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SiO2 core-shell nanoparticles
Solid State Communications 132 (2004) 71–74 www.elsevier.com/locate/ssc
Complex permeability of FeNi3/SiO2 core-shell nanoparticles N.J. Tanga,b,*, W. Zhonga, H.Y. Jianga, Z.D. Hana, W.Q. Zoua, Y.W. Dua a
National Laboratory of Solid State Microstructures, Physics Department, Nanjing University, Nanjing 210093, China b Materials Department, Nanchang Institute of Aeronautical Technology, Nanchang 330043, China Received 3 July 2004; received in revised form 13 July 2004; accepted 17 July 2004 by P. Wachter Available online 30 July 2004
Abstract In this paper, a simple sol–gel combined hydrogen reduction synthesis of FeNi3/SiO2 core-shell nanoparticles was reported. The real part m 0 of the permeability for FeNi3/SiO2 nanoparticles does not decrease in the frequency range measured, even up to at least 1 GHz, simultaneously, the loss of m 00 is very small. Our method provides a promising route to achieve soft magnetic material with good magnetic properties, especially in the high-frequency range. q 2004 Elsevier Ltd. All rights reserved. PACS: 75.50.Bb; 75.60.-d; 81.20.Ka Keywords: A. Soft magnetic materials; A. FeNi; A. SiO2-coat; D. Permeability
1. Introduction For the electromagnetic device applications at high frequencies, ferrite is commonly used. Besides the low price, they have the advantage of high resistivity and the possibility of making cores of various forms. However, they exhibit a negative thermal coefficient of resistivity and have a relatively low Curie temperature. Researchers have long been searching for soft magnetic materials with high saturation magnetization, high permeability, and low energy losses. FeNi3 is good soft magnetic material with both high saturation magnetization and low coercivity [1]. However, due to its metallic characteristics, the eddy current generation severely limits its application at high frequencies. It is, therefore, expectable that one may obtain high permeability in magnetic nanocomposite by coating an insulating shell on the surface of its soft magnetic nanoparticle cores. Recently, FeNi [1] coating with SiO2 * Corresponding author. Address: National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China. Tel.: C86-25-83594588; fax: C86-25-83595535. E-mail address: [email protected] (N.J. Tang). 0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2004.07.048
has been synthesized, and its permeability is independent of frequency even in high frequency range. SiO2-coated transition metal and alloy nanoparticles have been prepared by many methods, but the coating process by introduction of amorphous silica layer on the surface of either metal [2]/ alloys [1] or precursor precipitates [3] from aqueous solution is not easy to control, especially the dosage of coating silica. In this paper, we report coating FeNi3 nanoparticles with silica shells by a simple sol–gel combined hydrogen reduction method. This method is effective and the process is easy to control. The real part m 0 of permeability is almost independent of frequency up to at least 1 GHz, with value as high as 14. The dependence of magnetic properties on annealing temperatures is also investigated.
2. Experimental In this work, FeCl2$4H2O, citric acid monohydrate and ethanol absolute were used as the raw material, coordination agent, and solvent, respectively. In a typical experimental, 0.01 mol FeCl2$4H2O, 0.03 mol NiCl2$6H2O, 0.06 mol
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N.J. Tang et al. / Solid State Communications 132 (2004) 71–74
citric acid monohydrate were dissolved in 100 ml ethanol absolute, and stirred at 60 8C for 6 h. Then 0.8 ml tetraethyl silicate (TEOS) was added in the solution. After dried at 80 8C, the xerogel was heated at 450 8C for 3 h in air, and then the powder was reduced in H2 gas at different temperatures for 4 h. The phase identification and structural analysis of the sample were examined by X-ray powder diffraction (XRD) with Cu Ka radiation (Model D/Max-RA, Rigaku, Japan). The morphology of the samples was examined by direct observation via high-resolution transmission election microscopy (HRTEM) (Model JEOL-2010, Japan). Their magnetic properties were measured using a vibrating sample magnetometer (VSM) (Lakeshore, USA). Complex permeability spectra were measured with an impedance analyzer (Agilent4284A from 20 Hz to 1 MHz) and an impedance/material analyzer (Agilent4191B from 1 MHz to 1.8 GHz).
3. Results and discussion XRD plots of the sample reduced at different temperatures are presented in Fig. 1. For all the samples, some diffraction peaks can be seen at about 44.12, 51.4 and 75.648, which can be interpreted as the position of FeNi3 with a cubic structure. The absence of XRD peaks, characteristics of a-Fe (i.e. at 2q of 65.2 and 82.38), indicates that FeNi alloy is formed. Of all the samples, we detect only the FeNi3 phase without any other phase formation in the XRD patterns corresponding to iron or nickel oxides. The hydrogen reduction process efficiently converts the oxide particles into FeNi3. No crystalline SiO2 phase is found in all the samples. This is because silica as shell is a thin amorphous layer coating the iron core (Fig. 4b).
Fig. 1. XRD patterns of the FeNi3/SiO2 nanoparticles reduced at different temperatures.
The synthesized particles exhibit a dependence of magnetization on reduction temperatures. The influence of reduction temperature on magnetic properties of sample is shown in Fig. 2. Below 500 8C, with increasing reduction temperature, the specific magnetization increases and coercivity HC decreases very fast. However, above 500 8C, with further increasing the reduction temperature, the magnetization decreases and coercivity increases slightly. They reach a maximum (105.77 emu/g) and minimum (19.72 Oe), respectively, for the reduction temperature of 800 8C. The sample reduced at 400 8C has the highest coercivity and the lowest magnetization. The sample reduced at 400 8C maybe has an inner precursor compound core because the precursor compound has not been completely converted to FeNi3. Although, there are no other peaks except FeNi3 in the sample from Fig. 1a, we cannot deduce that there is no precursor compound core in the sample. So, the sample has the highest coercivity and the lowest magnetization. The size effect is believed to be the reason for the behavior of lower coercivity of the particles treated at higher temperature [4], according to the equation valid for multidomain particles Hc Z a C
b D
where D is particle size and a and b are constants. The effective FeNi3 particle size in the samples reduced at higher temperature should be larger than that in the samples reduced at lower temperature, due to the presence of the precursor compound core. On the other hand, because the particle size may increases with increasing the heating temperatures, the particle size of the samples heated at higher temperatures should larger than that of the samples heated at lower temperatures. So, the particle size in the samples reduced at higher temperature should be larger than that in the samples reduced at lower temperatures. Larger Ni3Fe particles exhibited lower coercivity. As a result, the sample reduced at higher temperature had lower coercivity.
Fig. 2. The variations of magnetization and coercivity as a function of reduction temperatures.
N.J. Tang et al. / Solid State Communications 132 (2004) 71–74
The complex permeability of the sample reduced at 800 8C, having the lowest HC, the highest MS, and good soft magnetic property, was measured. For complex permeability measurements, the powder sample was pressed into a ring. The frequency dependence of the complex permeability is shown in Fig. 3. There is a remarkable feature that the real part m 0 of the permeability for FeNi3/ SiO2 nanoparticles does not decrease in all the frequency range, even up to at least 1 GHz, which is much higher than the cut-off frequency of widely used in CoNi ferrite (10 MHz). No relaxation dispersion and resonance peak were observed in the frequency range measured. This can be explained by the insulation of SiO2 shell. That the relaxation dispersion appears at 10 MHz is because the size of the FeNi3 is large. For metal magnetic materials, the low cut-off frequency is probably related to the eddy currents due to poor insulation between particles. The total losses of a material can be related to its resistivity, which affects the losses of m 00 due to eddy current, induced under the action of a varying magnetic field. The current can lead to significant losses when the applied field is alternative current (AC), particularly at high frequencies, and can cause significant heating of the material. The insulation of the SiO2 shell between particles results in the increase of the resistivity of the sample, which may reduce the effect of eddy current in high frequency range, and make the real part m 0 of the permeability remain almost constant. If there is no SiO2 coating, direct metal contact would take place, the resulting eddy current would cause m 0 to decrease with frequency fast and imaginary part m 00 would reach a maximum at a lower frequency. As shown in Fig. 3, however, m 0 is almost unchanged and m 00 is still small up to 1 GHz for the sample, which indicates a good insulation of the metal particles by SiO2. In order to confirm the explanations above, transmission electron microscope (TEM) and HRTEM were
73
Fig. 4. The typical TEM and HRTEM images for the core-shell nanoparticles heated at 800 8C.
used to observe the nanoparticles. In Fig. 4(a) and (b), the TEM and HRTEM images demonstrate that the FeNi3 nanoparticles are full coverage of the amorphous silica shell with about 2 nm, which as an insulation layer coats FeNi3 core tightly. The interface between core and shell can be seen clearly. The coupling can be induced through direct exchange coupling and dipolar interaction. The direct exchange coupling is the main effect to improve the soft magnetic properties in permeability, which becomes more pronounced in samples with thin coatings and large densities. The dipolar interaction is a long-range interaction and exists in all samples, including those with low density, and is not the main contribution to improve the permeability. In this sample, the dipole reaction is main effect because each FeNi3 core is coated by silica shell, and the range between neighbor cores is too large, and direct exchange is weak. So, the m 0 is not very high. The permeability can be further significantly improved by optimizing the thickness of the shell and the density of the samples. [1]
4. Conclusion In conclusion, FeNi3/SiO2 nanoparticles are successfully synthesized by a simple sol–gel method. The silica shell is an insulating layer, which makes the m 0 of FeNi3/SiO2 constant and m 00 very small up to at least 1 GHz. The method provides us not only a good opportunity to produce a large quantity of core-shell nanoparticles using a simple procedure, but a promising route to achieve soft magnetic nanocomposite material with good soft magnetic property, especially in the high-frequency range.
Acknowledgements
Fig. 3. Frequency dependence of the complex permeability of the sample reduced at 800 8C.
This work was supported by the Chinese Ministry of Science and Technology, the National Key Project for Basic Research (grant G1999064508), and Sino-Israeli joint research project.
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N.J. Tang et al. / Solid State Communications 132 (2004) 71–74
References [1] Y.W. Zhao, C.Y. Ni, D. Kruczynski, X.K. Zhang, J.Q. Xiao, J. Phys. Chem. B 108 (2004) 3691. [2] Y. Kobayashi, M. Horie, M. Konno, B. Rodrı´guez-Gonza´lez, L.M. Liz-Marza´n, J. Phys. Chem. B 107 (2003) 7420.
[3] H.Y. Jiang, W. Zhong, N.J. Tang, X.S. Liu, Y.W. Du, Chin. Phys. Lett. 20 (2003) 1855. [4] M.Z. Wu, Y.D. Zhang, S. Hui, T.D. Xiao, S.H. Ge, W.A. Hines, J.I. Budnick, M.J. Yacaman, J. Appl. Phys. 92 (2002) 6809.
Complex permeability of FeNi3/SiO2 core-shell nanoparticles N.J. Tanga,b,*, W. Zhonga, H.Y. Jianga, Z.D. Hana, W.Q. Zoua, Y.W. Dua a
National Laboratory of Solid State Microstructures, Physics Department, Nanjing University, Nanjing 210093, China b Materials Department, Nanchang Institute of Aeronautical Technology, Nanchang 330043, China Received 3 July 2004; received in revised form 13 July 2004; accepted 17 July 2004 by P. Wachter Available online 30 July 2004
Abstract In this paper, a simple sol–gel combined hydrogen reduction synthesis of FeNi3/SiO2 core-shell nanoparticles was reported. The real part m 0 of the permeability for FeNi3/SiO2 nanoparticles does not decrease in the frequency range measured, even up to at least 1 GHz, simultaneously, the loss of m 00 is very small. Our method provides a promising route to achieve soft magnetic material with good magnetic properties, especially in the high-frequency range. q 2004 Elsevier Ltd. All rights reserved. PACS: 75.50.Bb; 75.60.-d; 81.20.Ka Keywords: A. Soft magnetic materials; A. FeNi; A. SiO2-coat; D. Permeability
1. Introduction For the electromagnetic device applications at high frequencies, ferrite is commonly used. Besides the low price, they have the advantage of high resistivity and the possibility of making cores of various forms. However, they exhibit a negative thermal coefficient of resistivity and have a relatively low Curie temperature. Researchers have long been searching for soft magnetic materials with high saturation magnetization, high permeability, and low energy losses. FeNi3 is good soft magnetic material with both high saturation magnetization and low coercivity [1]. However, due to its metallic characteristics, the eddy current generation severely limits its application at high frequencies. It is, therefore, expectable that one may obtain high permeability in magnetic nanocomposite by coating an insulating shell on the surface of its soft magnetic nanoparticle cores. Recently, FeNi [1] coating with SiO2 * Corresponding author. Address: National Laboratory of Solid State Microstructures, Nanjing University, Nanjing 210093, China. Tel.: C86-25-83594588; fax: C86-25-83595535. E-mail address: [email protected] (N.J. Tang). 0038-1098/$ - see front matter q 2004 Elsevier Ltd. All rights reserved. doi:10.1016/j.ssc.2004.07.048
has been synthesized, and its permeability is independent of frequency even in high frequency range. SiO2-coated transition metal and alloy nanoparticles have been prepared by many methods, but the coating process by introduction of amorphous silica layer on the surface of either metal [2]/ alloys [1] or precursor precipitates [3] from aqueous solution is not easy to control, especially the dosage of coating silica. In this paper, we report coating FeNi3 nanoparticles with silica shells by a simple sol–gel combined hydrogen reduction method. This method is effective and the process is easy to control. The real part m 0 of permeability is almost independent of frequency up to at least 1 GHz, with value as high as 14. The dependence of magnetic properties on annealing temperatures is also investigated.
2. Experimental In this work, FeCl2$4H2O, citric acid monohydrate and ethanol absolute were used as the raw material, coordination agent, and solvent, respectively. In a typical experimental, 0.01 mol FeCl2$4H2O, 0.03 mol NiCl2$6H2O, 0.06 mol
72
N.J. Tang et al. / Solid State Communications 132 (2004) 71–74
citric acid monohydrate were dissolved in 100 ml ethanol absolute, and stirred at 60 8C for 6 h. Then 0.8 ml tetraethyl silicate (TEOS) was added in the solution. After dried at 80 8C, the xerogel was heated at 450 8C for 3 h in air, and then the powder was reduced in H2 gas at different temperatures for 4 h. The phase identification and structural analysis of the sample were examined by X-ray powder diffraction (XRD) with Cu Ka radiation (Model D/Max-RA, Rigaku, Japan). The morphology of the samples was examined by direct observation via high-resolution transmission election microscopy (HRTEM) (Model JEOL-2010, Japan). Their magnetic properties were measured using a vibrating sample magnetometer (VSM) (Lakeshore, USA). Complex permeability spectra were measured with an impedance analyzer (Agilent4284A from 20 Hz to 1 MHz) and an impedance/material analyzer (Agilent4191B from 1 MHz to 1.8 GHz).
3. Results and discussion XRD plots of the sample reduced at different temperatures are presented in Fig. 1. For all the samples, some diffraction peaks can be seen at about 44.12, 51.4 and 75.648, which can be interpreted as the position of FeNi3 with a cubic structure. The absence of XRD peaks, characteristics of a-Fe (i.e. at 2q of 65.2 and 82.38), indicates that FeNi alloy is formed. Of all the samples, we detect only the FeNi3 phase without any other phase formation in the XRD patterns corresponding to iron or nickel oxides. The hydrogen reduction process efficiently converts the oxide particles into FeNi3. No crystalline SiO2 phase is found in all the samples. This is because silica as shell is a thin amorphous layer coating the iron core (Fig. 4b).
Fig. 1. XRD patterns of the FeNi3/SiO2 nanoparticles reduced at different temperatures.
The synthesized particles exhibit a dependence of magnetization on reduction temperatures. The influence of reduction temperature on magnetic properties of sample is shown in Fig. 2. Below 500 8C, with increasing reduction temperature, the specific magnetization increases and coercivity HC decreases very fast. However, above 500 8C, with further increasing the reduction temperature, the magnetization decreases and coercivity increases slightly. They reach a maximum (105.77 emu/g) and minimum (19.72 Oe), respectively, for the reduction temperature of 800 8C. The sample reduced at 400 8C has the highest coercivity and the lowest magnetization. The sample reduced at 400 8C maybe has an inner precursor compound core because the precursor compound has not been completely converted to FeNi3. Although, there are no other peaks except FeNi3 in the sample from Fig. 1a, we cannot deduce that there is no precursor compound core in the sample. So, the sample has the highest coercivity and the lowest magnetization. The size effect is believed to be the reason for the behavior of lower coercivity of the particles treated at higher temperature [4], according to the equation valid for multidomain particles Hc Z a C
b D
where D is particle size and a and b are constants. The effective FeNi3 particle size in the samples reduced at higher temperature should be larger than that in the samples reduced at lower temperature, due to the presence of the precursor compound core. On the other hand, because the particle size may increases with increasing the heating temperatures, the particle size of the samples heated at higher temperatures should larger than that of the samples heated at lower temperatures. So, the particle size in the samples reduced at higher temperature should be larger than that in the samples reduced at lower temperatures. Larger Ni3Fe particles exhibited lower coercivity. As a result, the sample reduced at higher temperature had lower coercivity.
Fig. 2. The variations of magnetization and coercivity as a function of reduction temperatures.
N.J. Tang et al. / Solid State Communications 132 (2004) 71–74
The complex permeability of the sample reduced at 800 8C, having the lowest HC, the highest MS, and good soft magnetic property, was measured. For complex permeability measurements, the powder sample was pressed into a ring. The frequency dependence of the complex permeability is shown in Fig. 3. There is a remarkable feature that the real part m 0 of the permeability for FeNi3/ SiO2 nanoparticles does not decrease in all the frequency range, even up to at least 1 GHz, which is much higher than the cut-off frequency of widely used in CoNi ferrite (10 MHz). No relaxation dispersion and resonance peak were observed in the frequency range measured. This can be explained by the insulation of SiO2 shell. That the relaxation dispersion appears at 10 MHz is because the size of the FeNi3 is large. For metal magnetic materials, the low cut-off frequency is probably related to the eddy currents due to poor insulation between particles. The total losses of a material can be related to its resistivity, which affects the losses of m 00 due to eddy current, induced under the action of a varying magnetic field. The current can lead to significant losses when the applied field is alternative current (AC), particularly at high frequencies, and can cause significant heating of the material. The insulation of the SiO2 shell between particles results in the increase of the resistivity of the sample, which may reduce the effect of eddy current in high frequency range, and make the real part m 0 of the permeability remain almost constant. If there is no SiO2 coating, direct metal contact would take place, the resulting eddy current would cause m 0 to decrease with frequency fast and imaginary part m 00 would reach a maximum at a lower frequency. As shown in Fig. 3, however, m 0 is almost unchanged and m 00 is still small up to 1 GHz for the sample, which indicates a good insulation of the metal particles by SiO2. In order to confirm the explanations above, transmission electron microscope (TEM) and HRTEM were
73
Fig. 4. The typical TEM and HRTEM images for the core-shell nanoparticles heated at 800 8C.
used to observe the nanoparticles. In Fig. 4(a) and (b), the TEM and HRTEM images demonstrate that the FeNi3 nanoparticles are full coverage of the amorphous silica shell with about 2 nm, which as an insulation layer coats FeNi3 core tightly. The interface between core and shell can be seen clearly. The coupling can be induced through direct exchange coupling and dipolar interaction. The direct exchange coupling is the main effect to improve the soft magnetic properties in permeability, which becomes more pronounced in samples with thin coatings and large densities. The dipolar interaction is a long-range interaction and exists in all samples, including those with low density, and is not the main contribution to improve the permeability. In this sample, the dipole reaction is main effect because each FeNi3 core is coated by silica shell, and the range between neighbor cores is too large, and direct exchange is weak. So, the m 0 is not very high. The permeability can be further significantly improved by optimizing the thickness of the shell and the density of the samples. [1]
4. Conclusion In conclusion, FeNi3/SiO2 nanoparticles are successfully synthesized by a simple sol–gel method. The silica shell is an insulating layer, which makes the m 0 of FeNi3/SiO2 constant and m 00 very small up to at least 1 GHz. The method provides us not only a good opportunity to produce a large quantity of core-shell nanoparticles using a simple procedure, but a promising route to achieve soft magnetic nanocomposite material with good soft magnetic property, especially in the high-frequency range.
Acknowledgements
Fig. 3. Frequency dependence of the complex permeability of the sample reduced at 800 8C.
This work was supported by the Chinese Ministry of Science and Technology, the National Key Project for Basic Research (grant G1999064508), and Sino-Israeli joint research project.
74
N.J. Tang et al. / Solid State Communications 132 (2004) 71–74
References [1] Y.W. Zhao, C.Y. Ni, D. Kruczynski, X.K. Zhang, J.Q. Xiao, J. Phys. Chem. B 108 (2004) 3691. [2] Y. Kobayashi, M. Horie, M. Konno, B. Rodrı´guez-Gonza´lez, L.M. Liz-Marza´n, J. Phys. Chem. B 107 (2003) 7420.
[3] H.Y. Jiang, W. Zhong, N.J. Tang, X.S. Liu, Y.W. Du, Chin. Phys. Lett. 20 (2003) 1855. [4] M.Z. Wu, Y.D. Zhang, S. Hui, T.D. Xiao, S.H. Ge, W.A. Hines, J.I. Budnick, M.J. Yacaman, J. Appl. Phys. 92 (2002) 6809.