fe/Pt]13 greatly increases beyond 319.4 kA/m at Ta = 350 °C, implying Ag, Ti or Bi underlayer cannot further decrease the ordering temperature of [Fe/Pt]13 film (350 °C). Compared with [Fe/Pt]13 films without any underlayer, Ag underlayer scarcely affects Hc of films. The Hc of Ti/[Fe/Pt]13 film keeps on rising with raising annealing temperature Ta up to 500 °C and declines by further increasing Ta. But the Hc of films is enhanced by Bi underlayer. In order to study the reason for the influences of those underlayers on magnetic performance, we carefully explored the structural change for the Fe/Pt multilayer films with Ag, Ti and Bi underlayers depending on Ta. The XRD patterns are demonstrated in Fig. 3 (a)–(c). The ordering parameter S [5] can be cited to quantitatively describe the ordering process. Fig. 3 (d) shows the variation of c/a ratio of FePt lattice with Ta. The c/ a ratio of all the films is round 1 at Ta = 300 °C, indicating that the FePt lattice is still remaining unity. It is clearly see that the L10-FePt superlattice diffraction (001) and (110) peaks appear in the XRD patterns for all films and the c/a ratio decreases below 0.99, suggesting the formation of the FePt lattice with tetragonality and the ordering temperature of Ag, Ti or Bi/[Fe/ Pt]13 films are 350 °C. As to Ag/[Fe/Pt]13 film, all L10-FePt superlattice peaks become stronger and the c/a ratio declines
C. Feng et al. / Thin Solid Films 515 (2007) 8009–8012
Fig. 3. The XRD patterns of Ag/[Fe/Pt]13 (a); Ti/[Fe/Pt]13 (b); Bi/[Fe/Pt]13 (c) depending on Ta. The curves are offset vertically for clarity. (d) The variation of c/a ratio of the FePt lattice with Ta.
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monotonously with a increase of Ta, indicating the formation of L10-FePt with higher S in the film. The strong Ag diffraction peaks also emerge in Fig. 3 (a). According to the Ag–Fe–Pt ternary phase diagrams [13], Ag does not form a solid solution with FePt and it is supposed to form L10-FePt and Ag compound phases in the film during annealing. Moreover, Ag/ [Fe/Pt]13 film has very close c/a ratio with that of [Fe/Pt]13 film, proving Ag has little effect on L10-FePt crystal lattice and subsequently on the Hc of [Fe/Pt]13 film. Our previous work showed that it was hardly to see apparent Ti diffraction peaks in the XRD patterns of a pure Ti film with 40 nm thickness (not shown here), indicating the poor crystallization of Ti. So we did not find any obvious peaks concerning to Ti in Fig. 3 (b). The L10-FePt superlattice peaks become stronger and the c/a ratio decreases with a raise of Ta below 500 °C. But a different trend happens to Ti/[Fe/Pt]13 films when Ta is higher than 500 °C. The FePt (001) peak decreases, (110) peak vanishes and (200) and (002) peaks merge into one fcc (200) peak. That suggests that the Ti underlayer does great damage to the L10-FePt phase at high Ta due to the diffusion of Ti atoms. It has been found from the Ti–Fe–Pt ternary phase diagrams that Ti can be solvent with Fe or Pt and may form many intermetallic compounds, such as FeTi, Fe2Ti, PtTi, PtTi3 and Pt8Ti [13]. Due to the diffusion and interfacial reactions during annealing [14,15], above various kinds of supplementary intermetallic compounds may be formed at high Ta, which was considered to induce the destruction of L10-FePt phase and subsequently the decreasing of Hc. Compared with [Fe/Pt]13 films, Bi/[Fe/Pt]13 films have much lower c/a ratio after annealing. This means that the L10FePt films with enhanced S can be realized by Bi underlayer. For example, the S of the Bi/[Fe/Pt]13 film annealed at 350 °C is 0.8 which is far higher than that of [Fe/Pt]13 film (S = 0.52). Compared with Fe and Pt atom, Bi atom possesses a large atomic radii and lower surface free energy. Bi atoms are not incorporated in the crystal lattice of FePt. We attribute the enhanced ordering process by Bi underlayer to the large amount of Bi diffusion during low-temperature annealing. To get a proof, the distribution of Bi atoms on the Bi/[Fe/Pt]13 film surface before and after annealing at Ta = 400 °C and 550 °C was studied by XPS. Fig. 4 shows the Bi high-solution XPS spectra. Very weak Bi peaks exist in Fig. 4 (a). This indicates that small amount or quantity of Bi atoms in the Bi underlayer migrates to the surface of FePt films after the deposition. Strong Bi 4f7/2 and 4f5/2 peaks are only found in Fig. 4 (c), implying that serious diffusion occurs with Ta rising up to 400 °C. Bi peaks are hardly to be seen in Fig. 4 (b), which suggests that a large amount of segregated Bi atoms at the surface disappears when Ta = 550 °C, indicating that Bi excretes from the film which may be attributed to sublimation due to its relatively low saturated vapor pressure [16]. This point can also be proved by the disappearance of Bi diffraction peaks in Fig. 3 (c). According to the report by Kitakami et al. [16], a lot of defects are produced in the CoPt film during Bi diffusion. Based on the knowledge of similar crystal lattice and parameters of L10– CoPt with L10-FePt, a lot of defects, which are favorable to the rearrangement of Fe and Pt atoms, are supposed to be produced during the Bi diffusion in this paper. This leads to the great
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2003008003), the Key Project of Science and Technology of the Ministry of Education (Grant No. 104023), the Program for New Century Excellent Talents in University (Grant No. NCEF04-0104) and National Natural Science foundation of China (Grant No.90607020, No.50471093 and No. 50571007). References
Fig. 4. The Bi high-resolution XPS spectra of Bi/[Fe/Pt]13 films surface. (a) Before annealing, (b) Ta = 550 °C, (c) Ta = 400 °C. The curves are offset vertically for clarity.
enhancement of the ordering degree. Subsequently the Hc of L10-FePt thin films is significantly increased. 4. Conclusions In summary, effects of Ag, Ti and Bi underlayers on the magnetic properties of annealed Fe/Pt multilayer films were investigated. Our results demonstrate that Ag underlayer has little effect on the Hc of the [Fe/Pt]13 film. The Hc of Ti/[Fe/ Pt]13 film keeps on rising with raising annealing temperature Ta up to 500 °C and declines by further increasing Ta. The Hc of [Fe/Pt]13 films is obviously enhanced by Bi underlayer, which are associated with Bi diffusion. Above results can be considered to be associated with the interaction between the underlayer and L10-FePt phase. Acknowledgements The present work was supported in part by the Ph. D. Programs foundation of Ministry of Education (Grant No.
[1] D. Weller, A. Moser, IEEE Trans. Magn. 35 (1999) 4423. [2] R. Wood, IEEE Trans. Magn. 36 (2000) 36. [3] C.M. Kuo, P.C. Kuo, H.C. Wu, Y.D. Yao, C.H. Lin, J. Appl. Phys. 85 (1999) 4886. [4] B.H. Li, C. Feng, T. Yang, J. Teng, Z.H. Zhai, G.H. Yu, F.W. Zhu, J. Phys. D: Appl. Phys. 39 (2006) 1018. [5] Y. Endo, N. Kikuchi, O. Kitakami, Y. Shimada, J. Appl. Phys. 89 (2001) 7065. [6] Y.N. Hsu, S. Jeong, D.E. Laughlin, J. Appl. Phys. 89 (2001) 7068. [7] S.C. Chen, P.C. Kuo, S.T. Kuo, A.C. Sun, C.T. Lie, C.Y. Chou, Mater. Sci. Eng. B, Solid-State Mater. Adv. Technol. 98 (2003) 244. [8] Y. Zhu, J.W. Cai, Appl. Phys. Lett. 87 (2005) 032504. [9] L.J. Zhang, J.W. Cai, F.B. Meng, Y.X. Li, Acta Phys. Sin. 55 (2006) 450 (in Chinese). [10] C. Feng, B.H. Li, G. Han, J. Teng, Y. Jiang, Q.L. Liu, G.H. Yu, Appl. Phys. Lett. 88 (2006) 232109. [11] G.H. Yu, M.H. Li, F.W. Zhu, H.W. Jiang, W.Y. Lai, C.L. Chai, Appl. Phys. Lett. 82 (2003) 94. [12] D.J. Sellmyer, C.P. Luo, M.L. Yan, Y. Liu, IEEE Trans. Magn. 37 (2001) 1286. [13] H. Okamoto, P.R. Subramanian, L. Kacprzak, Binary Alloy Phase Diagrams. Asm International. The Materials Information Society: 1992, Second edition, p.1273, p.1777. [14] S. Hsieh, D. Beck, T. Matsumoto, B.E. Koel, Thin Solid Films 466 (2004) 123. [15] M. Singh, I.P. Jain, Int. J. Hydrogen Energy 21 (1996) 367. [16] O. Kitakami, Y. Shimada, K. Oikawa, H. Daimon, K. Fukamichi, Appl. Phys. Lett. 78 (2001) 1104.
Effect of the underlayer (Ag, Ti or Bi) on the magnetic properties of Fe/Pt multilayer films C. Feng a , B.H. Li a,b , G. Han a , J. Teng a , Y. Jiang a , T. Yang a , G.H. Yu a,⁎ a
Department of Materials Physics and Chemistry, University of Science and Technology Beijing, Beijing 100083, China b Department of Mathematics and Physics, Beijing Technology and Business University, Beijing 100037, China Received 18 September 2006; received in revised form 11 January 2007; accepted 30 March 2007 Available online 11 April 2007
Abstract Fe/Pt multilayer films with an Ag, Ti or Bi underlayer were prepared by magnetron sputtering and effect of the underlayer on the magnetic properties of annealed Fe/Pt multilayer films was also investigated. Our results demonstrate that Ag underlayer has little effect on the in-plane coercivity Hc of the Fe/Pt multilayer film due to the formation of Ag and L10-FePt phases in the films. The Hc of Ti/[Fe/Pt]13 film keeps on rising with the increasing annealing temperature Ta up to 500 °C and then decreases with the further increasing Ta. The reason should be attributed to the formation of some intermetallic compounds between Ti and Fe; Pt at a high Ta which is harmful to the structure of L10-FePt. The Hc of [Fe/Pt]13 films is enhanced by the Bi underlayer, which can be understood by considering the promotion of the L10-FePt phase by Bi diffusion. © 2007 Elsevier B.V. All rights reserved. Keywords: L10-FePt thin films; Ordering temperature; Underlayer; Multilayer structure; Coercivity
1. Introduction Recently, due to the large uniaxial magnetocrystalline anisotropy (7 × 106 J/m3) [1,2] of the ordered L10-FePt alloy, FePt thin films have drawn considerable attention as a potential candidate for ultrahigh density recording media. The FePt thin film deposited at the room temperature has a disordered and face-centered cubic (fcc) structure which shows a soft magnetic behavior. Thus, it is necessary to deposit the film on a heated substrate or anneal it after deposition usually above 500 °C [3] to obtain an ordered and face-centered-tetragonal (fct) structure showing hard magnetic performance. But this high temperature process results in the undesirable large grain size and surface roughness which are drawbacks for improving the areal density. So how to reduce the ordering temperature of the FePt film and achieve a high coercivity are urgent problems [4]. Recently, several attempts have been made to lower the ordering temperature with the introduction of underlayers, such as Ag, Ti, Ta and AuCu [5–9]. Endo et al. [5] achieved the lowtemperature ordering of FePt film at 300 °C with the ⁎ Corresponding author. Tel.: +86 10 62332342; fax: +86 10 62334950. E-mail address: [email protected] (G.H. Yu). 0040-6090/$ - see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2007.03.181
introduction of Fe/Pt multilayer structure, but the coercivity of films was not high enough. Hsu et al. [6] found that an Ag underlayer was beneficial for reducing the ordering temperature of FePt films, and Chen et al. [7] reported that a Ti underlayer enhanced the coercivity of FePt films and obtained fine FePt grains. Our previous work [10] showed that Bi underlayer can accelerate ordering process of FePt films. But until now, the systematic research works on effect of Ag, Ti or Bi underlayer on Fe/Pt multilayer films are scarcely reported. In this paper, the L10-FePt films with Fe/Pt multilayer structure and an Ag, Ti or Bi underlayer simultaneously were prepared. Effect of the underlayer on the magnetic properties of annealed Fe/Pt multilayer films was investigated. 2. Experimental details All films were prepared by a magnetron sputtering system with a base pressure of 5 × 10− 5 Pa. Endo et al. [5] reported that the Fe/Pt multilayers films showed the highest coercivity when Fe and Pt layer have equal thickness. So, in this paper, the films with the structure of Ag, Ti or Bi (40 nm)/[Fe(1.5 nm)/Pt (1.5 nm)]13 and [Fe(1.5 nm)/Pt(1.5 nm)]13 were deposited on glass substrates by magnetron sputtering Ag, Ti, Bi, Fe and Pt
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targets with high purity (99.99%). The composition of FePt in all films was measured to be about Fe48Pt52 by inductively coupled plasma atomic emission spectrometer with the error less than 1%. To conveniently compare with above Fe/Pt multilayer films, FePt (40 nm) films with composition of Fe48Pt52 were deposited on glass at the same condition. All films were grown on 250 °C heated clean glass substrates. Ar pressure was kept at 0.45 Pa during sputtering. The as-deposited films were annealed at the temperature of 300–550 °C in a vacuum of 3 × 10− 5 Pa for 20 min. Magnetic properties were measured using an alternating gradient field magnetometer (AGM Micromag2900) with an applied in-plane field up to 1432 kA/m. The crystal structures of all films were identified by a MXT21VAHF X-ray diffractometer (XRD) using Cu–Kα radiation (40 kV × 200 mA) and a graphite monochromator using a continuous scanning mode with rate of 8° (2θ)/min. The chemical status of surface elements was studied using X-ray photoelectron spectroscopy (XPS). An Mg Kα radiation was used with the X-ray source running at 14.5 kV. The energy analyzer was operated at constant pass energy of 50 eV. The XPS detectable depth d = 3λ sin α, where λ and α are respectively inelastic mean-free paths (IMFPs) for photoelectrons and a take off angle for photoelectrons with respect to the samples surface plane [11]. For an Mg Kα radiation source, the IMFPs for Bi 4f in Bi are about 2.15 nm. So the detectable depth d of Bi atoms is 6.45 nm when α = 90°. 3. Results and discussion Fig. 1 shows the dependence of the in-plane coercivity Hc for FePt (40 nm) and [Fe (1.5 nm)/Pt (1.5 nm)]13 films on annealing temperature Ta. Ta ranges from 300 to 550 °C, and the annealing time is 20 min. It has been reported that the Hc of a FePt film is mainly affected with its ordering degree [12]. From Fig. 1, a Ta around 500 °C is required for realizing a high Hc = 437.8 kA/m for the FePt film. On the other hand, Ta = 350 °C is high enough for the [Fe/Pt]13 film to remarkably increase Hc to 421.9 kA/m. Moreover, the Hc of [Fe/Pt]13 films
Fig. 1. The dependence of the in-plane coercivity Hc for FePt (40 nm) and [Fe (1.5 nm)/Pt (1.5 nm)]13 films on annealing temperature Ta for 20 min. Ta ranges from 300 to 550 °C.
Fig. 2. The variation of Hc for Ag, Ti or Bi (40 nm)/[Fe (1.5 nm)/Pt (1.5 nm)]13 and [Fe (1.5 nm)/Pt (1.5 nm)]13 films with Ta for 20 min. Ta ranges from 300 to 550 °C.
is far larger than that of the FePt film at the same Ta. In other words, the low-temperature ordering L10-FePt films with enhanced coercivity have been achieved by introduction of Fe/Pt multilayer structure. As reported by Endo et al. [5], the structure of Fe/Pt multilayer structure offers an extra driven force—interfacial energy. So the appreciable reduction of ordering temperature is correlated with rapid diffusion at the interface of Fe/Pt, and subsequently the faster corruption of the multilayer structure which leads to form the better ordered L10FePt phase and the enhanced Hc of films. So it was based on [Fe/Pt]13 film that effect of underlayer on magnetic performance was studied below. Fig. 2 shows the variation of Hc for Ag, Ti or Bi (40 nm)/[Fe (1.5 nm)/Pt (1.5 nm)]13 and [Fe (1.5 nm)/Pt (1.5 nm)]13 films with Ta. Ta ranges from 300 to 550 °C, and the annealing time is 20 min. As shown in the figure, the Hc of Ag, Ti or Bi/[Fe/Pt]13 greatly increases beyond 319.4 kA/m at Ta = 350 °C, implying Ag, Ti or Bi underlayer cannot further decrease the ordering temperature of [Fe/Pt]13 film (350 °C). Compared with [Fe/Pt]13 films without any underlayer, Ag underlayer scarcely affects Hc of films. The Hc of Ti/[Fe/Pt]13 film keeps on rising with raising annealing temperature Ta up to 500 °C and declines by further increasing Ta. But the Hc of films is enhanced by Bi underlayer. In order to study the reason for the influences of those underlayers on magnetic performance, we carefully explored the structural change for the Fe/Pt multilayer films with Ag, Ti and Bi underlayers depending on Ta. The XRD patterns are demonstrated in Fig. 3 (a)–(c). The ordering parameter S [5] can be cited to quantitatively describe the ordering process. Fig. 3 (d) shows the variation of c/a ratio of FePt lattice with Ta. The c/ a ratio of all the films is round 1 at Ta = 300 °C, indicating that the FePt lattice is still remaining unity. It is clearly see that the L10-FePt superlattice diffraction (001) and (110) peaks appear in the XRD patterns for all films and the c/a ratio decreases below 0.99, suggesting the formation of the FePt lattice with tetragonality and the ordering temperature of Ag, Ti or Bi/[Fe/ Pt]13 films are 350 °C. As to Ag/[Fe/Pt]13 film, all L10-FePt superlattice peaks become stronger and the c/a ratio declines
C. Feng et al. / Thin Solid Films 515 (2007) 8009–8012
Fig. 3. The XRD patterns of Ag/[Fe/Pt]13 (a); Ti/[Fe/Pt]13 (b); Bi/[Fe/Pt]13 (c) depending on Ta. The curves are offset vertically for clarity. (d) The variation of c/a ratio of the FePt lattice with Ta.
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monotonously with a increase of Ta, indicating the formation of L10-FePt with higher S in the film. The strong Ag diffraction peaks also emerge in Fig. 3 (a). According to the Ag–Fe–Pt ternary phase diagrams [13], Ag does not form a solid solution with FePt and it is supposed to form L10-FePt and Ag compound phases in the film during annealing. Moreover, Ag/ [Fe/Pt]13 film has very close c/a ratio with that of [Fe/Pt]13 film, proving Ag has little effect on L10-FePt crystal lattice and subsequently on the Hc of [Fe/Pt]13 film. Our previous work showed that it was hardly to see apparent Ti diffraction peaks in the XRD patterns of a pure Ti film with 40 nm thickness (not shown here), indicating the poor crystallization of Ti. So we did not find any obvious peaks concerning to Ti in Fig. 3 (b). The L10-FePt superlattice peaks become stronger and the c/a ratio decreases with a raise of Ta below 500 °C. But a different trend happens to Ti/[Fe/Pt]13 films when Ta is higher than 500 °C. The FePt (001) peak decreases, (110) peak vanishes and (200) and (002) peaks merge into one fcc (200) peak. That suggests that the Ti underlayer does great damage to the L10-FePt phase at high Ta due to the diffusion of Ti atoms. It has been found from the Ti–Fe–Pt ternary phase diagrams that Ti can be solvent with Fe or Pt and may form many intermetallic compounds, such as FeTi, Fe2Ti, PtTi, PtTi3 and Pt8Ti [13]. Due to the diffusion and interfacial reactions during annealing [14,15], above various kinds of supplementary intermetallic compounds may be formed at high Ta, which was considered to induce the destruction of L10-FePt phase and subsequently the decreasing of Hc. Compared with [Fe/Pt]13 films, Bi/[Fe/Pt]13 films have much lower c/a ratio after annealing. This means that the L10FePt films with enhanced S can be realized by Bi underlayer. For example, the S of the Bi/[Fe/Pt]13 film annealed at 350 °C is 0.8 which is far higher than that of [Fe/Pt]13 film (S = 0.52). Compared with Fe and Pt atom, Bi atom possesses a large atomic radii and lower surface free energy. Bi atoms are not incorporated in the crystal lattice of FePt. We attribute the enhanced ordering process by Bi underlayer to the large amount of Bi diffusion during low-temperature annealing. To get a proof, the distribution of Bi atoms on the Bi/[Fe/Pt]13 film surface before and after annealing at Ta = 400 °C and 550 °C was studied by XPS. Fig. 4 shows the Bi high-solution XPS spectra. Very weak Bi peaks exist in Fig. 4 (a). This indicates that small amount or quantity of Bi atoms in the Bi underlayer migrates to the surface of FePt films after the deposition. Strong Bi 4f7/2 and 4f5/2 peaks are only found in Fig. 4 (c), implying that serious diffusion occurs with Ta rising up to 400 °C. Bi peaks are hardly to be seen in Fig. 4 (b), which suggests that a large amount of segregated Bi atoms at the surface disappears when Ta = 550 °C, indicating that Bi excretes from the film which may be attributed to sublimation due to its relatively low saturated vapor pressure [16]. This point can also be proved by the disappearance of Bi diffraction peaks in Fig. 3 (c). According to the report by Kitakami et al. [16], a lot of defects are produced in the CoPt film during Bi diffusion. Based on the knowledge of similar crystal lattice and parameters of L10– CoPt with L10-FePt, a lot of defects, which are favorable to the rearrangement of Fe and Pt atoms, are supposed to be produced during the Bi diffusion in this paper. This leads to the great
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C. Feng et al. / Thin Solid Films 515 (2007) 8009–8012
2003008003), the Key Project of Science and Technology of the Ministry of Education (Grant No. 104023), the Program for New Century Excellent Talents in University (Grant No. NCEF04-0104) and National Natural Science foundation of China (Grant No.90607020, No.50471093 and No. 50571007). References
Fig. 4. The Bi high-resolution XPS spectra of Bi/[Fe/Pt]13 films surface. (a) Before annealing, (b) Ta = 550 °C, (c) Ta = 400 °C. The curves are offset vertically for clarity.
enhancement of the ordering degree. Subsequently the Hc of L10-FePt thin films is significantly increased. 4. Conclusions In summary, effects of Ag, Ti and Bi underlayers on the magnetic properties of annealed Fe/Pt multilayer films were investigated. Our results demonstrate that Ag underlayer has little effect on the Hc of the [Fe/Pt]13 film. The Hc of Ti/[Fe/ Pt]13 film keeps on rising with raising annealing temperature Ta up to 500 °C and declines by further increasing Ta. The Hc of [Fe/Pt]13 films is obviously enhanced by Bi underlayer, which are associated with Bi diffusion. Above results can be considered to be associated with the interaction between the underlayer and L10-FePt phase. Acknowledgements The present work was supported in part by the Ph. D. Programs foundation of Ministry of Education (Grant No.
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