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Iron nanoparticles coated with graphite nanolayers and carbon nanotubes
Diamond and Related Materials 13 (2004) 1270–1273
Iron nanoparticles coated with graphite nanolayers and carbon nanotubes Hisato Tokoroa, Shigeo Fujiia,*, Takeo Okub a Hitachi Metals, Limited, Advanced Electronics Research Laboratory, Mikajiri 5200, Kumagaya, Saitama, 360-0843, Japan Osaka University, Nanoscience and Nanotechnology Center, Institute of Scientific and Industrial Research, Mihogaoka 8-1, Ibaraki, Osaka, 567-0047, Japan
b
Abstract Fe nanoparticles coated with graphite nanolayers have been synthesized by annealing mixtures of hematite and carbon under nitrogen atmosphere. Hematite is reduced to Fe3 O4 , FeO, and finally to Fe completely at 1200 8C. High resolution electron micrographs reveal that the Fe particles are ;200 nm in diameter and are coated with graphite nanolayers of ;30 nm. The particles exhibit high magnetization of 100–170 emuyg and soft magnetic properties with coercivity of ;15 Oe, and excellent corrosion resistance as well as oxidation resistance. It is noted that carbon nanotubes with diameter of ;100 nm and length of a few micrometers are also synthesized during the process. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Graphite; Coatings; Nanotubes
1. Introduction Magnetic nanoparticles such as iron oxides have been applied to magnetic fluids w1x, magnetic recording media w2x and magnetic beads w3x. Though Fe, Co, Fe based alloys show superior magnetic properties with high saturation magnetization, their fine particles are easy to be oxidized in air. This prevents them from wide variety of practical applications. Therefore, many studies on coating them with inorganic layers have been devoted to overcome this problem. Fe and Fe–Co nanoparticles encapsulation in C formed by an arc discharge method have been proposed w4–8x. The particles are uniformly encapsulated with a total C layer less than 10-nm thickness w5,7x. However, this method is regarded as a gas phase reaction and not supposed to be suitable for mass production. We have developed a new technique to synthesize Fe nanoparticles coated with C nanolayers by means of annealing mixtures of hematite and carbon powders. Their morphology and stability of magnetic properties against corrosion is presented in this article. *Corresponding author. Tel.: q81-48-531-1623; fax: q81-48-5337102. E-mail address: [email protected] (S. Fujii).
2. Experimental Hematite (a-Fe2O3) and carbon black (C) powders were used as starting materials with the average particle diameter for each powder being 30 nm and 20 nm, respectively. These powders were mixed well where weight ratio of a-Fe2O3 to C (sa-Fe2O3 yC) was 1, 1.5, 2.3, 3, 4, and annealed, respectively, in a usual furnace. The mixture were heated from room temperature to 900, 1000 and 1200 8C at heating rate of 3 8Cy min, and kept for 2 h at each temperature in nitrogen atmosphere. An X-ray diffractometer with Cu Ka radiation under an applied power of 50 kV=250 mA was employed to detect metallurgical phases. The reaction process between a-Fe2O3 and C was confirmed by a differential thermal analysis (DTA-TG) in the temperature range of 25–1400 8C at heating rate of 10 8Cymin under nitrogen atmosphere. Magnetic properties of samples after annealing were measured by a vibrating sample magnetometer (VSM) under a field of 20 kOe. The morphology of the sample mixed with a-Fe2O3 yCs2.3 and annealed at 1200 8C was investigated by high-resolution transmission electron microscopy (HRTEM). Qualitative analysis of particle composition was carried out using energy dispersive X-ray analysis (EDX) equipped with HRTEM. In order to test oxidation resistance of the
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.10.058
H. Tokoro et al. / Diamond and Related Materials 13 (2004) 1270–1273
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Fig. 2. DTA-TG curve of a mixture of a-Fe2O3 and C (a-Fe2O3yCs 1); heat-treatment is conducted from 25 to 1400 8C in nitrogen atmosphere.
to iron. The following chemical formula is proposed as follows: Fig. 1. X-ray diffraction profiles of samples mixed with a-Fe2O3yCs 2.3 and annealed at 900 8C (a) and 1000 8C (b); each peak is assigned to phases shown in the upper frame.
sample used in HRTEM observation, temperature dependence of magnetization was also measured by the VSM under a field of 10 kOe from room temperature to 800 8C at heating rate of 10 8Cymin in air and Ar atmosphere. Moreover, this sample was also exposed to environment with temperature of 120 8C and humidity of 100% for a predetermined time. Commercially available carbonyl iron powders with an average diameter of 3 mm were used as a reference. 3. Results and discussion Fig. 1 shows X-ray diffraction profiles of samples after annealing mixture of hematite and carbon powders (a-Fe2O3 yCs2.3) at 900 and 1000 8C for 2 h in nitrogen. A broad peak appears approximately 2u of 258 in Fig. 1a and b is assigned to graphite carbon. The sample annealed at 900 8C has phases of Fe3O4, FeO and Fe. However, there is no peak of hematite that is used as a starting material. An X-ray diffraction profile of the sample annealed at 1000 8C shows existence of a-Fe and a little amount of g-Fe phase. Any peak of iron oxides disappears. It is confirmed that hematite is reduced to iron by annealing. A DTA-TG is employed in order to examine a reaction process of Fe2O3 with C (Fig. 2). A thermal gravity (TG) decreases with increasing temperature from the onset of heating but a sudden loss of mass takes place when the temperature exceeds 1000 8C. This rapid weight loss means endothermic reaction according to a DTA curve, which suggests that C reduces iron oxides
Fe2O3qxC™2Feq(xy3)Cq3CO≠ (x)3).
(1)
It is considered that C probably dissolves in Fe during the heating, and precipitate after cooling. Table 1 shows the magnetic properties of samples annealed at 1200 8C for 2 h, in which a-Fe2O3 must be completely reduced to Fe. The samples exhibit soft magnetic properties with coercivity of 14–20 Oe. Saturation magnetization naturally increases with increasing the relative amount of a-Fe2O3 in the starting material with maximum saturation magnetization of 172 emuyg, 78% of saturation magnetization of bulk iron w9x. Fig. 3 shows an HRTEM image of a metallic particle with a diameter of ;200 nm found in the sample prepared by annealing the mixture powder of a-Fe2O3 y Cs2.3 at 1200 8C. It is not really fine with 200-nm diameter. Carbon encapsulation over a thickness of approximately 30 nm is also found. EDX analysis makes it clear that the particle is composed of Fe and that the coating is C. The diameters of approximately 300 particles have been measured giving an average diameter of 200 nm. Table 1 Magnetic properties (saturation magnetization Ms and coercivity Hc under the DC field of 20 kOe) of powder samples prepared by mixing a-Fe2O3 and C powders at various weight ratios and annealing at 1200 8C a-Fe2O3 yC
Ms (emuyg)
Hc (Oe)
4.0 3.0 2.3 1.5 1.0
172 164 141 111 95
14 14 16 18 20
H. Tokoro et al. / Diamond and Related Materials 13 (2004) 1270–1273
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Fig. 4. Variation of saturation magnetization due to heating in air (solid line) and Ar atmosphere (dotted line); saturation magnetization, Ms, is normalized as Ms(T)yMs (300 K).
Fig. 3. HRTEM image of a Fe nanoparticle coated with C nanolayers.
Both the sample as shown in Fig. 3, FeyC, and carbonyl iron (control sample) were exposed to an environment with humidity of 100% and temperature of 120 8C in order to investigate corrosion resistance. Table 2 shows the oxygen content in the samples before and after 12 and 24 h exposure. Oxygen content in the carbonyl iron powders increases to 2.8 wt.% and to 3.7 wt.% after 12 h and 24 h exposure, respectively. However, FeyC powders do not show any change of oxygen content. This means that C encapsulation does indeed improve corrosion resistance of the Fe cores. Variation of a Ms due to heat treatment of FeyC powders in air and Ar atmosphere was also examined in order to test oxidation resistance (Fig. 4). Heating rate was 10 8Cymin, and the Ms was measured under the field of 10 kOe. The Ms values are normalized by Ms at 25 8C, which is expressed as Ms(T)yMs(25 8C). Both the samples show similar behavior with a sudden decrease in magnetization for temperature at approximately 780 8C. There is no difference between in air and Ar atmosphere. It is obvious that carbon nanolayers improve oxidation resistance. The Curie temperature of the Fe nanoparticles is roughly estimated to be near 780 8C, which is almost the same as that of bulk iron w9x.
It is noted that structures similar to carbon nanotubes (CNTs) were also found in the same sample as shown in Fig. 3 (Fig. 5). These CNTs had a diameter of 100 nm, length of a few micrometers and a multi-walled structure. They have iron particles at the tip of tubes, which is confirmed by an EDX analysis. Some techniques that synthesize CNTs by using Fe or iron oxide nanoparticles have been reported w10,11x. It can be considered that Fe reduced by annealing acts as a catalyst and induces growth of CNTs in this study as well. A detailed morphology of them will be studied and presented elsewhere in near future. The generation of carbon nanolayers could be explained as mechanism of ‘dissolution and precipita-
Table 2 Oxygen content included in samples before and after 12 and 14 h test; ‘FeyC’ represent the sample in Fig. 3. Oxygen content is expressed as weight percent, wt.%. Sample powders
Carbonyl Fe (3 mm) FeyC (This work)
Oxygen content (wt.%) Test time (h) 0
12
24
0.3 0.7
2.8 0.3
3.7 0.4
Fig. 5. HRTEM image of CNT found in our sample; dark contrast is iron particles.
H. Tokoro et al. / Diamond and Related Materials 13 (2004) 1270–1273
tion’ w12x. Namely, iron oxides are reduced to iron during annealing and carbon can dissolve in iron above 740 8C according to the Fe–C phase diagram w13x. The carbon precipitates on the surface of iron particles and generates nanolayers under slow cooling (3 8Cymin or so), because solubility of carbon into iron decreases with temperature. The precipitated carbon would make graphite structure. In the precipitation, hexagonal rings of carbon would nucleate along the surface of iron particles at the first instance, and then grow to graphene sheets. 4. Conclusions Fe nanoparticles with an average diameter of ;200 nm inside C multilayer capsules ;30 nm thick have been successfully prepared by annealing mixtures of aFe2O3 and C at 1200 8C. The produced material exhibits high saturation magnetization with maximum value of 172 emuyg and soft magnetic properties with coercivity of ;15 Oe, and shows an excellent oxidation resistance as well as corrosion resistance. Our technique can also synthesize CNTs with a diameter of ;100 nm and a length of a few micrometers. The proposed method is simple and inexpensive and could be a promising way
1273
to produce Fe nanoparticles coated with C as well as CNTs. References w1x J. Ding, W.F. Miao, P.G. McCormick, R. Street, Appl. Phys. Lett. 67 (1995) 3804. w2x F.E. Kruis, H. Fissan, A. Peled, J. Aerosol Sci. 29 (1998) 511. w3x Q. Liu, Z. Xu, J.A. Finch, R. Egerton, Chem. Mater. 10 (1998) 3936. w4x Y. Yoshida, S. Shida, T. Ohsuna, N. Shiraga, J. Appl. Phys. 76 (1994) 721. w5x S. Tomita, M. Hikita, M. Fujii, S. Hayashi, K. Yamamoto, Chem. Phys. Lett. 316 (2000) 361. w6x M. Jiang, X.G. Zhang, Y. Liu, G.M. Hao, J. Lin, Mater. Sci. Eng. B87 (2001) 66. w7x Z.D. Zhang, J.G. Zheng, I. Skorvanek, G.H. Wen, J. Kovac, F.W. Wang, et al., J. Phys.: Condens. Matter 13 (2001) 1921. w8x V.P. Dravid, J.J. Host, M.H. Teng, B. Elliott, J. Hwang, D.L. Johnson, et al., Nature 374 (1995) 602. w9x National Astronomical Observatory, Chronological Scientific Tables 2000, Maruzen Co. Ltd, Tokyo, 1999, p. 491. w10x H. Ago, S. Ohshima, K. Uchida, M. Yumura, J. Phys. Chem. B 105 (2001) 10453. w11x Y. Homma, T. Yamashita, P. Finnie, M. Tomita, T. Ogino, Jpn. J. Appl. Phys. 41 (2002) L89. w12x Y. Saito, T. Yoshikawa, M. Okuda, N. Fujimoto, S. Yamamuro, K. Wakoh, et al., Chem. Phys. Lett. 212 (1993) 379. w13x T.B. Massalski, H. Okamoto, P.R. Subramanian, L. Kacprzak, W.W. Scott Jr., M.A. Fleming, Binary Alloy Phase Diagrams, 1, American Society for Metals, USA, 1986, p. 562.
Iron nanoparticles coated with graphite nanolayers and carbon nanotubes Hisato Tokoroa, Shigeo Fujiia,*, Takeo Okub a Hitachi Metals, Limited, Advanced Electronics Research Laboratory, Mikajiri 5200, Kumagaya, Saitama, 360-0843, Japan Osaka University, Nanoscience and Nanotechnology Center, Institute of Scientific and Industrial Research, Mihogaoka 8-1, Ibaraki, Osaka, 567-0047, Japan
b
Abstract Fe nanoparticles coated with graphite nanolayers have been synthesized by annealing mixtures of hematite and carbon under nitrogen atmosphere. Hematite is reduced to Fe3 O4 , FeO, and finally to Fe completely at 1200 8C. High resolution electron micrographs reveal that the Fe particles are ;200 nm in diameter and are coated with graphite nanolayers of ;30 nm. The particles exhibit high magnetization of 100–170 emuyg and soft magnetic properties with coercivity of ;15 Oe, and excellent corrosion resistance as well as oxidation resistance. It is noted that carbon nanotubes with diameter of ;100 nm and length of a few micrometers are also synthesized during the process. 䊚 2003 Elsevier B.V. All rights reserved. Keywords: Nanoparticles; Graphite; Coatings; Nanotubes
1. Introduction Magnetic nanoparticles such as iron oxides have been applied to magnetic fluids w1x, magnetic recording media w2x and magnetic beads w3x. Though Fe, Co, Fe based alloys show superior magnetic properties with high saturation magnetization, their fine particles are easy to be oxidized in air. This prevents them from wide variety of practical applications. Therefore, many studies on coating them with inorganic layers have been devoted to overcome this problem. Fe and Fe–Co nanoparticles encapsulation in C formed by an arc discharge method have been proposed w4–8x. The particles are uniformly encapsulated with a total C layer less than 10-nm thickness w5,7x. However, this method is regarded as a gas phase reaction and not supposed to be suitable for mass production. We have developed a new technique to synthesize Fe nanoparticles coated with C nanolayers by means of annealing mixtures of hematite and carbon powders. Their morphology and stability of magnetic properties against corrosion is presented in this article. *Corresponding author. Tel.: q81-48-531-1623; fax: q81-48-5337102. E-mail address: [email protected] (S. Fujii).
2. Experimental Hematite (a-Fe2O3) and carbon black (C) powders were used as starting materials with the average particle diameter for each powder being 30 nm and 20 nm, respectively. These powders were mixed well where weight ratio of a-Fe2O3 to C (sa-Fe2O3 yC) was 1, 1.5, 2.3, 3, 4, and annealed, respectively, in a usual furnace. The mixture were heated from room temperature to 900, 1000 and 1200 8C at heating rate of 3 8Cy min, and kept for 2 h at each temperature in nitrogen atmosphere. An X-ray diffractometer with Cu Ka radiation under an applied power of 50 kV=250 mA was employed to detect metallurgical phases. The reaction process between a-Fe2O3 and C was confirmed by a differential thermal analysis (DTA-TG) in the temperature range of 25–1400 8C at heating rate of 10 8Cymin under nitrogen atmosphere. Magnetic properties of samples after annealing were measured by a vibrating sample magnetometer (VSM) under a field of 20 kOe. The morphology of the sample mixed with a-Fe2O3 yCs2.3 and annealed at 1200 8C was investigated by high-resolution transmission electron microscopy (HRTEM). Qualitative analysis of particle composition was carried out using energy dispersive X-ray analysis (EDX) equipped with HRTEM. In order to test oxidation resistance of the
0925-9635/04/$ - see front matter 䊚 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.diamond.2003.10.058
H. Tokoro et al. / Diamond and Related Materials 13 (2004) 1270–1273
1271
Fig. 2. DTA-TG curve of a mixture of a-Fe2O3 and C (a-Fe2O3yCs 1); heat-treatment is conducted from 25 to 1400 8C in nitrogen atmosphere.
to iron. The following chemical formula is proposed as follows: Fig. 1. X-ray diffraction profiles of samples mixed with a-Fe2O3yCs 2.3 and annealed at 900 8C (a) and 1000 8C (b); each peak is assigned to phases shown in the upper frame.
sample used in HRTEM observation, temperature dependence of magnetization was also measured by the VSM under a field of 10 kOe from room temperature to 800 8C at heating rate of 10 8Cymin in air and Ar atmosphere. Moreover, this sample was also exposed to environment with temperature of 120 8C and humidity of 100% for a predetermined time. Commercially available carbonyl iron powders with an average diameter of 3 mm were used as a reference. 3. Results and discussion Fig. 1 shows X-ray diffraction profiles of samples after annealing mixture of hematite and carbon powders (a-Fe2O3 yCs2.3) at 900 and 1000 8C for 2 h in nitrogen. A broad peak appears approximately 2u of 258 in Fig. 1a and b is assigned to graphite carbon. The sample annealed at 900 8C has phases of Fe3O4, FeO and Fe. However, there is no peak of hematite that is used as a starting material. An X-ray diffraction profile of the sample annealed at 1000 8C shows existence of a-Fe and a little amount of g-Fe phase. Any peak of iron oxides disappears. It is confirmed that hematite is reduced to iron by annealing. A DTA-TG is employed in order to examine a reaction process of Fe2O3 with C (Fig. 2). A thermal gravity (TG) decreases with increasing temperature from the onset of heating but a sudden loss of mass takes place when the temperature exceeds 1000 8C. This rapid weight loss means endothermic reaction according to a DTA curve, which suggests that C reduces iron oxides
Fe2O3qxC™2Feq(xy3)Cq3CO≠ (x)3).
(1)
It is considered that C probably dissolves in Fe during the heating, and precipitate after cooling. Table 1 shows the magnetic properties of samples annealed at 1200 8C for 2 h, in which a-Fe2O3 must be completely reduced to Fe. The samples exhibit soft magnetic properties with coercivity of 14–20 Oe. Saturation magnetization naturally increases with increasing the relative amount of a-Fe2O3 in the starting material with maximum saturation magnetization of 172 emuyg, 78% of saturation magnetization of bulk iron w9x. Fig. 3 shows an HRTEM image of a metallic particle with a diameter of ;200 nm found in the sample prepared by annealing the mixture powder of a-Fe2O3 y Cs2.3 at 1200 8C. It is not really fine with 200-nm diameter. Carbon encapsulation over a thickness of approximately 30 nm is also found. EDX analysis makes it clear that the particle is composed of Fe and that the coating is C. The diameters of approximately 300 particles have been measured giving an average diameter of 200 nm. Table 1 Magnetic properties (saturation magnetization Ms and coercivity Hc under the DC field of 20 kOe) of powder samples prepared by mixing a-Fe2O3 and C powders at various weight ratios and annealing at 1200 8C a-Fe2O3 yC
Ms (emuyg)
Hc (Oe)
4.0 3.0 2.3 1.5 1.0
172 164 141 111 95
14 14 16 18 20
H. Tokoro et al. / Diamond and Related Materials 13 (2004) 1270–1273
1272
Fig. 4. Variation of saturation magnetization due to heating in air (solid line) and Ar atmosphere (dotted line); saturation magnetization, Ms, is normalized as Ms(T)yMs (300 K).
Fig. 3. HRTEM image of a Fe nanoparticle coated with C nanolayers.
Both the sample as shown in Fig. 3, FeyC, and carbonyl iron (control sample) were exposed to an environment with humidity of 100% and temperature of 120 8C in order to investigate corrosion resistance. Table 2 shows the oxygen content in the samples before and after 12 and 24 h exposure. Oxygen content in the carbonyl iron powders increases to 2.8 wt.% and to 3.7 wt.% after 12 h and 24 h exposure, respectively. However, FeyC powders do not show any change of oxygen content. This means that C encapsulation does indeed improve corrosion resistance of the Fe cores. Variation of a Ms due to heat treatment of FeyC powders in air and Ar atmosphere was also examined in order to test oxidation resistance (Fig. 4). Heating rate was 10 8Cymin, and the Ms was measured under the field of 10 kOe. The Ms values are normalized by Ms at 25 8C, which is expressed as Ms(T)yMs(25 8C). Both the samples show similar behavior with a sudden decrease in magnetization for temperature at approximately 780 8C. There is no difference between in air and Ar atmosphere. It is obvious that carbon nanolayers improve oxidation resistance. The Curie temperature of the Fe nanoparticles is roughly estimated to be near 780 8C, which is almost the same as that of bulk iron w9x.
It is noted that structures similar to carbon nanotubes (CNTs) were also found in the same sample as shown in Fig. 3 (Fig. 5). These CNTs had a diameter of 100 nm, length of a few micrometers and a multi-walled structure. They have iron particles at the tip of tubes, which is confirmed by an EDX analysis. Some techniques that synthesize CNTs by using Fe or iron oxide nanoparticles have been reported w10,11x. It can be considered that Fe reduced by annealing acts as a catalyst and induces growth of CNTs in this study as well. A detailed morphology of them will be studied and presented elsewhere in near future. The generation of carbon nanolayers could be explained as mechanism of ‘dissolution and precipita-
Table 2 Oxygen content included in samples before and after 12 and 14 h test; ‘FeyC’ represent the sample in Fig. 3. Oxygen content is expressed as weight percent, wt.%. Sample powders
Carbonyl Fe (3 mm) FeyC (This work)
Oxygen content (wt.%) Test time (h) 0
12
24
0.3 0.7
2.8 0.3
3.7 0.4
Fig. 5. HRTEM image of CNT found in our sample; dark contrast is iron particles.
H. Tokoro et al. / Diamond and Related Materials 13 (2004) 1270–1273
tion’ w12x. Namely, iron oxides are reduced to iron during annealing and carbon can dissolve in iron above 740 8C according to the Fe–C phase diagram w13x. The carbon precipitates on the surface of iron particles and generates nanolayers under slow cooling (3 8Cymin or so), because solubility of carbon into iron decreases with temperature. The precipitated carbon would make graphite structure. In the precipitation, hexagonal rings of carbon would nucleate along the surface of iron particles at the first instance, and then grow to graphene sheets. 4. Conclusions Fe nanoparticles with an average diameter of ;200 nm inside C multilayer capsules ;30 nm thick have been successfully prepared by annealing mixtures of aFe2O3 and C at 1200 8C. The produced material exhibits high saturation magnetization with maximum value of 172 emuyg and soft magnetic properties with coercivity of ;15 Oe, and shows an excellent oxidation resistance as well as corrosion resistance. Our technique can also synthesize CNTs with a diameter of ;100 nm and a length of a few micrometers. The proposed method is simple and inexpensive and could be a promising way
1273
to produce Fe nanoparticles coated with C as well as CNTs. References w1x J. Ding, W.F. Miao, P.G. McCormick, R. Street, Appl. Phys. Lett. 67 (1995) 3804. w2x F.E. Kruis, H. Fissan, A. Peled, J. Aerosol Sci. 29 (1998) 511. w3x Q. Liu, Z. Xu, J.A. Finch, R. Egerton, Chem. Mater. 10 (1998) 3936. w4x Y. Yoshida, S. Shida, T. Ohsuna, N. Shiraga, J. Appl. Phys. 76 (1994) 721. w5x S. Tomita, M. Hikita, M. Fujii, S. Hayashi, K. Yamamoto, Chem. Phys. Lett. 316 (2000) 361. w6x M. Jiang, X.G. Zhang, Y. Liu, G.M. Hao, J. Lin, Mater. Sci. Eng. B87 (2001) 66. w7x Z.D. Zhang, J.G. Zheng, I. Skorvanek, G.H. Wen, J. Kovac, F.W. Wang, et al., J. Phys.: Condens. Matter 13 (2001) 1921. w8x V.P. Dravid, J.J. Host, M.H. Teng, B. Elliott, J. Hwang, D.L. Johnson, et al., Nature 374 (1995) 602. w9x National Astronomical Observatory, Chronological Scientific Tables 2000, Maruzen Co. Ltd, Tokyo, 1999, p. 491. w10x H. Ago, S. Ohshima, K. Uchida, M. Yumura, J. Phys. Chem. B 105 (2001) 10453. w11x Y. Homma, T. Yamashita, P. Finnie, M. Tomita, T. Ogino, Jpn. J. Appl. Phys. 41 (2002) L89. w12x Y. Saito, T. Yoshikawa, M. Okuda, N. Fujimoto, S. Yamamuro, K. Wakoh, et al., Chem. Phys. Lett. 212 (1993) 379. w13x T.B. Massalski, H. Okamoto, P.R. Subramanian, L. Kacprzak, W.W. Scott Jr., M.A. Fleming, Binary Alloy Phase Diagrams, 1, American Society for Metals, USA, 1986, p. 562.