- Email: [email protected]
Y2O3 nanocomposites
ARTICLE IN PRESS
Journal of Magnetism and Magnetic Materials 271 (2004) L147–L152
Letter to the Editor
A GHz range electromagnetic wave absorber with wide bandwidth made of FeCo/Y2O3 nanocomposites Jiu Rong Liua, Masahiro Itoha, Jianzhuang Jiangb, Ken-ichi Machidaa,* a
Collaborative Research Center for Advanced Science and Technology, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Key Laboratory for Colloid and Interface Chemistry of State Education Ministry, Department of Chemistry, Shandong University, Jinan 250100, China
b
Received 16 June 2003; received in revised form 2 December 2003
Abstract Nanocomposites consisting of fine particles of FeCo (B30 nm) and Y2O3 (B15 nm) have been prepared from an intermetallic compound Y2(Fe0.5Co0.5)17 by melt-spinning techniques and subsequent hydrogenation-disproportionation and oxidation treatments. The electromagnetic (EM) wave absorption properties were characterized in a frequency range of 0.05–20.05 GHz. The measured real part (e0r ) and imaginary part (e00r ) of the relative permittivity were small and approximately constant in the 1–10 GHz range. The imaginary part of relative permeability m00r exhibited two resonant peaks and a high value over the 1–10 GHz range. As a result, the resin composites with 80 wt% FeCo/Y2O3 powders exhibited excellent EM wave absorption properties (reflection loss RLo20 dB) in a frequency range of 2.7–8.1 GHz, with thickness of 2.2–5.7 mm, respectively. A minimum RL of 43 dB was observed at 5.6 GHz with an absorber thickness of 3 mm. r 2004 Elsevier B.V. All rights reserved. PACS: 72.80.Tm; 74.25.Ha; 75.20.En Keywords: Melt spinning; Nanocomposites; Electromagnetic wave absorption; Magnetic anisotropy; Matching frequency
1. Introduction With development of electromagnetic (EM) wave communication devices, EM wave absorbing materials continue to attract much attention as EM interference-suppressing coatings, self-concealing technology and microwave darkrooms. Among the candidates for such applications, soft metallic magnets are a promising and interesting *Corresponding author. Tel./fax: +81-6-6879-4209. E-mail address: [email protected] (K.-i. Machida).
set of materials. For magnetic EM wave absorbers, the complex permeability (m0 mr ¼ m0 jm00 ) and permittivity (e0 er ¼ e0 je00 ) of materials determine reflection and attenuation characteristics of absorbers, in which mr is the relative complex permeability and er is the relative complex permittivity; and there is a relationship between the thickness (dm ) and magnetic loss (m00r ) of absorbers given by dm ¼ c=2pfm m00r ;
ð1Þ
where c is velocity of light and fm the matching frequency. Since metallic magnetic materials have
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.1386
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large saturation magnetization and the Snoek’s limit is at a high frequency [1–3], their complex permeability values remain high over a wide frequency range. Therefore, it is possible to make thin absorbers from such kinds of materials. However, the high-frequency permeability of these materials decreases due to the eddy current loss induced by EM waves. For this reason, it is better to use smaller particles which are isolated by insulating materials, such as epoxy resin. Sugimoto et al. [4,5] have reported the good EM wave absorption properties of the a-Fe/SmO composites derived from rare-earth intermetallic compounds in the 0.73–1.30 GHz range, which were prepared by an arc-melting technique. It is well known that melt-spinning techniques are an excellent way to produce homogenous microstructured materials with attractive properties for a variety of applications [6–8]. We also have reported that the a-Fe/Y2O3 composite materials prepared by melt-spinning techniques showed good EM wave absorption properties in the 2.0– 3.5 GHz range because of fine particle size of a-Fe (B20 nm) [9]. One criterion for selecting suitable magnetic EM wave absorption materials is the location of its natural ferromagnetic resonance frequency, which should be matched with the frequency of EM waves to be absorbed. For spin rotation, the general model developed by Snoek [3], predicts that the natural resonance frequency (fr ) is related to the anisotropy field HA by 2pfr ¼ gHA ;
ð2Þ
where g is the gyromagnetic ratio. Gutierrez et al. [10] have reported that MBE-grown FeCo films have large magnetocrystalline anisotropy constants K1. The purpose of this study was to investigate the effect of substitution a-Fe by Co, to form FeCo/Y2O3 nanocomposite materials, on the EM wave absorption properties and compare them with that of a-Fe/Y2O3. The nanocomposite materials FeCo/Y2O3 were prepared by the hydrogenation-disproportionation and the subsequent oxidation for Y2(Fe0.5Co0.5)17, which was prepared by melt-spinning techniques. The EM absorption properties of the resultant fine powder were measured in the 0.05–20.05 GHz range.
2. Experimental Intermetallic compound ingots of Y2(Fe0.5Co0.5)17 were first prepared from Y, Co and Fe metals (>99.9% in purity) by means of induction melting in Ar. Alloy ribbons of Y2(Fe0.5Co0.5)17 with 1.5 mm in width and about 50 mm in thickness were prepared from the ingots of Y2(Fe0.5Co0.5)17 by the single-roller melt-spun apparatus at a roll surface velocity of 20 m/s. After dry ball-milling in Ar, the Y2(Fe0.5Co0.5)17 powders with a particle size distribution of 2–4 mm, were heated at 873 K in H2 for 2 h under 2 MPa, and then were heated at 553 K for 2 h in an O2 stream. The powders were characterized by X-ray diffraction (XRD), and the microstructures were analyzed by a high-resolution scanning electron microscope (HITACHI S-5000) and transmission electron microscope (HITACHI H-800). Magnetization hysteresis curves of the powdered samples were recorded by a vibration sample magnetometer (Tamakawa, TM-VSM2014-MHR-Type) in a field up to 71.6 MA m1. Epoxy resin composites were prepared by homogenously mixing epoxy resin XN-5861 (Nippon Pelnox Corp.) with the 80 wt% composite powder and pressing into cylindrical shaped compacts. These compacts were cured by heating at 453 K for 30 min, and then they were cut into toroidal shaped samples of 7.00 mm outer diameter and 3.04 mm inner diameter. The scattering parameters (S11 ; S21 ) of the toroidal shaped sample were measured using a Hewlett-Packard 8720B network analyzer. The relative permeability (mr ) and permittivity (er ) values were determined from the scattering parameters as measured in the frequency range of 0.05–20.05 GHz [11]. The reflection loss (RL) curves were calculated from the relative permeability and permittivity at given frequency and absorber thickness with the following equations: Zin ¼ Z0 ðmr =er Þ1=2 tanhfjð2pfd=cÞðmr er Þ1=2 g;
ð3Þ
RL ¼ 20 logjðZin Z0 Þ=ðZin þ Z0 Þj;
ð4Þ
where f is the frequency of the EM wave, d the thickness of an absorber, c the velocity of light, Z0 the impedance of air, and Zin the input impedance
ARTICLE IN PRESS J.R. Liu et al. / Journal of Magnetism and Magnetic Materials 271 (2004) L147–L152
of absorber. In this investigation, the situation when RLo20 dB, is considered as an adequate EM absorption (the case when RL=20 dB, is comparable to the 99% of EM wave absorption), and is called a ‘‘matching situation’’. The absorber thickness and frequency in the matching situation are defined as matching thickness (dm ) and matching frequency (fm ), respectively. The attenuation of the EM wave and reflectance of an absorber at the surface were determined from the frequency dependency of RL.
3. Results and discussion Fig. 1 shows a set of typical XRD patterns measured on the hydrogenated and oxidized Y2(Fe0.5Co0.5)17 powders, together with the starting powder. It is confirmed from Fig. 1(a) that the starting powder prepared by using the meltspinning technique was Y2(Fe0.5Co0.5)17. After the treatment of hydrogenation-disproportionation as shown in Fig. 1(b), the Y2(Fe0.5Co0.5)17 peaks disappeared and two sets of peaks, for both the FeCo alloy and YH2 phases, appeared. After oxidizing the FeCo/YH2 powder, composite ma-
Fig. 1. The XRD pattern of the powder: (a) as-obtained, (b) after hydrogenation-disproportionation at 873 K for 2 h in H2 under 2 MPa, and (c) after oxidizing the sample (b) in O2 at 553 K for 2 h.
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terials of FeCo/Y2O3 were formed. The XRD pattern characteristic of FeCo/Y2O3 composites is shown in Fig. 1(c), not only the peaks of Y2O3 and FeCo clearly are observed, but also the intensity sequence for the peaks of FeCo alloy agrees with the standard powder diffraction measurements [12]. The mean crystalline size of the Y2O3 and FeCo particles was evaluated from the line broadening of the XRD peaks using Scherrer’s formula to be about 15 and 30 nm, respectively. From the TEM photographs of FeCo/Y2O3 composite powder, the particles are roughly of spherical shape with a narrow particle size distribution. Most FeCo grains appear as clusters and chains aggregated several crystallites with the size of 20– 40 nm (Fig. 2). This observation was in good agreement with the crystalline size estimated from XRD peaks by Scherrer’s formula. The frequency dependence of relative permittivity for resin composites of 80 wt% FeCo/Y2O3 is shown in Fig. 3. The real part e0r and imaginary part e00r of relative permittivity were almost constant (e0r ¼ B14 and e00r ¼ B0:6) in the 1– 10 GHz range, in which the relative permittivity (er ¼ e0r je00r ) showed less variation than the relative permeability. However, the Sm2Fe17 resin composites as reported by Sugimoto et al. [5], showed a high relative complex permittivity and nearly behaved as a metal conductor. The
Fig. 2. A TEM microgaph of FeCo/Y2O3 powder.
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Fig. 3. The relative permittivity as a function of frequency for the resin composites with 80 wt% FeCo/Y2O3 powder.
Fig. 4. The variations of m0r and m00r as a function of frequency for the resin composites with 80 wt% FeCo/Y2O3 or a-Fe/Y2O3 powder.
electrical resistivity of FeCo/Y2O3 composites was measured to be about 200 Om. The high resistivity of the nanocomposites is attributed to the microstructure of FeCo/Y2O3 nanoparticles, of which the Y2O3 phase plays a role as an insulator embeded among FeCo particles. The real part m0r and imaginary part m00r of relative permeability are plotted as a function of frequency in Fig. 4. For FeCo/Y2O3 resin composites, the real part of permeability m0r declined from 3.9 to 0.9 with increasing frequency. However, the imaginary part of permeability m00r showed two dispersion peaks and m00r value retained a high over the 1–10 GHz range, and then decreased gradually in the higher-frequency range (Fig. 4). This interesting resonance phenomenon can be explained as follows: as shown in Fig. 2, most nanometer FeCo crystallites form clusters and chains, so that the permeability spectra of FeCo/ Y2O3 composites comes from the cooperative effect of domain wall resonance and spin rotational resonance. The real part of relative permeability m0r of FeCo/Y2O3 (Fig. 4) exhibited almost the same variation with frequency in the 0.05– 10 GHz range as a-Fe/Y2O3; however, the peak of imaginary part of relative permeability m00r is broader in frequency. This can be attributed to the partial substitution of Co in the crystal lattice of a-Fe resulting in increasing the HA value. We
have employed the singular point detection method [13] to measure the anisotropy fields in FeCo/ Y2O3 nanocomposites, which exhibited large anisotropy fields (HA ¼ B0:24 T). The resonance frequency (fr ) of 8.5 GHz, estimated according to Eq. (2), is larger than that of a-Fe, which has an fr of 1.6 GHz [5]. As a result, the nanocomposite powder has a remarkable feature of EM wave absorption in the 1–10 GHz range. Fig. 5(a) shows a typical relationship between RL and frequency for the resin composites with 80 wt% FeCo/Y2O3 powder. First, the minimum RL was found to move toward the lowerfrequency region with increasing thickness. Second, the RL values of resin composites less than 20 dB were obtained in the 2.78.1 GHz frequency range. In particular, a minimum RL value of 43 dB was observed at 5.6 GHz with a matching thickness (dm ) of 3 mm, and the minimum dm value of 2.2 mm was obtained at 8.1 GHz. The EM wave absorption properties of FeCo/ Y2O3, a-Fe/SmO, and a-Fe/Y2O3 resin composites prepared under the optimized conditions are summarized in Table 1. Comparing FeCo/Y2O3 with a-Fe/Y2O3 sample [9], the minimum reflection point shifted to the higher frequency from 2.6 to 5.6 GHz, and moreover the EM wave absorption bandwidth, with RLo20 dB, is broadened from 1.5 to 5.4 GHz
ARTICLE IN PRESS J.R. Liu et al. / Journal of Magnetism and Magnetic Materials 271 (2004) L147–L152
range (see Fig. 5). This is attributed to the increase in HA due to the partial substitution of a-Fe by Co resulting in the increase of the natural resonance frequency (see Fig. 4). Qin et al. [14] have reported the coercivity (Hc ) of FeCo alloy nanowire arrays with particle size of 30 nm reaching a high value (B2800 Oe). Such large coercivity was ascribed to the high-shape magnetic anisotropy of FeCo
Fig. 5. Frequency dependence of RL for the resin composites at different thickness with: (a) 80 wt% FeCo/Y2O3 powders, and (b) a-Fe/Y2O3 powder.
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magnetic grains by them. We have also measured the magnetic hysteresis loops of a-Fe/Y2O3 and FeCo/Y2O3 as shown in Fig. 6. The coercivity values of a-Fe/Y2O3 and FeCo/Y2O3 are very small (B300 Oe). This result can be explained by the lack of a shape effect on the magnetic anisotropy due to the roughly spherical shaped FeCo particles prepared by our method (Fig. 2). At the same time, it suggests that the higher HA value of FeCo/Y2O3 than that of a-Fe/Y2O3 comes from the partial substitution of Co in the crystal lattice of a-Fe. For this reason, the EM absorption of FeCo/Y2O3 resin composites is shifted to the higher-frequency range.
Fig. 6. Magnetic hysteresis loops for the FeCo/Y2O3 and a-Fe/ Y2O3 nanocomposites powder.
Table 1 Electromagnetic wave absorption properties of a-Fe/SmO, a-Fe/Y2O3 and FeCo/Y2O3 resin composites with 80 wt% powder Sample (80 wt%)
Apparatus
Mean particle Electromagnetic wave absorption properties of resin composites Ref. size of magnet (nm) Minimum RL dm (mm) fm (GHz) Frequency value (dB) (RLo20 dB) (minimum RL) range (GHz) (RLo20 dB)
a-Fe/SmO a-Fe/Y2O3 FeCo/Y2O3
Arc melting Melt spun Melt spun
B30 B20 B30
52 36 43
7.9–13.1 3–5 2.2–5.7
0.95 2.6 5.6
0.73–1.30 2–3.5 2.7–8.1
[5] [9] This work
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4. Conclusion
References
In conclusion, the FeCo/Y2O3 resin composites exhibited good EM absorption properties due to the low relative permittivity and high-permeability value in 1–10 GHz range. Compared with a-Fe/Y2O3, the FeCo/Y2O3 resin composites showed higher EM absorption frequency and wider absorption bandwidth (RLo20 dB) due to the larger magnetic anisotropy of FeCo nanoparticles. Our work suggests that the FeCo/Y2O3 can be used as good EM absorption material in the 2.7–8.1 GHz range.
[1] S. Yoshida, J. Magn. Soc. Japan 22 (1998) 1353. [2] S. Yoshida, M. Sato, E. Sugawara, Y. Shimada, J. Appl. Phys. 85 (1999) 4636. [3] J.L. Snoek, Physica 14 (1948) 207. [4] T. Maeda, S. Sugimoto, T. Kagotani, D. Book, M. Homma, H. Ota, Y. Houjou, Mater. Trans. 41 (2000) 1172. [5] S. Sugimoto, T. Maeda, D. Book, T. Kagotani, K. Inomata, M. Homma, H. Ota, Y. Houjou, R. Sato, J. Alloys Compounds 330 (2002) 301. [6] H. Kanekiyo, M. Uehara, S. Hirosawa, IEEE Trans. Magn. 29 (1993) 2863. [7] A. Manaf, R.A. Buckley, H.A. Davis, J. Magn. Magn. Mater. 128 (1993) 302. [8] W.C. Chang, S.H. Wu, B.M. Ma, C.O. Bounds, J. Appl. Phys. 81 (1997) 4453. [9] J.R. Liu, M. Itoh, K. Machida, Chem. Lett. 32 (2003) 394. [10] C.J. Gutierrez, J.J. Krebs, G.A. Prinz, Appl. Phys. Lett. 61 (1992) 2467. [11] W.B. Weir, Proc. IEEE 62 (1974) 33. [12] J. Chamberlain, Can. J. Earth Sci. 2 (1966) 188. [13] G. Asti, S. Rinaldi, J. Appl. Phys. 45 (1974) 3600. [14] D.H. Qin, L. Cao, Q.Y. Sun, Y. Huang, H.L. Li, Chem. Phys. Lett. 358 (2002) 484.
Acknowledgements The authors wish to thank Mr. T. Surutome (Sumitomo Special Metals Co., Ltd.) for measurements of the EM wave absorption properties for a series of samples. This work was supported by Grant-in-Aid for Science Research No. 15205025 from the Ministry of Education, Science, Sports and Culture of Japan.
Journal of Magnetism and Magnetic Materials 271 (2004) L147–L152
Letter to the Editor
A GHz range electromagnetic wave absorber with wide bandwidth made of FeCo/Y2O3 nanocomposites Jiu Rong Liua, Masahiro Itoha, Jianzhuang Jiangb, Ken-ichi Machidaa,* a
Collaborative Research Center for Advanced Science and Technology, Osaka University, 2-1 Yamadaoka, Suita, Osaka 565-0871, Japan Key Laboratory for Colloid and Interface Chemistry of State Education Ministry, Department of Chemistry, Shandong University, Jinan 250100, China
b
Received 16 June 2003; received in revised form 2 December 2003
Abstract Nanocomposites consisting of fine particles of FeCo (B30 nm) and Y2O3 (B15 nm) have been prepared from an intermetallic compound Y2(Fe0.5Co0.5)17 by melt-spinning techniques and subsequent hydrogenation-disproportionation and oxidation treatments. The electromagnetic (EM) wave absorption properties were characterized in a frequency range of 0.05–20.05 GHz. The measured real part (e0r ) and imaginary part (e00r ) of the relative permittivity were small and approximately constant in the 1–10 GHz range. The imaginary part of relative permeability m00r exhibited two resonant peaks and a high value over the 1–10 GHz range. As a result, the resin composites with 80 wt% FeCo/Y2O3 powders exhibited excellent EM wave absorption properties (reflection loss RLo20 dB) in a frequency range of 2.7–8.1 GHz, with thickness of 2.2–5.7 mm, respectively. A minimum RL of 43 dB was observed at 5.6 GHz with an absorber thickness of 3 mm. r 2004 Elsevier B.V. All rights reserved. PACS: 72.80.Tm; 74.25.Ha; 75.20.En Keywords: Melt spinning; Nanocomposites; Electromagnetic wave absorption; Magnetic anisotropy; Matching frequency
1. Introduction With development of electromagnetic (EM) wave communication devices, EM wave absorbing materials continue to attract much attention as EM interference-suppressing coatings, self-concealing technology and microwave darkrooms. Among the candidates for such applications, soft metallic magnets are a promising and interesting *Corresponding author. Tel./fax: +81-6-6879-4209. E-mail address: [email protected] (K.-i. Machida).
set of materials. For magnetic EM wave absorbers, the complex permeability (m0 mr ¼ m0 jm00 ) and permittivity (e0 er ¼ e0 je00 ) of materials determine reflection and attenuation characteristics of absorbers, in which mr is the relative complex permeability and er is the relative complex permittivity; and there is a relationship between the thickness (dm ) and magnetic loss (m00r ) of absorbers given by dm ¼ c=2pfm m00r ;
ð1Þ
where c is velocity of light and fm the matching frequency. Since metallic magnetic materials have
0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.1386
ARTICLE IN PRESS L148
J.R. Liu et al. / Journal of Magnetism and Magnetic Materials 271 (2004) L147–L152
large saturation magnetization and the Snoek’s limit is at a high frequency [1–3], their complex permeability values remain high over a wide frequency range. Therefore, it is possible to make thin absorbers from such kinds of materials. However, the high-frequency permeability of these materials decreases due to the eddy current loss induced by EM waves. For this reason, it is better to use smaller particles which are isolated by insulating materials, such as epoxy resin. Sugimoto et al. [4,5] have reported the good EM wave absorption properties of the a-Fe/SmO composites derived from rare-earth intermetallic compounds in the 0.73–1.30 GHz range, which were prepared by an arc-melting technique. It is well known that melt-spinning techniques are an excellent way to produce homogenous microstructured materials with attractive properties for a variety of applications [6–8]. We also have reported that the a-Fe/Y2O3 composite materials prepared by melt-spinning techniques showed good EM wave absorption properties in the 2.0– 3.5 GHz range because of fine particle size of a-Fe (B20 nm) [9]. One criterion for selecting suitable magnetic EM wave absorption materials is the location of its natural ferromagnetic resonance frequency, which should be matched with the frequency of EM waves to be absorbed. For spin rotation, the general model developed by Snoek [3], predicts that the natural resonance frequency (fr ) is related to the anisotropy field HA by 2pfr ¼ gHA ;
ð2Þ
where g is the gyromagnetic ratio. Gutierrez et al. [10] have reported that MBE-grown FeCo films have large magnetocrystalline anisotropy constants K1. The purpose of this study was to investigate the effect of substitution a-Fe by Co, to form FeCo/Y2O3 nanocomposite materials, on the EM wave absorption properties and compare them with that of a-Fe/Y2O3. The nanocomposite materials FeCo/Y2O3 were prepared by the hydrogenation-disproportionation and the subsequent oxidation for Y2(Fe0.5Co0.5)17, which was prepared by melt-spinning techniques. The EM absorption properties of the resultant fine powder were measured in the 0.05–20.05 GHz range.
2. Experimental Intermetallic compound ingots of Y2(Fe0.5Co0.5)17 were first prepared from Y, Co and Fe metals (>99.9% in purity) by means of induction melting in Ar. Alloy ribbons of Y2(Fe0.5Co0.5)17 with 1.5 mm in width and about 50 mm in thickness were prepared from the ingots of Y2(Fe0.5Co0.5)17 by the single-roller melt-spun apparatus at a roll surface velocity of 20 m/s. After dry ball-milling in Ar, the Y2(Fe0.5Co0.5)17 powders with a particle size distribution of 2–4 mm, were heated at 873 K in H2 for 2 h under 2 MPa, and then were heated at 553 K for 2 h in an O2 stream. The powders were characterized by X-ray diffraction (XRD), and the microstructures were analyzed by a high-resolution scanning electron microscope (HITACHI S-5000) and transmission electron microscope (HITACHI H-800). Magnetization hysteresis curves of the powdered samples were recorded by a vibration sample magnetometer (Tamakawa, TM-VSM2014-MHR-Type) in a field up to 71.6 MA m1. Epoxy resin composites were prepared by homogenously mixing epoxy resin XN-5861 (Nippon Pelnox Corp.) with the 80 wt% composite powder and pressing into cylindrical shaped compacts. These compacts were cured by heating at 453 K for 30 min, and then they were cut into toroidal shaped samples of 7.00 mm outer diameter and 3.04 mm inner diameter. The scattering parameters (S11 ; S21 ) of the toroidal shaped sample were measured using a Hewlett-Packard 8720B network analyzer. The relative permeability (mr ) and permittivity (er ) values were determined from the scattering parameters as measured in the frequency range of 0.05–20.05 GHz [11]. The reflection loss (RL) curves were calculated from the relative permeability and permittivity at given frequency and absorber thickness with the following equations: Zin ¼ Z0 ðmr =er Þ1=2 tanhfjð2pfd=cÞðmr er Þ1=2 g;
ð3Þ
RL ¼ 20 logjðZin Z0 Þ=ðZin þ Z0 Þj;
ð4Þ
where f is the frequency of the EM wave, d the thickness of an absorber, c the velocity of light, Z0 the impedance of air, and Zin the input impedance
ARTICLE IN PRESS J.R. Liu et al. / Journal of Magnetism and Magnetic Materials 271 (2004) L147–L152
of absorber. In this investigation, the situation when RLo20 dB, is considered as an adequate EM absorption (the case when RL=20 dB, is comparable to the 99% of EM wave absorption), and is called a ‘‘matching situation’’. The absorber thickness and frequency in the matching situation are defined as matching thickness (dm ) and matching frequency (fm ), respectively. The attenuation of the EM wave and reflectance of an absorber at the surface were determined from the frequency dependency of RL.
3. Results and discussion Fig. 1 shows a set of typical XRD patterns measured on the hydrogenated and oxidized Y2(Fe0.5Co0.5)17 powders, together with the starting powder. It is confirmed from Fig. 1(a) that the starting powder prepared by using the meltspinning technique was Y2(Fe0.5Co0.5)17. After the treatment of hydrogenation-disproportionation as shown in Fig. 1(b), the Y2(Fe0.5Co0.5)17 peaks disappeared and two sets of peaks, for both the FeCo alloy and YH2 phases, appeared. After oxidizing the FeCo/YH2 powder, composite ma-
Fig. 1. The XRD pattern of the powder: (a) as-obtained, (b) after hydrogenation-disproportionation at 873 K for 2 h in H2 under 2 MPa, and (c) after oxidizing the sample (b) in O2 at 553 K for 2 h.
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terials of FeCo/Y2O3 were formed. The XRD pattern characteristic of FeCo/Y2O3 composites is shown in Fig. 1(c), not only the peaks of Y2O3 and FeCo clearly are observed, but also the intensity sequence for the peaks of FeCo alloy agrees with the standard powder diffraction measurements [12]. The mean crystalline size of the Y2O3 and FeCo particles was evaluated from the line broadening of the XRD peaks using Scherrer’s formula to be about 15 and 30 nm, respectively. From the TEM photographs of FeCo/Y2O3 composite powder, the particles are roughly of spherical shape with a narrow particle size distribution. Most FeCo grains appear as clusters and chains aggregated several crystallites with the size of 20– 40 nm (Fig. 2). This observation was in good agreement with the crystalline size estimated from XRD peaks by Scherrer’s formula. The frequency dependence of relative permittivity for resin composites of 80 wt% FeCo/Y2O3 is shown in Fig. 3. The real part e0r and imaginary part e00r of relative permittivity were almost constant (e0r ¼ B14 and e00r ¼ B0:6) in the 1– 10 GHz range, in which the relative permittivity (er ¼ e0r je00r ) showed less variation than the relative permeability. However, the Sm2Fe17 resin composites as reported by Sugimoto et al. [5], showed a high relative complex permittivity and nearly behaved as a metal conductor. The
Fig. 2. A TEM microgaph of FeCo/Y2O3 powder.
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Fig. 3. The relative permittivity as a function of frequency for the resin composites with 80 wt% FeCo/Y2O3 powder.
Fig. 4. The variations of m0r and m00r as a function of frequency for the resin composites with 80 wt% FeCo/Y2O3 or a-Fe/Y2O3 powder.
electrical resistivity of FeCo/Y2O3 composites was measured to be about 200 Om. The high resistivity of the nanocomposites is attributed to the microstructure of FeCo/Y2O3 nanoparticles, of which the Y2O3 phase plays a role as an insulator embeded among FeCo particles. The real part m0r and imaginary part m00r of relative permeability are plotted as a function of frequency in Fig. 4. For FeCo/Y2O3 resin composites, the real part of permeability m0r declined from 3.9 to 0.9 with increasing frequency. However, the imaginary part of permeability m00r showed two dispersion peaks and m00r value retained a high over the 1–10 GHz range, and then decreased gradually in the higher-frequency range (Fig. 4). This interesting resonance phenomenon can be explained as follows: as shown in Fig. 2, most nanometer FeCo crystallites form clusters and chains, so that the permeability spectra of FeCo/ Y2O3 composites comes from the cooperative effect of domain wall resonance and spin rotational resonance. The real part of relative permeability m0r of FeCo/Y2O3 (Fig. 4) exhibited almost the same variation with frequency in the 0.05– 10 GHz range as a-Fe/Y2O3; however, the peak of imaginary part of relative permeability m00r is broader in frequency. This can be attributed to the partial substitution of Co in the crystal lattice of a-Fe resulting in increasing the HA value. We
have employed the singular point detection method [13] to measure the anisotropy fields in FeCo/ Y2O3 nanocomposites, which exhibited large anisotropy fields (HA ¼ B0:24 T). The resonance frequency (fr ) of 8.5 GHz, estimated according to Eq. (2), is larger than that of a-Fe, which has an fr of 1.6 GHz [5]. As a result, the nanocomposite powder has a remarkable feature of EM wave absorption in the 1–10 GHz range. Fig. 5(a) shows a typical relationship between RL and frequency for the resin composites with 80 wt% FeCo/Y2O3 powder. First, the minimum RL was found to move toward the lowerfrequency region with increasing thickness. Second, the RL values of resin composites less than 20 dB were obtained in the 2.78.1 GHz frequency range. In particular, a minimum RL value of 43 dB was observed at 5.6 GHz with a matching thickness (dm ) of 3 mm, and the minimum dm value of 2.2 mm was obtained at 8.1 GHz. The EM wave absorption properties of FeCo/ Y2O3, a-Fe/SmO, and a-Fe/Y2O3 resin composites prepared under the optimized conditions are summarized in Table 1. Comparing FeCo/Y2O3 with a-Fe/Y2O3 sample [9], the minimum reflection point shifted to the higher frequency from 2.6 to 5.6 GHz, and moreover the EM wave absorption bandwidth, with RLo20 dB, is broadened from 1.5 to 5.4 GHz
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range (see Fig. 5). This is attributed to the increase in HA due to the partial substitution of a-Fe by Co resulting in the increase of the natural resonance frequency (see Fig. 4). Qin et al. [14] have reported the coercivity (Hc ) of FeCo alloy nanowire arrays with particle size of 30 nm reaching a high value (B2800 Oe). Such large coercivity was ascribed to the high-shape magnetic anisotropy of FeCo
Fig. 5. Frequency dependence of RL for the resin composites at different thickness with: (a) 80 wt% FeCo/Y2O3 powders, and (b) a-Fe/Y2O3 powder.
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magnetic grains by them. We have also measured the magnetic hysteresis loops of a-Fe/Y2O3 and FeCo/Y2O3 as shown in Fig. 6. The coercivity values of a-Fe/Y2O3 and FeCo/Y2O3 are very small (B300 Oe). This result can be explained by the lack of a shape effect on the magnetic anisotropy due to the roughly spherical shaped FeCo particles prepared by our method (Fig. 2). At the same time, it suggests that the higher HA value of FeCo/Y2O3 than that of a-Fe/Y2O3 comes from the partial substitution of Co in the crystal lattice of a-Fe. For this reason, the EM absorption of FeCo/Y2O3 resin composites is shifted to the higher-frequency range.
Fig. 6. Magnetic hysteresis loops for the FeCo/Y2O3 and a-Fe/ Y2O3 nanocomposites powder.
Table 1 Electromagnetic wave absorption properties of a-Fe/SmO, a-Fe/Y2O3 and FeCo/Y2O3 resin composites with 80 wt% powder Sample (80 wt%)
Apparatus
Mean particle Electromagnetic wave absorption properties of resin composites Ref. size of magnet (nm) Minimum RL dm (mm) fm (GHz) Frequency value (dB) (RLo20 dB) (minimum RL) range (GHz) (RLo20 dB)
a-Fe/SmO a-Fe/Y2O3 FeCo/Y2O3
Arc melting Melt spun Melt spun
B30 B20 B30
52 36 43
7.9–13.1 3–5 2.2–5.7
0.95 2.6 5.6
0.73–1.30 2–3.5 2.7–8.1
[5] [9] This work
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4. Conclusion
References
In conclusion, the FeCo/Y2O3 resin composites exhibited good EM absorption properties due to the low relative permittivity and high-permeability value in 1–10 GHz range. Compared with a-Fe/Y2O3, the FeCo/Y2O3 resin composites showed higher EM absorption frequency and wider absorption bandwidth (RLo20 dB) due to the larger magnetic anisotropy of FeCo nanoparticles. Our work suggests that the FeCo/Y2O3 can be used as good EM absorption material in the 2.7–8.1 GHz range.
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Acknowledgements The authors wish to thank Mr. T. Surutome (Sumitomo Special Metals Co., Ltd.) for measurements of the EM wave absorption properties for a series of samples. This work was supported by Grant-in-Aid for Science Research No. 15205025 from the Ministry of Education, Science, Sports and Culture of Japan.