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Structure evolution, magnetic domain structures and magnetic properties of CoPt–C nanocomposite films
ARTICLE IN PRESS
Physica B 351 (2004) 77–82
Structure evolution, magnetic domain structures and magnetic properties of CoPt–C nanocomposite films Hao Wanga,*, Fujun Yanga, Mingzhe Hua, Bin Zhoua, Haoshuang Gua, M.F. Chiahb, W.Y. Cheungb, S.P. Wongb b
a Faculty of Physics and Electronic Technology, Hubei University, Wuhan 430062, PR China Department of Electronic Engineering and Materials Science and Technology Research Center, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong, PR China
Received 25 February 2004; received in revised form 23 April 2004; accepted 7 May 2004
Abstract CoPt–C nanocomposite thin films with various carbon concentrations were prepared by a pulsed filtered vacuum arc deposition technique. Annealing was performed in vacuum for 1 h at various temperatures. The structure evolution and magnetic properties of the films were characterized by non-Rutherford backscattering spectrometry, X-ray diffraction, magnetic force microscopy and vibrating sample magnetometer. The as-deposited samples are magnetic soft with facecentral-cubic phase CoPt nanocrystals encapsulated in carbon matrix. After annealing at a sufficiently high temperature, depending on the carbon composition, the CoPt nanocrystals transform to a face-centered-tetragonal phase. For the film with a particular composition of (Co0.44Pt0.56)0.55C0.45, the coercivity is observed to increase with increasing annealing temperature, up to a value of 7 kOe at an annealing temperature of 750 C, and the grain size increases from 9 to 20 nm. r 2004 Elsevier B.V. All rights reserved. PACS: 75.70.Ak; 75.50.Ss; 75.70.Kw Keywords: High-density magnetic recording; CoPt–C nanocomposite films; Ferromagnetic; Magnetic domain structure
1. Introduction In recent years, high uniaxial magnetocrystalline anisotropy (Ku) nanocomposite films have gained increasing attentions, due to their potential appli*Corresponding author. Tel.: +86-27-6251-2839; fax: +8627-8866-2550. E-mail address: [email protected] (Hao Wang).
cations as ultra-high density magnetic recording media [1–4]. In the case of ultra-high density magnetic recording, media with larger coercivities, smaller grain sizes and weakly intergrain exchange coupling are required to yield adequate high signal-to-noise ratio. However, for grain size less than 10 nm, the issue of thermal instability originated from superparamagnetism has to be considered. To avoid this problem, high Ku
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.05.014
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materials would be required to achieve smaller stable grains. The structure characteristics of nanocomposite films may satisfy the requirements for ultra-high-density magnetic recording media. For example of films consisting of magnetic grains encapsulated in carbon, the most important advantage of carbon encapsulation is the increase of the effective distance of neighboring magnetic grains so that the exchange coupling between them is significantly weakened or eliminated [5]. Grains separation by carbon layer may also play a role of interfering the increase of grain size and the coalescence of neighboring grains. Face-central-tetragonal (FCT) phase of nearequiatomic composition CoPt- and FePt-based nanocomposite films [2–4, 6–12] have been studied extensively in a last few years due to their very high Ku values. CoPt–C nanocomposite films of coercivities of up to 12 kOe with grain sizes of 7– 20 nm have been obtained at various C concentrations produced at suitable annealing temperatures [2,7]. The magnetic properties for (Co86Cr14)Pt–C films were reported to be better than those of CoPt–C films, with coercivity values ranging from 2 to 10 kOe and remanence ratio close to 1 [8]. In CoPt–B films with a wide range of B composition [9], the minimum grain size achieved is about 20 nm for the 20 vol% B sample after annealing at 600 C with a coercivity of only about 3 kOe. Larger coercivities up to 10 kOe can be obtained after annealing at 650–700 C but the grain sizes grew to close to 30 nm. In CoPt–Ag nanocomposite films, coercivity values in the range of 1–17 kOe with grain sizes in the range of 7–100 nm have been obtained by adjusting the annealing conditions and the relative CoPt–Ag composition [10]. For CoPt–SiO2 nanocomposite films with a wide range of CoPt composition [11], coercivities in the range of 0.07–2.3 kOe were obtained when the grain sizes were in the range of 3–8 nm. An overview of various high Hc nanocomposite films has appeared more recently [4]. In this work, we have employed the pulsed filtered vacuum arc deposition technique to prepare CoPt–C nanocomposite films and studied their structure evolution, magnetic domain structures and magnetic properties.
2. Experiments CoPt–C nanocomposite thin films about 40– 50 nm thick with various C concentration were prepared by a 3-source pulsed filtered vacuum arc deposition system. The details of the system have been described elsewhere in detail for the preparation of Co–C films [13–17]. In this work, three adjacent sources with pure graphite, platinum and cobalt as the cathode materials were operated simultaneously in a pulse mode with a pulse duration of 2.5 ms. Of the three sources, the platinum source was positioned at the center. Thermally grown SiO2 films of about 100 nm on Si (1 0 0) wafers were used as the substrate. The substrate was placed at the center of the chamber facing to the platinum source and a negative bias voltage of 80 V was applied. The composition of the films was varied by adjusting the arc discharge conditions and was monitored by the integrated charges arriving at the sample holder from the respective arc sources. After deposition, thermal annealing was performed in a vacuum furnace (o10 3 Pa) for 1 h at various temperatures. The composition and thickness of the CoPt–C films were determined by non-Rutherford backscattering spectrometry (NRBS) using a 2 MV tandem accelerator. The NRBS was performed with a beam of 3.5 MeV 4He2+ ions, which have a non-Rutherford resonance scattering cross-section with the C atoms so that the C detection sensibility is greatly enhanced to achieve a more accurate determination of the film composition. The structural properties were analyzed by X-ray diffraction (XRD). The magnetic properties were characterized by a vibrating sample magnetometer (VSM). The magnetic domain structures were investigated by magnetic force microscopy (MFM). MFM imaging were performed using the tapping and lift modes under ambient conditions, using a Nanoscope III scanning probe microscope with a vertically magnetized hard Co-alloy-coated silicon tip.
3. Results and discussion There are two phases of CoPt, namely, face centered-cubic (FCC) CoPt and FCT CoPt. The
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(fct 110)
Intensity (a.u.)
(fct 001)
(111)
Ta=650 °C
Ta=600 °C
Ta=550 °C
20
40
60 2θ
80
100
Fig. 1. XRD patterns for (Co0.44Pt0.56)0.55C0.45 films after annealing at various temperatures in vacuum for 1 h.
25 20
d (nm)
FCT phase is corresponding to a larger Ku value. In order to obtain the ordered FCT phase from disordered FCC phase, high-temperature annealing is required to overcome the energy barrier for superlattice ordering. Fig. 1 shows the XRD patterns of (Co0.44Pt0.56)0.55C0.45 films after annealing at various temperatures. The XRD patterns for FCC CoPt and FCT CoPt are quite similar except that there are several additional peaks for the FCT phase such as FCT (0 0 1) and FCT (1 1 0) [12]. The other peaks in the XRD patterns can be contributed by both the FCC and FCT phases. The as-deposited films were FCC CoPt nanocrystals encapsulated in carbon matrix. After annealing at 550 C, as shown in Fig. 1, still only those peaks from FCC CoPt could be observed. While the annealing temperature (Ta) increasing to 600 C, broad FCT (0 0 1) and FCT (1 1 0) peaks emerged, indicating the formation of the FCT CoPt phase. These two peaks became sharper as Ta increased to 650 C, indicating more and larger FCT CoPt grains are formed. The grain size is estimated by Scherrer’s formula using the (1 1 1) peak width from the XRD patterns. Shown in Fig. 2 is the grain size d against annealing temperature for CoPt–C films with various compositions as indicated. Since the observed (1 1 1) peak is contributed by both the FCC and FCT phases in a wide annealing temperature range, the average peak must be broader than each individual
79
15 10 5
(Co0.42Pt0.58)0.88C0.12
0
(Co0.44Pt0.56)0.55C0.45 (Co0.4Pt0.6)0.4C0.6 500
550
600 650 Ta (°C)
700
750
Fig. 2. Grain size d against annealing temperature for samples of various compositions as indicated.
one, and the grain size taken from the average width is probably smaller than the real value. The grain size of CoPt nanocrystals in the films generally increases with Ta due to thermal growth of the grains. It is also seen that higher carbon content will result in smaller as-deposited grain size as well as suppression of grain growth upon annealing. In fact, XRD results also showed that for the film with the highest carbon content, (Co0.40Pt0.60)0.40C0.60, the FCT peaks were observed only after Ta equal to or higher than 700 C. Shown in Fig. 3 are the MFM images of the film with a composition of (Co0.44Pt0.56)0.55C0.45 before and after annealing. The bright and dark regions in the MFM images are associated with domains of which the vertical component of the magnetization vector is parallel or anti-parallel to the film normal. As shown in Fig. 3a, a long-range stripe domain structure is observed in the as-deposited CoPt–C sample, indicating strong inter-grains exchange coupling. After annealing at 650 C, a short-range dispersed domain structure is observed in Fig. 3b, indicating the magnetic grains growth and separation from carbon matrix and the weakened inter-grains exchange coupling. The sharp change in the domain structures as presented in the MFM images is dominated by the change of nanostructure, the CoPt grains growth and separation with increasing annealing temperature in the
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Fig. 3. MFM images of (Co0.44Pt0.56)0.55C0.45 films; (a) as-deposited and (b) Ta=650 C.
M (x102 emu/cm3)
4
0 Ta=550 °C Ta=600 °C Ta=650 °C
-4
Ta=750 °C -10
-5
0 H (kOe)
5
10
Fig. 4. Magnetic hysteresis loops measured at 300 K for the (Co0.44Pt0.56)0.55C0.45 films after annealing at the temperature as indicated.
films. The phase transition from FCC CoPt phase to the high Ku value FCT CoPt phase in the films as revealed by XRD results, also plays an important role in this evolution. Fig. 4 shows the in-plane magnetic hysteresis loops of the (Co0.44Pt0.56)0.55C0.45 films after annealing at various temperatures in vacuum for 1 h. It is seen from Fig. 4 that as Ta increases, there is no significant change of the saturation magnetization of the films at a value of about 300 emu/ cm3. The film still showed a soft ferromagnetic characteristic even after annealing at 550 C with a
coercivity of about 100 Oe and a saturation magnetic field of about 200 Oe, corresponding to the FCC CoPt phase in the film. After annealing at a higher temperature of 600 C, the film became magnetically harder exhibiting a coercivity of 760 Oe. The increase in coercivity is attributed to the formation of the FCT CoPt phase. The coercivity increased further with Ta increasing, up to a value of 7 kOe at Ta=750 C. This is also understandable to be the result of FCT CoPt grain growth and more complete FCC to FCT CoPt phase transformation. The magnetic moment was found to increase with decreasing C content. Among this batch of films in the present study, the (Co0.42Pt0.58)0.88C0.12 film showed the highest magnetic moment of about 470 emu/cm3 because it had the highest CoPt concentration. However, as shown in Fig. 5, the coercivity against annealing temperature for the films with different composition showed more complicated behavior. For the (Co0.42Pt0.58)0.88C0.12 film, the coercivity increased with Ta from 500 C to 600 C, and then decreased at 650 C, even though it had the highest CoPt concentration. At Ta=650 C, the coercivity of the (Co0.42Pt0.58)0.88C0.12 film was measured to be only about 500 Oe, smaller than that of the (Co0.44Pt0.56)0.55C0.45 film, which exhibited a coercivity of about 2.1 kOe, though the XRD patterns of both films showed the formation of FCT CoPt phase. This is probably because the carbon content of the (Co0.42Pt0.58)0.88C0.12 film is too small to provide sufficient and effective encapsulation of
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(Co0.42Pt0.58)0.88C0.12 (Co0.44Pt0.56)0.55C0.45
Coercivity (kOe)
6
(Co0.4Pt0.6)0.4C0.6 4
2
0 500
550
600 650 Ta (°C)
700
750
Fig. 5. Room temperature coercivity against annealing temperature for the films with various compositions as indicated.
the CoPt grains. Hence, the grain size grew rapidly with increasing Ta, up to 21 nm at Ta=650 C, causing the formation of multi-domain structures in the film. In contrast, the film with the highest carbon content, (Co0.4Pt0.6)0.4C0.6, exhibited the smallest coercivity. In (Co0.4Pt0.6)0.4C0.6, the stoichiometry of CoPt is far from the equal atomic ratio and CoPt content is lower than those of the (Co0.44Pt0.56)0.55C0.45, so coercivity values are significant lower than those of the other samples. These results show that the grain size and coercivity strongly depend on the carbon content, thermal treatment conditions as well as the stoichiometry of CoPt. It was estimated that for applications as ultrahigh density recording media, a material should have a coercivity of about 4–5 kOe and isolated grains with a size smaller than that of about 10– 12 nm [18]. From the preliminary results obtained so far in the present study, the grain size is about 17 nm to achieve such a high coercivity value. Continued effort to further optimize the carbon content and annealing condition is on-going in our study aiming at increasing the coercivity and reducing the grain size of the films.
4. Summary In summary, we have prepared CoPt–C nanocomposite films of various carbon concentrations
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by pulsed filtered vacuum arc deposition and studied their properties using various characterization techniques. Diversified magnetic properties were observed in these films, depending on the composition and thermal annealing conditions. Both X-ray diffraction and MFM analyses confirmed the formation of nano-crystallites of FCT CoPt phase in the carbon matrix after annealing at a sufficiently high temperature, depending on the carbon concentration. The dependence of the CoPt grain size on the composition and annealing temperature, and their correlation with the coercivity have also been studied. For example, the coercivity of the (Co0.44Pt0.56)0.55C0.45 film was observed to increase with increasing annealing temperature, up to a value of 7 kOe at Ta=750 C, and the grain size increased from 9 to 20 nm. Further optimization in the carbon composition and thermal annealing processes is in progress aiming at improving the properties of CoPt–C thin films for applications as ultra-high density recording media.
Acknowledgements This work is supported in part by the National Nature Science Foundation of China under Grant No. 50371056, Excellent Creative Research Team Project of Hubei Province and Hubei University, Key project of Educational Department of Hubei Province, and the Research Grants Council of Hong Kong SAR (Ref. No. CUHK4216/00E).
References [1] K. Shibata, Mater. Trans. 44 (2003) 1542. [2] M. Yu, Y. Liu, A. Moser, D. Weller, D.J. Sellmyer, Appl. Phys. Lett. 75 (1999) 3992. [3] H.S. Ko, A. Perumal, S.C. Shin, Appl. Phys. Lett. 82 (2003) 2311. [4] H. Wang, S.P. Wong, Chapter 10, in: H.S. Nalwa (Ed.), Magnetic Nanostructures, American Scientific Publishers, Stevenson Ranch, CA 91381, USA, 2002, ISBN: 1-58883000-4, and the references therein. [5] T. Hayashi, et al., Nature 381 (1996) 772. [6] T. Suzuki, Mater. Trans. 44 (2003) 1535.
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[7] S. Stavroyiannis, I. Panagiotopoulos, D. Niarchos, J.A. Christodoulides, Y. Zhang, G.C. Hadjipanayis, J. Magn. Magn. Mater. 193 (1999) 181. [8] H. Zeng, M.L. Yan, Y. Liu, D.J. Sellmyer, J. Appl. Phys. 89 (2001) 810. [9] V. Karanasos, I. Panagiotopoulos, D. Niarchos, H. Okumura, G.C. Hadjipanayis, J. Appl. Phys. 90 (2001) 3112. [10] S. Stavroyiannis, I. Panagiotopoulos, D. Niarchos, J.A. Christodoulides, Y. Zhang, G.C. Hadjipanayis, J. Appl. Phys. 85 (1999) 4304. [11] A. Kikitsu, A. Murayama, K. Hyomi, C.M. Falco, J. Appl. Phys. 87 (2000) 6944. [12] M. Watanabe, T. Masumoto, D.H. Ping, K. Hono, Appl. Phys. Lett. 76 (2000) 3971.
[13] S.P. Wong, H. Wang, N. Ke, W.Y. Cheung, M.F. Chiah, D.H. Chen, Q.J. Li, H. Liu, X.X. Zhang, Int. J. Mod. Phys. B 14 (2000) 321. [14] H. Wang, S.P. Wong, W.Y. Cheung, N. Ke, W.F. Lau, M.F. Chiah, X.X. Zhang, Mater. Sci. Eng. C 16 (2001) 145. [15] H. Wang, S.P. Wong, W.Y. Cheung, N. Ke, M.F. Chiah, H. Liu, X.X. Zhang, J. Appl. Phys. 88 (2000) 2063. [16] H. Wang, S.P. Wong, W.Y. Cheung, N. Ke, G.H. Wen, J. Appl. Phys. 88 (2000) 4919. [17] H. Wang, M.F. Chiah, W.Y. Cheung, S.P. Wong, Phys. Lett. A 316 (2003) 122. [18] D. Weller, A. Moser, L. Folks, M.E. Best, W. Lee, M.F. Toney, M. Schwickert, J. Thiele, M.F. Doerner, IEEE Trans. Magn. 36 (2000) 10.
Physica B 351 (2004) 77–82
Structure evolution, magnetic domain structures and magnetic properties of CoPt–C nanocomposite films Hao Wanga,*, Fujun Yanga, Mingzhe Hua, Bin Zhoua, Haoshuang Gua, M.F. Chiahb, W.Y. Cheungb, S.P. Wongb b
a Faculty of Physics and Electronic Technology, Hubei University, Wuhan 430062, PR China Department of Electronic Engineering and Materials Science and Technology Research Center, The Chinese University of Hong Kong, Shatin, N.T., Hong Kong, PR China
Received 25 February 2004; received in revised form 23 April 2004; accepted 7 May 2004
Abstract CoPt–C nanocomposite thin films with various carbon concentrations were prepared by a pulsed filtered vacuum arc deposition technique. Annealing was performed in vacuum for 1 h at various temperatures. The structure evolution and magnetic properties of the films were characterized by non-Rutherford backscattering spectrometry, X-ray diffraction, magnetic force microscopy and vibrating sample magnetometer. The as-deposited samples are magnetic soft with facecentral-cubic phase CoPt nanocrystals encapsulated in carbon matrix. After annealing at a sufficiently high temperature, depending on the carbon composition, the CoPt nanocrystals transform to a face-centered-tetragonal phase. For the film with a particular composition of (Co0.44Pt0.56)0.55C0.45, the coercivity is observed to increase with increasing annealing temperature, up to a value of 7 kOe at an annealing temperature of 750 C, and the grain size increases from 9 to 20 nm. r 2004 Elsevier B.V. All rights reserved. PACS: 75.70.Ak; 75.50.Ss; 75.70.Kw Keywords: High-density magnetic recording; CoPt–C nanocomposite films; Ferromagnetic; Magnetic domain structure
1. Introduction In recent years, high uniaxial magnetocrystalline anisotropy (Ku) nanocomposite films have gained increasing attentions, due to their potential appli*Corresponding author. Tel.: +86-27-6251-2839; fax: +8627-8866-2550. E-mail address: [email protected] (Hao Wang).
cations as ultra-high density magnetic recording media [1–4]. In the case of ultra-high density magnetic recording, media with larger coercivities, smaller grain sizes and weakly intergrain exchange coupling are required to yield adequate high signal-to-noise ratio. However, for grain size less than 10 nm, the issue of thermal instability originated from superparamagnetism has to be considered. To avoid this problem, high Ku
0921-4526/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.physb.2004.05.014
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materials would be required to achieve smaller stable grains. The structure characteristics of nanocomposite films may satisfy the requirements for ultra-high-density magnetic recording media. For example of films consisting of magnetic grains encapsulated in carbon, the most important advantage of carbon encapsulation is the increase of the effective distance of neighboring magnetic grains so that the exchange coupling between them is significantly weakened or eliminated [5]. Grains separation by carbon layer may also play a role of interfering the increase of grain size and the coalescence of neighboring grains. Face-central-tetragonal (FCT) phase of nearequiatomic composition CoPt- and FePt-based nanocomposite films [2–4, 6–12] have been studied extensively in a last few years due to their very high Ku values. CoPt–C nanocomposite films of coercivities of up to 12 kOe with grain sizes of 7– 20 nm have been obtained at various C concentrations produced at suitable annealing temperatures [2,7]. The magnetic properties for (Co86Cr14)Pt–C films were reported to be better than those of CoPt–C films, with coercivity values ranging from 2 to 10 kOe and remanence ratio close to 1 [8]. In CoPt–B films with a wide range of B composition [9], the minimum grain size achieved is about 20 nm for the 20 vol% B sample after annealing at 600 C with a coercivity of only about 3 kOe. Larger coercivities up to 10 kOe can be obtained after annealing at 650–700 C but the grain sizes grew to close to 30 nm. In CoPt–Ag nanocomposite films, coercivity values in the range of 1–17 kOe with grain sizes in the range of 7–100 nm have been obtained by adjusting the annealing conditions and the relative CoPt–Ag composition [10]. For CoPt–SiO2 nanocomposite films with a wide range of CoPt composition [11], coercivities in the range of 0.07–2.3 kOe were obtained when the grain sizes were in the range of 3–8 nm. An overview of various high Hc nanocomposite films has appeared more recently [4]. In this work, we have employed the pulsed filtered vacuum arc deposition technique to prepare CoPt–C nanocomposite films and studied their structure evolution, magnetic domain structures and magnetic properties.
2. Experiments CoPt–C nanocomposite thin films about 40– 50 nm thick with various C concentration were prepared by a 3-source pulsed filtered vacuum arc deposition system. The details of the system have been described elsewhere in detail for the preparation of Co–C films [13–17]. In this work, three adjacent sources with pure graphite, platinum and cobalt as the cathode materials were operated simultaneously in a pulse mode with a pulse duration of 2.5 ms. Of the three sources, the platinum source was positioned at the center. Thermally grown SiO2 films of about 100 nm on Si (1 0 0) wafers were used as the substrate. The substrate was placed at the center of the chamber facing to the platinum source and a negative bias voltage of 80 V was applied. The composition of the films was varied by adjusting the arc discharge conditions and was monitored by the integrated charges arriving at the sample holder from the respective arc sources. After deposition, thermal annealing was performed in a vacuum furnace (o10 3 Pa) for 1 h at various temperatures. The composition and thickness of the CoPt–C films were determined by non-Rutherford backscattering spectrometry (NRBS) using a 2 MV tandem accelerator. The NRBS was performed with a beam of 3.5 MeV 4He2+ ions, which have a non-Rutherford resonance scattering cross-section with the C atoms so that the C detection sensibility is greatly enhanced to achieve a more accurate determination of the film composition. The structural properties were analyzed by X-ray diffraction (XRD). The magnetic properties were characterized by a vibrating sample magnetometer (VSM). The magnetic domain structures were investigated by magnetic force microscopy (MFM). MFM imaging were performed using the tapping and lift modes under ambient conditions, using a Nanoscope III scanning probe microscope with a vertically magnetized hard Co-alloy-coated silicon tip.
3. Results and discussion There are two phases of CoPt, namely, face centered-cubic (FCC) CoPt and FCT CoPt. The
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(fct 110)
Intensity (a.u.)
(fct 001)
(111)
Ta=650 °C
Ta=600 °C
Ta=550 °C
20
40
60 2θ
80
100
Fig. 1. XRD patterns for (Co0.44Pt0.56)0.55C0.45 films after annealing at various temperatures in vacuum for 1 h.
25 20
d (nm)
FCT phase is corresponding to a larger Ku value. In order to obtain the ordered FCT phase from disordered FCC phase, high-temperature annealing is required to overcome the energy barrier for superlattice ordering. Fig. 1 shows the XRD patterns of (Co0.44Pt0.56)0.55C0.45 films after annealing at various temperatures. The XRD patterns for FCC CoPt and FCT CoPt are quite similar except that there are several additional peaks for the FCT phase such as FCT (0 0 1) and FCT (1 1 0) [12]. The other peaks in the XRD patterns can be contributed by both the FCC and FCT phases. The as-deposited films were FCC CoPt nanocrystals encapsulated in carbon matrix. After annealing at 550 C, as shown in Fig. 1, still only those peaks from FCC CoPt could be observed. While the annealing temperature (Ta) increasing to 600 C, broad FCT (0 0 1) and FCT (1 1 0) peaks emerged, indicating the formation of the FCT CoPt phase. These two peaks became sharper as Ta increased to 650 C, indicating more and larger FCT CoPt grains are formed. The grain size is estimated by Scherrer’s formula using the (1 1 1) peak width from the XRD patterns. Shown in Fig. 2 is the grain size d against annealing temperature for CoPt–C films with various compositions as indicated. Since the observed (1 1 1) peak is contributed by both the FCC and FCT phases in a wide annealing temperature range, the average peak must be broader than each individual
79
15 10 5
(Co0.42Pt0.58)0.88C0.12
0
(Co0.44Pt0.56)0.55C0.45 (Co0.4Pt0.6)0.4C0.6 500
550
600 650 Ta (°C)
700
750
Fig. 2. Grain size d against annealing temperature for samples of various compositions as indicated.
one, and the grain size taken from the average width is probably smaller than the real value. The grain size of CoPt nanocrystals in the films generally increases with Ta due to thermal growth of the grains. It is also seen that higher carbon content will result in smaller as-deposited grain size as well as suppression of grain growth upon annealing. In fact, XRD results also showed that for the film with the highest carbon content, (Co0.40Pt0.60)0.40C0.60, the FCT peaks were observed only after Ta equal to or higher than 700 C. Shown in Fig. 3 are the MFM images of the film with a composition of (Co0.44Pt0.56)0.55C0.45 before and after annealing. The bright and dark regions in the MFM images are associated with domains of which the vertical component of the magnetization vector is parallel or anti-parallel to the film normal. As shown in Fig. 3a, a long-range stripe domain structure is observed in the as-deposited CoPt–C sample, indicating strong inter-grains exchange coupling. After annealing at 650 C, a short-range dispersed domain structure is observed in Fig. 3b, indicating the magnetic grains growth and separation from carbon matrix and the weakened inter-grains exchange coupling. The sharp change in the domain structures as presented in the MFM images is dominated by the change of nanostructure, the CoPt grains growth and separation with increasing annealing temperature in the
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Fig. 3. MFM images of (Co0.44Pt0.56)0.55C0.45 films; (a) as-deposited and (b) Ta=650 C.
M (x102 emu/cm3)
4
0 Ta=550 °C Ta=600 °C Ta=650 °C
-4
Ta=750 °C -10
-5
0 H (kOe)
5
10
Fig. 4. Magnetic hysteresis loops measured at 300 K for the (Co0.44Pt0.56)0.55C0.45 films after annealing at the temperature as indicated.
films. The phase transition from FCC CoPt phase to the high Ku value FCT CoPt phase in the films as revealed by XRD results, also plays an important role in this evolution. Fig. 4 shows the in-plane magnetic hysteresis loops of the (Co0.44Pt0.56)0.55C0.45 films after annealing at various temperatures in vacuum for 1 h. It is seen from Fig. 4 that as Ta increases, there is no significant change of the saturation magnetization of the films at a value of about 300 emu/ cm3. The film still showed a soft ferromagnetic characteristic even after annealing at 550 C with a
coercivity of about 100 Oe and a saturation magnetic field of about 200 Oe, corresponding to the FCC CoPt phase in the film. After annealing at a higher temperature of 600 C, the film became magnetically harder exhibiting a coercivity of 760 Oe. The increase in coercivity is attributed to the formation of the FCT CoPt phase. The coercivity increased further with Ta increasing, up to a value of 7 kOe at Ta=750 C. This is also understandable to be the result of FCT CoPt grain growth and more complete FCC to FCT CoPt phase transformation. The magnetic moment was found to increase with decreasing C content. Among this batch of films in the present study, the (Co0.42Pt0.58)0.88C0.12 film showed the highest magnetic moment of about 470 emu/cm3 because it had the highest CoPt concentration. However, as shown in Fig. 5, the coercivity against annealing temperature for the films with different composition showed more complicated behavior. For the (Co0.42Pt0.58)0.88C0.12 film, the coercivity increased with Ta from 500 C to 600 C, and then decreased at 650 C, even though it had the highest CoPt concentration. At Ta=650 C, the coercivity of the (Co0.42Pt0.58)0.88C0.12 film was measured to be only about 500 Oe, smaller than that of the (Co0.44Pt0.56)0.55C0.45 film, which exhibited a coercivity of about 2.1 kOe, though the XRD patterns of both films showed the formation of FCT CoPt phase. This is probably because the carbon content of the (Co0.42Pt0.58)0.88C0.12 film is too small to provide sufficient and effective encapsulation of
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(Co0.42Pt0.58)0.88C0.12 (Co0.44Pt0.56)0.55C0.45
Coercivity (kOe)
6
(Co0.4Pt0.6)0.4C0.6 4
2
0 500
550
600 650 Ta (°C)
700
750
Fig. 5. Room temperature coercivity against annealing temperature for the films with various compositions as indicated.
the CoPt grains. Hence, the grain size grew rapidly with increasing Ta, up to 21 nm at Ta=650 C, causing the formation of multi-domain structures in the film. In contrast, the film with the highest carbon content, (Co0.4Pt0.6)0.4C0.6, exhibited the smallest coercivity. In (Co0.4Pt0.6)0.4C0.6, the stoichiometry of CoPt is far from the equal atomic ratio and CoPt content is lower than those of the (Co0.44Pt0.56)0.55C0.45, so coercivity values are significant lower than those of the other samples. These results show that the grain size and coercivity strongly depend on the carbon content, thermal treatment conditions as well as the stoichiometry of CoPt. It was estimated that for applications as ultrahigh density recording media, a material should have a coercivity of about 4–5 kOe and isolated grains with a size smaller than that of about 10– 12 nm [18]. From the preliminary results obtained so far in the present study, the grain size is about 17 nm to achieve such a high coercivity value. Continued effort to further optimize the carbon content and annealing condition is on-going in our study aiming at increasing the coercivity and reducing the grain size of the films.
4. Summary In summary, we have prepared CoPt–C nanocomposite films of various carbon concentrations
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by pulsed filtered vacuum arc deposition and studied their properties using various characterization techniques. Diversified magnetic properties were observed in these films, depending on the composition and thermal annealing conditions. Both X-ray diffraction and MFM analyses confirmed the formation of nano-crystallites of FCT CoPt phase in the carbon matrix after annealing at a sufficiently high temperature, depending on the carbon concentration. The dependence of the CoPt grain size on the composition and annealing temperature, and their correlation with the coercivity have also been studied. For example, the coercivity of the (Co0.44Pt0.56)0.55C0.45 film was observed to increase with increasing annealing temperature, up to a value of 7 kOe at Ta=750 C, and the grain size increased from 9 to 20 nm. Further optimization in the carbon composition and thermal annealing processes is in progress aiming at improving the properties of CoPt–C thin films for applications as ultra-high density recording media.
Acknowledgements This work is supported in part by the National Nature Science Foundation of China under Grant No. 50371056, Excellent Creative Research Team Project of Hubei Province and Hubei University, Key project of Educational Department of Hubei Province, and the Research Grants Council of Hong Kong SAR (Ref. No. CUHK4216/00E).
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