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Surfactant-assisted preparation of single-crystalline Fe3O4 nanowires under low magnetic field
Materials Letters 61 (2007) 1629 – 1632 www.elsevier.com/locate/matlet
Surfactant-assisted preparation of single-crystalline Fe3O4 nanowires under low magnetic field Jingang Zhang a,b , Jiafu Chen a,⁎, Zhaoxu Wang a,b a
Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China (USTC), Hefei, Anhui, 230026, PR China b Department of Polymer Science and Engineering, USTC, PR China Received 10 March 2006; accepted 22 July 2006 Available online 11 August 2006
Abstract Fe3O4 nanowires were successfully synthesized from ferrous chloride (FeCl2·4H2O) and diamine hydrate (H4N2·H2O) via the surfactantassisted redox hydrothermal process induced by low magnetic field. The products as-prepared were characterized by X-ray diffraction, TEM and HRTEM. The mechanism for the formation of single-crystalline Fe3O4 nanowires was discussed based on the oriented growth of magnetic materials. © 2006 Elsevier B.V. All rights reserved. Keywords: Fe3O4; Nanowires; Magnetic field-induced; Surfactant-assisted
1. Introduction Recently, considerable attention has been drawn towards onedimensional (1D) nanostructure materials, such as nanotubes, nanorods, and nanowires, because of their unique chemical and physical properties as a result of their low dimensionality and quantum confinement effect [2,3], especially, anisotropic magnetic nanoparticles have been found to possess interesting magnetic properties because of their shape anisotropy [4], and much effort has been made to study their potential applications in ultrahigh-density magnetic storage devices [5,6], nanodevices [7,8] and so on. Their reduced size and large surface-to-volume ratios also give them some novel magnetic [9], electronic [10], and optical [11] properties which are different from those of their bulk counterparts. Till now, many approaches have been developed for the preparation of magnetic materials such as magnetic metals, alloys, and metal oxides, for example, hard-template process (using the anodic aluminum oxide template) [12–14], softtemplate (lecithin) [15,16], redox methods and so on. It's noticed that most of these studies tend to get poly-crystalline products, acicular particles or uniform spherical forms. As it's known, ⁎ Corresponding author. E-mail address: [email protected] (J. Chen). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.07.088
magnetic materials possess easy magnetic axes. Fe3O4, for example, has easy magnetic axes along the [111] and [110] directions [17]. Therefore, it's believed that external magnetic field might induce an oriented growth of magnetic materials, which can result to the formation of 1D nanostructures. Chen's group did the magnetic field-induced growth of singlecrystalline Fe3O4 nanowires, but in their case, the maximum magnetic field applied was 0.35 T [1], and below this magnetic field, it's hard to get a large amount of single-crystalline Fe3O4 nanowires. So, it's very interesting and exciting to explore the synthesis of single-crystalline Fe3O4 nanowires at a very low magnetic field. Of course, experimental conditions need to be carefully selected and well controlled for the 1D nanostructure of magnetic material. In this letter, we report the preparation of single-crystalline Fe3O4 nanowires by means of a simple surfactant-assisted redox hydrothermal route under a low magnetic field (at 0.035 T). Based on the oriented growth of magnetic materials, the mechanism for the formation of Fe3O4 nanowires is also proposed. 2. Experimental procedure The chemical reagents used in the work are ferrous chloride (FeCl2·4H2O), diamine hydrate (H4N2·H2O, 50% water), the surfactant polyvinyl pyrrolidone (PVP) and sodium hydroxide
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Fig. 1. XRD pattern of the as-grown Fe3O4 nanowires.
(NaOH). All chemicals are of analytical grade. The reaction cells were self-made with permanent magnet under a Teflon (poly(tetrafluoroethylene)) lining stainless autoclave. The
strength of the magnetic field on the inner lower surface of the cell was 0.035 T. Distilled water, degassed with N2 for half an hour, was used for the preparation of PVP and Fe2+ (0.1 mol/ L) aqueous solution. The solution (30 mL) was put into the reaction cell, NaOH (0.6 g) was dissolved into H4N2·H2O (5 mL) and then the mixture was slowly dropped into the autoclave. During the experiment, N2 continuously bubbled the solution to prevent the oxidation of Fe2+ in the system. The autoclave was put into an oven, kept at 130 °C for 6 h, and then cooled to room temperature naturally. The products were filtered and washed several times with distilled water and absolute ethanol, and finally dried in a vacuum oven at 25 °C for 12 h. The sample was characterized by XRD using an X-ray diffractometer with high-intensity Cu Kα radiation (λ = 1.5418 Å), recorded at a scanning rate of 0.05° per second with the 2θ ranging from 25° to 70°. Transmission electron microscope (TEM) images were taken by a Hitachi model H-800, using an accelerating voltage of 200 kV. High-resolution transmission
Fig. 2. TEM images of the samples obtained in zero magnetic field and no PVA (A); 0.035 T magnetic field and PVA-assisted (B); zero magnetic and PVA-assisted (C); 0.035 T magnetic field and no PVA (D).
J. Zhang et al. / Materials Letters 61 (2007) 1629–1632
electron microscope (HRTEM) images were taken by JEOL2010 with an accelerating voltage of 200 kV.2 3. Results and discussion As shown in Fig. 1, the XRD pattern indicates that the as-grown products in the reaction are pure Fe3O4, although the diffraction peaks are broadened owing to their small size. All the diffraction peaks can be indexed by the cubic structure of Fe3O4 (JCPDS card no. 85-1436). No other peaks for impurities are detected. However, the XRD pattern cannot provide enough evidence to confirm the formation of Fe3O4, since there is a little difference between the XRD patterns of Fe3O4 and that of γ-Fe2O3. The black color of the sample as-prepared suggested the formation of Fe3O4 but not of γ-Fe2O3. On the basis of the Scherrer equation [18] we can figure it out that the average diameter of the Fe3O4 nanowires is 25–35 nm, and the length of the nanowires is about 0.5–1.0 μm. Fig. 2 shows the TEM images of the samples prepared under different conditions. It's found that Fe3O4 particles are square and hexagonal in shape without external magnetic field and not surfactant-assisted (Fig. 2A). When PVP but not an external magnetic field was applied, the shape of the Fe3O4 particles formed (seen in Fig. 2C) is also similar to that of the particles shown in Fig. 2A. Furthermore, the reaction process under low external magnetic field but not PVP-assisted, the final Fe3O4 particles are still square and hexagonal as shown in Fig. 2D. When the reaction system was performed in a low magnetic field (0.035 T) and was PVPassisted, the morphology of the sample would change drastically, and a large amount of Fe3O4 nanowires was prepared (seen in Fig. 2B), clearly revealing the co-effect of magnetic field and surfactant-assisted on the nucleation and oriented growth of Fe3O4 nanowires. These results show that the external magnetic field should influence the shape of ferromagnetic materials, and the surfactant should be necessary in preparing one-dimensional nanoparticles under lower strength of the magnetic field. This conclusion confirmed the case reported by Chen's group [1] in which they could not also get a large amount of Fe3O4 nanowires which was not surfactant-assisted under a magnetic field lower than 0.35 T. Electron Spin Resonance (ESR) experimental results (to be processed and reported elsewhere) show that the magnetic properties of these Fe3O4
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nanowires are a little different from those of bulk Fe3O4 or Fe3O4 clusters, and it can be observed that there is a co-existing ferromagnetic and paramagnetic state at room temperature (300 K) for these Fe3O4 nanowires, but only a ferromagnetic state for bulk Fe3O4 or Fe3O4 clusters in general. The HRTEM image of the boxed area in Fig. 3A further supports the single-crystalline nature of these Fe3O4 nanowires (Fig. 3B). The lattice fringe (∼0.48 nm) observed in this image well agrees with the separation between the (111) lattice planes. Combined with the ED results, it is confirmed that the nanowires have grown along the [110] direction, one of the easy magnetic axes of Fe3O4. As mentioned above, if there were no magnetic field and surfactant PVP during the reaction process, the shape of the particles we got was square and hexagonal. So the formation of the wire or rod of magnetite is speculated to result from the cooperation of the magnetic fieldinduced effect and the soft-template effect of PVP. We could surmise that while the reaction cell was put into an external magnetic field during the hydrothermal process, the Fe3O4 particles would take on a directional array along the direction of the magnetic field at first, and then the order state of the nucleus might result in the ordinal growth which yields the nanowires or nanorods. As we know, non-ionized surfactant molecules can form chain-like structures due to their assembly in water. Therefore, it can be supposed that many 1D liquid reaction places are formed when PVP and water are mixed together. The growth of Fe3O4 is strictly limited in such 1D separated field along the directional space. Owing to the inhibition of the surfactant molecules, the induced effect of the external magnetic field can be easily actualized, so the equal effect only requires much lower intensity of the magnetic field. It's hoped that more interesting results would be obtained if the strength of the magnetic field could be enhanced higher in our case. Though a detailed analysis of the magnetic effect is too complex to explain clearly, we may suggest that the magnetic field could induce nanoparticles growth along the easy magnetic axis of Fe3O4 in aqueous solution. But referring to the former literature, it's easily understood that only under a higher strength of the external magnetic field (up to 0.35 T) [1] can nanowires or nanorods be prepared successfully without a surfactant, because the soft-template-assisted function is by no means substituted under a lower magnetic field.
Fig. 3. HRTEM images of the sample, the inset shows an ED pattern of the individual Fe3O4 nanowires. The strength of the magnetic field is 0.035 T in the nucleation process.
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4. Conclusion In summary, single-crystalline Fe3O4 nanoparticles were successfully synthesized through a hydrothermal route at 130 °C for 6 h induced by an external magnetic field (0.035 T) and PVP-assisted. These nanoparticles prepared with diameters of 25–35 nm and lengths of about 0.5–1.0 μm grew along the easy magnetization axis [110] of Fe3O4. It's believed that the formation of the wire or rod of magnetite is the result of the cooperation of the magnetic field-induced effect and the soft-template effect of PVP. The synthesis route used here may stimulate technological interest and may also have many applications in magnetic materials. Acknowledgements This work was financially supported by the Department of Basic Research, Ministry of Science and Technology, P.R.C., the Grants-in-aids for the special projects of the significant basic research (2003CCA03700). This study was also supported by NSFC (Project No. 20473082). References [1] J. Wang, Q.W. Chen, C. Zeng, B.Y. Hou, Adv. Mater. 16 (2004) 137. [2] J.T. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435. [3] F. Duda, J. Kohnoff, M. Parrinello, Phys. Rev. Lett. 69 (1992) 1272.
[4] S.J. Park, S. Kim, S. Lee, Z.G. Khim, K. Char, T. Hyeon, J. Am. Chem. Soc. 122 (2000) 8581. [5] C.J. Murphy, N.R. Jana, Adv. Mater. 14 (2002) 80. [6] N. Cordente, M. Respaud, F. Senocq, M.J. Casanove, C. Amiens, B. Chaudret, Nano Lett. 10 (2001) 565. [7] J.F. Wang, M.S. Gudiksen, X.F. Duan, Y. Cui, C.M. Lieber, Science 293 (2001) 1445. [8] H. Kind, H.Q. Yan, B. Messer, M. Law, P.D. Yang, Adv. Mater. 14 (2002) 158. [9] T. Thurn-Albrecht, J. Schotter, C.A. Kastle, N. Emley, T. Shibauchi, L. Krusin-Elbaum, K. Guarini, C.T. Black, M.T. Tuoninen, T.P. Russell, Science 290 (2000) 2126. [10] W.J. Liang, M. Bockrath, D. Bozovic, J.H. Hafner, M. Tinkham, H. Park, Nature 411 (2001) 665. [11] X.F. Duan, Y. Huang, Y. Cui, J. Wang, C.M. Lieber, Nature 409 (2001) 6816. [12] (a) M. Tanase, D.M. Silevitch, A. Hultgren, L.A. Baner, P.C. Searson, G.J. Meyer, D.H. Reich, J. Appl. Phys. 91 (2002) 8549; (b) S.H. Ge, C. Li, X. Ma, W. Li, L. Xi, C.X. Li, J. Appl. Phys. 90 (2001) 509. [13] S.G. Yang, H. Zhu, D.L. Yu, Z.Q. Jin, S.L. Tang, Y.W. Du, J. Magn. Magn. Mater. 222 (2000) 97. [14] H.R. Khan, K. Petrikowski, Mater. Sci. Eng., C, Biomim. Mater., Sens. Syst. 19 (2002) 345. [15] J.L. Cain, D.E. Nikles, IEEE Trans. Magn. 3 (1997) 3718. [16] M. Chen, D.E. Nikles, J. Appl. Phys. 85 (1999) 5540. [17] Y.L. Li, G.D. Li, Physics of Ferrite, Science Publishing Corporation, Beijing, 1978, 381 pp. [18] C.N.J. Wagner, E.N. Aqua, Adv. X-ray Anal. 7 (1964) 46.
Surfactant-assisted preparation of single-crystalline Fe3O4 nanowires under low magnetic field Jingang Zhang a,b , Jiafu Chen a,⁎, Zhaoxu Wang a,b a
Hefei National Laboratory for Physical Sciences at Microscale, University of Science and Technology of China (USTC), Hefei, Anhui, 230026, PR China b Department of Polymer Science and Engineering, USTC, PR China Received 10 March 2006; accepted 22 July 2006 Available online 11 August 2006
Abstract Fe3O4 nanowires were successfully synthesized from ferrous chloride (FeCl2·4H2O) and diamine hydrate (H4N2·H2O) via the surfactantassisted redox hydrothermal process induced by low magnetic field. The products as-prepared were characterized by X-ray diffraction, TEM and HRTEM. The mechanism for the formation of single-crystalline Fe3O4 nanowires was discussed based on the oriented growth of magnetic materials. © 2006 Elsevier B.V. All rights reserved. Keywords: Fe3O4; Nanowires; Magnetic field-induced; Surfactant-assisted
1. Introduction Recently, considerable attention has been drawn towards onedimensional (1D) nanostructure materials, such as nanotubes, nanorods, and nanowires, because of their unique chemical and physical properties as a result of their low dimensionality and quantum confinement effect [2,3], especially, anisotropic magnetic nanoparticles have been found to possess interesting magnetic properties because of their shape anisotropy [4], and much effort has been made to study their potential applications in ultrahigh-density magnetic storage devices [5,6], nanodevices [7,8] and so on. Their reduced size and large surface-to-volume ratios also give them some novel magnetic [9], electronic [10], and optical [11] properties which are different from those of their bulk counterparts. Till now, many approaches have been developed for the preparation of magnetic materials such as magnetic metals, alloys, and metal oxides, for example, hard-template process (using the anodic aluminum oxide template) [12–14], softtemplate (lecithin) [15,16], redox methods and so on. It's noticed that most of these studies tend to get poly-crystalline products, acicular particles or uniform spherical forms. As it's known, ⁎ Corresponding author. E-mail address: [email protected] (J. Chen). 0167-577X/$ - see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2006.07.088
magnetic materials possess easy magnetic axes. Fe3O4, for example, has easy magnetic axes along the [111] and [110] directions [17]. Therefore, it's believed that external magnetic field might induce an oriented growth of magnetic materials, which can result to the formation of 1D nanostructures. Chen's group did the magnetic field-induced growth of singlecrystalline Fe3O4 nanowires, but in their case, the maximum magnetic field applied was 0.35 T [1], and below this magnetic field, it's hard to get a large amount of single-crystalline Fe3O4 nanowires. So, it's very interesting and exciting to explore the synthesis of single-crystalline Fe3O4 nanowires at a very low magnetic field. Of course, experimental conditions need to be carefully selected and well controlled for the 1D nanostructure of magnetic material. In this letter, we report the preparation of single-crystalline Fe3O4 nanowires by means of a simple surfactant-assisted redox hydrothermal route under a low magnetic field (at 0.035 T). Based on the oriented growth of magnetic materials, the mechanism for the formation of Fe3O4 nanowires is also proposed. 2. Experimental procedure The chemical reagents used in the work are ferrous chloride (FeCl2·4H2O), diamine hydrate (H4N2·H2O, 50% water), the surfactant polyvinyl pyrrolidone (PVP) and sodium hydroxide
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Fig. 1. XRD pattern of the as-grown Fe3O4 nanowires.
(NaOH). All chemicals are of analytical grade. The reaction cells were self-made with permanent magnet under a Teflon (poly(tetrafluoroethylene)) lining stainless autoclave. The
strength of the magnetic field on the inner lower surface of the cell was 0.035 T. Distilled water, degassed with N2 for half an hour, was used for the preparation of PVP and Fe2+ (0.1 mol/ L) aqueous solution. The solution (30 mL) was put into the reaction cell, NaOH (0.6 g) was dissolved into H4N2·H2O (5 mL) and then the mixture was slowly dropped into the autoclave. During the experiment, N2 continuously bubbled the solution to prevent the oxidation of Fe2+ in the system. The autoclave was put into an oven, kept at 130 °C for 6 h, and then cooled to room temperature naturally. The products were filtered and washed several times with distilled water and absolute ethanol, and finally dried in a vacuum oven at 25 °C for 12 h. The sample was characterized by XRD using an X-ray diffractometer with high-intensity Cu Kα radiation (λ = 1.5418 Å), recorded at a scanning rate of 0.05° per second with the 2θ ranging from 25° to 70°. Transmission electron microscope (TEM) images were taken by a Hitachi model H-800, using an accelerating voltage of 200 kV. High-resolution transmission
Fig. 2. TEM images of the samples obtained in zero magnetic field and no PVA (A); 0.035 T magnetic field and PVA-assisted (B); zero magnetic and PVA-assisted (C); 0.035 T magnetic field and no PVA (D).
J. Zhang et al. / Materials Letters 61 (2007) 1629–1632
electron microscope (HRTEM) images were taken by JEOL2010 with an accelerating voltage of 200 kV.2 3. Results and discussion As shown in Fig. 1, the XRD pattern indicates that the as-grown products in the reaction are pure Fe3O4, although the diffraction peaks are broadened owing to their small size. All the diffraction peaks can be indexed by the cubic structure of Fe3O4 (JCPDS card no. 85-1436). No other peaks for impurities are detected. However, the XRD pattern cannot provide enough evidence to confirm the formation of Fe3O4, since there is a little difference between the XRD patterns of Fe3O4 and that of γ-Fe2O3. The black color of the sample as-prepared suggested the formation of Fe3O4 but not of γ-Fe2O3. On the basis of the Scherrer equation [18] we can figure it out that the average diameter of the Fe3O4 nanowires is 25–35 nm, and the length of the nanowires is about 0.5–1.0 μm. Fig. 2 shows the TEM images of the samples prepared under different conditions. It's found that Fe3O4 particles are square and hexagonal in shape without external magnetic field and not surfactant-assisted (Fig. 2A). When PVP but not an external magnetic field was applied, the shape of the Fe3O4 particles formed (seen in Fig. 2C) is also similar to that of the particles shown in Fig. 2A. Furthermore, the reaction process under low external magnetic field but not PVP-assisted, the final Fe3O4 particles are still square and hexagonal as shown in Fig. 2D. When the reaction system was performed in a low magnetic field (0.035 T) and was PVPassisted, the morphology of the sample would change drastically, and a large amount of Fe3O4 nanowires was prepared (seen in Fig. 2B), clearly revealing the co-effect of magnetic field and surfactant-assisted on the nucleation and oriented growth of Fe3O4 nanowires. These results show that the external magnetic field should influence the shape of ferromagnetic materials, and the surfactant should be necessary in preparing one-dimensional nanoparticles under lower strength of the magnetic field. This conclusion confirmed the case reported by Chen's group [1] in which they could not also get a large amount of Fe3O4 nanowires which was not surfactant-assisted under a magnetic field lower than 0.35 T. Electron Spin Resonance (ESR) experimental results (to be processed and reported elsewhere) show that the magnetic properties of these Fe3O4
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nanowires are a little different from those of bulk Fe3O4 or Fe3O4 clusters, and it can be observed that there is a co-existing ferromagnetic and paramagnetic state at room temperature (300 K) for these Fe3O4 nanowires, but only a ferromagnetic state for bulk Fe3O4 or Fe3O4 clusters in general. The HRTEM image of the boxed area in Fig. 3A further supports the single-crystalline nature of these Fe3O4 nanowires (Fig. 3B). The lattice fringe (∼0.48 nm) observed in this image well agrees with the separation between the (111) lattice planes. Combined with the ED results, it is confirmed that the nanowires have grown along the [110] direction, one of the easy magnetic axes of Fe3O4. As mentioned above, if there were no magnetic field and surfactant PVP during the reaction process, the shape of the particles we got was square and hexagonal. So the formation of the wire or rod of magnetite is speculated to result from the cooperation of the magnetic fieldinduced effect and the soft-template effect of PVP. We could surmise that while the reaction cell was put into an external magnetic field during the hydrothermal process, the Fe3O4 particles would take on a directional array along the direction of the magnetic field at first, and then the order state of the nucleus might result in the ordinal growth which yields the nanowires or nanorods. As we know, non-ionized surfactant molecules can form chain-like structures due to their assembly in water. Therefore, it can be supposed that many 1D liquid reaction places are formed when PVP and water are mixed together. The growth of Fe3O4 is strictly limited in such 1D separated field along the directional space. Owing to the inhibition of the surfactant molecules, the induced effect of the external magnetic field can be easily actualized, so the equal effect only requires much lower intensity of the magnetic field. It's hoped that more interesting results would be obtained if the strength of the magnetic field could be enhanced higher in our case. Though a detailed analysis of the magnetic effect is too complex to explain clearly, we may suggest that the magnetic field could induce nanoparticles growth along the easy magnetic axis of Fe3O4 in aqueous solution. But referring to the former literature, it's easily understood that only under a higher strength of the external magnetic field (up to 0.35 T) [1] can nanowires or nanorods be prepared successfully without a surfactant, because the soft-template-assisted function is by no means substituted under a lower magnetic field.
Fig. 3. HRTEM images of the sample, the inset shows an ED pattern of the individual Fe3O4 nanowires. The strength of the magnetic field is 0.035 T in the nucleation process.
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4. Conclusion In summary, single-crystalline Fe3O4 nanoparticles were successfully synthesized through a hydrothermal route at 130 °C for 6 h induced by an external magnetic field (0.035 T) and PVP-assisted. These nanoparticles prepared with diameters of 25–35 nm and lengths of about 0.5–1.0 μm grew along the easy magnetization axis [110] of Fe3O4. It's believed that the formation of the wire or rod of magnetite is the result of the cooperation of the magnetic field-induced effect and the soft-template effect of PVP. The synthesis route used here may stimulate technological interest and may also have many applications in magnetic materials. Acknowledgements This work was financially supported by the Department of Basic Research, Ministry of Science and Technology, P.R.C., the Grants-in-aids for the special projects of the significant basic research (2003CCA03700). This study was also supported by NSFC (Project No. 20473082). References [1] J. Wang, Q.W. Chen, C. Zeng, B.Y. Hou, Adv. Mater. 16 (2004) 137. [2] J.T. Hu, T.W. Odom, C.M. Lieber, Acc. Chem. Res. 32 (1999) 435. [3] F. Duda, J. Kohnoff, M. Parrinello, Phys. Rev. Lett. 69 (1992) 1272.
[4] S.J. Park, S. Kim, S. Lee, Z.G. Khim, K. Char, T. Hyeon, J. Am. Chem. Soc. 122 (2000) 8581. [5] C.J. Murphy, N.R. Jana, Adv. Mater. 14 (2002) 80. [6] N. Cordente, M. Respaud, F. Senocq, M.J. Casanove, C. Amiens, B. Chaudret, Nano Lett. 10 (2001) 565. [7] J.F. Wang, M.S. Gudiksen, X.F. Duan, Y. Cui, C.M. Lieber, Science 293 (2001) 1445. [8] H. Kind, H.Q. Yan, B. Messer, M. Law, P.D. Yang, Adv. Mater. 14 (2002) 158. [9] T. Thurn-Albrecht, J. Schotter, C.A. Kastle, N. Emley, T. Shibauchi, L. Krusin-Elbaum, K. Guarini, C.T. Black, M.T. Tuoninen, T.P. Russell, Science 290 (2000) 2126. [10] W.J. Liang, M. Bockrath, D. Bozovic, J.H. Hafner, M. Tinkham, H. Park, Nature 411 (2001) 665. [11] X.F. Duan, Y. Huang, Y. Cui, J. Wang, C.M. Lieber, Nature 409 (2001) 6816. [12] (a) M. Tanase, D.M. Silevitch, A. Hultgren, L.A. Baner, P.C. Searson, G.J. Meyer, D.H. Reich, J. Appl. Phys. 91 (2002) 8549; (b) S.H. Ge, C. Li, X. Ma, W. Li, L. Xi, C.X. Li, J. Appl. Phys. 90 (2001) 509. [13] S.G. Yang, H. Zhu, D.L. Yu, Z.Q. Jin, S.L. Tang, Y.W. Du, J. Magn. Magn. Mater. 222 (2000) 97. [14] H.R. Khan, K. Petrikowski, Mater. Sci. Eng., C, Biomim. Mater., Sens. Syst. 19 (2002) 345. [15] J.L. Cain, D.E. Nikles, IEEE Trans. Magn. 3 (1997) 3718. [16] M. Chen, D.E. Nikles, J. Appl. Phys. 85 (1999) 5540. [17] Y.L. Li, G.D. Li, Physics of Ferrite, Science Publishing Corporation, Beijing, 1978, 381 pp. [18] C.N.J. Wagner, E.N. Aqua, Adv. X-ray Anal. 7 (1964) 46.