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2009, 113, 6902–6905 Published on Web 04/08/2009
Room Temperature Ferromagnetism in Single-Crystalline Fe5Si3 Nanowires Kwanyong Seo,† Sunghun Lee,† Younghun Jo,‡ Myung-Hwa Jung,§ Jinhee Kim,¶ David G. Churchill,† and Bongsoo Kim*,† Department of Chemistry, KAIST, Daejeon 305-701, Korea, Nano Materials Research Team, KBSI, Daejeon 305-333, Korea, Department of Physics, Sogang UniVersity, Seoul 121-742, Korea, and Center for Nanoscience and Quantum Metrology, KRISS, Daejeon 305-600, Korea ReceiVed: March 5, 2009; ReVised Manuscript ReceiVed: March 27, 2009
We have investigated electrical and magnetic properties of single-crystalline Fe5Si3 nanowires. The nanowire ensemble shows ferromagnetic properties with a high Tc of 380 K, small coercivity, and no remanence in zero field at room temperature. Such magnetic properties of the single-crystalline nanowires should give a chance to realize not only novel nanospintronic devices but also biomedical applications. Electrical transport measurements on single Fe5Si3 nanowire device show metallic properties with low resistivity of 487 µ · Ω · cm. Fe5Si3 nanowires are the first example of single-crystalline metallic ferromagnet with a Tc higher than room temperature. Introduction One dimensional nanowires (NWs) possessing highly polarized spins can play important roles in fabricating practical nanospintronic devices, which has been expected to overcome the technical limits in conventional charge-based electronic devices, in addition to their fundamental importance. By employing the spin degree of freedom, the spintronic device can have nonvolatility, increased data processing speed, decreased electric power consumption, and higher integration densities than conventional devices.1 Because spins are scattered by grain boundaries, carriers, and impurities, spin diffusion length is strongly dependent on the crystallinity of the material.2 Thus for nanospintronics applications it is highly important that NWs have both ferromagnetic properties above room temperature and single-crystalline nature. Single-crystalline dilute magnetic semiconductor (DMS) NWs which are ferromagnetic and semiconducting have been successfully synthesized by doping semiconducting materials with magnetic impurities such as Mn, Fe, or Co,3 and related applications of these DMS NWs for spin FET and spin LED have been investigated.4 However, defect-free single-crystalline metallic NWs that are ferromagnetic above room temperature have not yet been realized in spite of intense efforts by many research groups. Such NWs can be effectively used for spin injection into semiconductors, superconductors, and paramagnetic metals, sometimes through a tunnelling barrier.1 Hence, synthesis of those NWs is strongly required for future nanospintronic applications. A series of Fe-Si alloys has attracted much attention because of their great variety of physical properties leading to many possible applications. FeSi, which is the most stable phase in a series of Fe-Si alloys, has been found to be a Kondo insulator.5 * To whom correspondence should be addressed. E-mail: bongsoo@ kaist.ac.kr. Fax: +82-42-350-2810. † KAIST. ‡ KBSI. § Sogang University. ¶ KRISS.
10.1021/jp902010j CCC: $40.75
The silicon-rich phase in the solid solution series such as β-FeSi2 is a direct band gap material and can be used as a silicon based light emitting diode (LED).6 Moreover, high temperature ferromagnetism is observed in iron-rich phases such as Fe3Si (Tc ) 840 K) and Fe5Si3 (Tc ) 380 K).7 Recently, Fe5Si3 was successfully synthesized in a NW form by vapor phase method although it is metastable in bulk,8 because thermodynamically metastable modifications can be stabilized in nanostructures.9 The single-crystalline Fe5Si3 NWs have been a promising candidate for nanospintronic applications and attract much research interests because of their high Tc in the bulk form. Here, we report the magnetic and electrical properties of singlecrystalline Fe5Si3 NWs for nanoscale device applications. The results show clear ferromagnetic properties with hysteresis loop at room temperature and metallic properties with very low electrical resistivity of 487 µ · Ω · cm. The magnetic properties of single-crystalline Fe5Si3 NWs with high Tc, very small coercive field, and almost zero remanence should provide opportunities to realize novel nanospintronic devices as well as biomedical applications. Experimental Section Fe5Si3 NWs were synthesized using a 1 in. diameter quartz tube in a horizontal hot-wall two zone furnace. The temperature of both the zones was independently controlled. Anhydrous FeI2 precursor is evaporated at a low temperature and transported to the high temperature zone by carrier gas, where Fe is deposited after the dissociation of the precursor on the preheated substrate. The upstream (US) zone and downstream (DS) zone were used for vaporization of precursor and NW growth, respectively. A rectangular Si wafer (45 mm length and 15 mm width) kept at the DS zone was the source of Si. The Fe5Si3 NWs were grown on c-plane sapphire substrates placed on the Si wafer. Anhydrous FeI2 powder (50 mg from Sigma-Aldrich, 99.999%) in an alumina boat was placed at the center of the US zone. The system was purged with Ar gas for 30 min before the start of 2009 American Chemical Society
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each reaction to maintain an inert atmosphere. The carrier argon gas was supplied through a mass-flow controller at the rate of 200 sccm. The DS zone was initially heated to 900 °C. After reaching the target temperature, the temperature of the US zone was raised to 500 °C. The reaction time was varied between 5 and 30 min. X-ray diffraction (XRD) patterns of the specimen were recorded on a Rigaku D/max-rc (12 kW) diffractometer operated at 40 kV and 80 mA with filtered 0.15405 nm Cu KR radiation. Field emission scanning electron microscope (FESEM) images of Fe5Si3 NWs were taken on a Phillips XL30S. Transmission electron microscope (TEM) image and selected area electron diffraction (SAED) pattern were taken on a JEOL JEM-2100F TEM operated at 200 kV. After nanostructures were dispersed in ethanol, a drop of the solution was put on the holey carbon coated copper grid for the preparation of TEM analysis. The total mass of Fe5Si3 NWs was obtained by subtracting the mass of Al2O3 substrate from the mass of the NWs on the substrate. To weigh the mass of the substrate only, we used sonication process to remove the NWs from the substrate which was used for SQUID measurement. After the sonication the Al2O3 substrate became clean. Results and Discussion Single-crystalline Fe5Si3 NWs were grown on a sapphire substrate at 900 °C by a vapor transport method as we reported in a previous paper.8 The morphology of the Fe5Si3 NWs was examined by scanning electron microscope (SEM) and transmission electron microscope (TEM). The SEM image in Figure 1a shows the morphology of the NWs, which are tens of micrometers long and 100-200 nm in diameter. Figure 1b shows that the NWs have a clean surface. No secondary growth or extra structural features were observed. The selected area electron diffraction (SAED) pattern of the NW shows a regular spot pattern, reflecting the single-crystalline nature of the NWs. The diffraction pattern is fully indexed to the hexagonal Fe5Si3 structure and shows that the NW has a [001] growth direction. More importantly, we have significantly increased the density of the NWs (Figure 1a) by careful optimization of reaction condition to investigate the tiny magnetization. Figure 1c shows the XRD pattern of the as grown NWs ensemble, in which all the diffraction peaks are indexed to the hexagonal Fe5Si3 structure (JCPDS file: 65-3593). The highest intensity peak corresponding to the Fe5Si3 (211) plane is located very close to the peak of the FeSi (210) plane. However, other peaks such as (102), (300), and (112) peaks in the pattern clearly indicate that the NWs are composed of hexagonal Fe5Si3. Detailed magnetic properties of the as-grown Fe5Si3 NW ensemble on a sapphire substrate have been studied by using a superconducting quantum interference device (SQUID) magnetometer. Figure 2 displays the temperature dependent magnetization M(T) curves measured under applied magnetic fields of 100 and 500 Oe, respectively, after zero-field cooling (ZFC) and field cooling (FC). The ZFC data are slightly different from the FC data because the applied field of 500 Oe is not enough to align the ferromagnetic component completely. Figure 3 shows that the magnetization is fully saturated above HS ) 5000 Oe. The extrapolated line of high temperature data intersects the temperature axis around 380 K, which is the Curie temperature of Fe5Si3 NWs. This value agrees well with the reported Curie temperature of bulk Fe5Si3.7 Note that giant magnetoresistance (GMR) has been observed at room temperature in nanogranular structures of Fe5Si3 with an average grain size of 100 nm.10
Figure 1. (a) Representative SEM image of Fe5Si3 NWs. Scale bar: 10 µm. (b) Representative high-resolution TEM image and SAED pattern. The labeled distance of 0.48 nm corresponds to the (002) planes, and the arrow shows the [001] growth direction of the NW. The SAED pattern is indexed for a hexagonal Fe5Si3 NW. Scale bar: 2 nm. (c) XRD spectrum of the Fe5Si3 NWs on a sapphire substrate.
The ferromagnetic nature of Fe5Si3 NWs is manifested in the hysteresis loops. In Figure 3, the M(H) curve shows the hysteresis loops investigated at different temperatures of 5, 300, and 370 K, respectively. As expected from the M(T) data, we obtained typical ferromagnetic curves below the Curie temperature of 380 K. The saturation magnetization and the coercive field at 5 K are about 46 emu/g and 100 Oe, respectively. The relatively weak ferromagnetic behavior at 370 K is due to the reduction of magnetization just below the Curie temperature. Such single-crystalline ferromagnetic NWs can be employed in versatile applications in the field of spintronics such as spin injection electrodes in tunneling magnetoresistance (TMR) devices by utilizing both electrons and spins.11 Interestingly, for 65 and 500 nm thick films, the coercive fields at room temperature are 200 and 97 Oe, respectively.12 On the other hand, for Fe5Si3 NWs synthesized here the coercive field disappears at room temperature. Superparamagnetic nanomaterials that have ferromagnetic-like properties but very small coercivity and zero remanence have attracted much attention for their biomedical applications. Although the Fe5Si3 NWs are not superparamagnetic, they are soft ferromagnetic with very small coercivity and zero remanence, thus available for potential biomedical applications such as MRI contrast enhancement agents, hyperthermia, and drug delivery,13-16 in addition to the spintronic applications. Note that these magnetic properties of Fe5Si3 NWs would be able to prevent embolism due to
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Figure 2. Plot of M vs T obtained from Fe5Si3 NW ensemble at applied fields of (a) 100 Oe and (b) 500 Oe. Squares and circles represent the ZFC and FC data, respectively. The extrapolated line of high temperature data intersects the temperature axis around 380 K, which indicates the Curie temperature of Fe5Si3. Dashed line is a guide to the eye.
Figure 4. Electrical properties of the Fe5Si3 NW. (a) SEM image of the Fe5Si3 NW device. Scale bar: 1 µm. (b) I-V curves recorded on the Fe5Si3 NW device, with four-probe (line 1) and two-probe (line 2) measurements, respectively. (c) Temperature-dependent resistance curve. It shows typical metallic behavior.
Figure 3. Plot of M as function of H obtained from the Fe5Si3 NW ensemble at 5, 300, and 370 K. The inset shows the loops on an enlarged scale (the axis labels remain the same).
aggregation of NWs for in vivo applications in animals and humans after switching off the applied magnetic field.13 Furthermore, the biocompatible silica layer formed on the surface of Fe5Si3 NWs in the synthetic process would make them more appropriate for bioapplications.8,17 Electrical properties of Fe5Si3 NWs have been also investigated. Figure 4 shows the electrical transport data for the single Fe5Si3 NW device fabricated by e-beam lithography. The linear behavior of current-voltage (I-V) characteristics indicates the ohmic contact between the NW and electrodes. The values of electrical resistivity at room temperature are estimated to be
881 and 487 µ · Ω · cm measured with two- and four-probe techniques, respectively. The higher value of two-probe resistivity is due to the contact resistance between the NW and electrodes. Figure 4c shows the electrical resistivity (R) as a function of temperature measured by the two-probe technique. The temperature derivative of resistivity is negative over the whole measured temperature range, implying the metallic nature of Fe5Si3 NW. This result is quite different from the electrical properties of FeSi NW, which is classified as a Kondo insulator.18 We note that the resistivity weakly depends on the temperature. The change of electrical resistivity between 2 and 300 K is only 8%. At very low temperatures near 0 K, the resistivity tends to decrease slightly with an increasing temperature. This result might be attributed to the contact resistance in the two-probe measurement configuration. Summary In summary, Fe5Si3 NWs have been prepared by a vapor transport method with no catalyst. The NW ensemble shows ferromagnetic properties with a high Tc of 380 K, very small
Letters coercivity, and no remanence in zero field at room temperature. This observation suggests that our newly fabricated Fe5Si3 NWs can be employed in biomedical applications as well as nanospintronic applications. Electrical transport measurements on single Fe5Si3 NW show metallic properties with low resistivity of 487 µ · Ω · cm. These Fe5Si3 NWs are the first example of single-crystalline metallic ferromagnet with a Tc higher than room temperature. Acknowledgment. This research was supported by KOSEF through NRL (ROA-2007-000-20127-0), SRC through the center for intelligent Nano-Bio Materials (R11-2005-008-030011-0) and CNMT (Code No. 08K1501-02210) from MEST, Korea. References and Notes (1) Wolf, S. A.; Awschalom, D. D.; Buhrman, R. A.; Daughton, J. M.; Molna´r, S. von; Roukes, M. L.; Chtchelkanova, A. Y.; Treger, D. M. Science 2001, 294, 1488. (2) Ikegami, T.; Kawayama, I.; Tonouchi, M.; Nakao, S.; Yamashita, Y.; Tada, H. Appl. Phys. Lett. 2008, 92, 153304. (3) (a) Ohno, H. Science 1998, 281, 951. (b) Stamplecoskie, K. G.; Ju, L.; Farvid, S. S.; Radovanovic, P. V. Nano Lett. 2008, 8, 2674. (4) (a) Choi, H.; Seong, H.; Chang, J.; Park, Y.; Kim, J.; He, R.; Kuykendall, T.; Yang, P. AdV. Mater. 2005, 17, 1351. (b) Liang, W.; Yuhas, B. D.; Yang, P. Nano Lett. 2009, 9, 892. (c) Ham, M.-H.; Oh, D.-K.; Myoung, J.-M. J. Phys. Chem. C 2007, 111, 11480. (d) Xu, C.; Chun, J.; Lee, H. J.; Jeong, Y. H.; Han, S.-E.; Kim, J.-J.; Kim, D. E. J. Phys. Chem. C 2007, 111, 1180. (5) (a) Paschen, S.; Felder, E.; Chernikov, M. A.; Degiorgi, L.; Schwer, H.; Ott, H. R.; Young, D. P.; Sarrao, J. L.; Fisk, Z. Phy. ReV. B 1997, 56, 12916. (b) Aeppli, G.; Fisk, Z. Comments Condens. Mater. Phys. 1992,
J. Phys. Chem. C, Vol. 113, No. 17, 2009 6905 16, 155. (c) Schlesinger, Z.; Fisk, Z.; Zhang, H.-T.; Maple, M. B.; DiTusa, J. F.; Aeppli, G. Phys. ReV. Lett. 1993, 71, 1748. (d) Sluchanko, N. E.; Glushkov, V. V.; Demishev, S. V.; Menovsky, A. A.; Weckhuysen, L.; Moshchalkov, V. V. Phys. ReV. B 2002, 65, 064404. (6) Leong, D.; Harry, M.; Reeson, K. J.; Homewood, K. P. Nature (London) 1997, 387, 686. (7) (a) Hines, W. A.; Menotti, A. H.; Budnick, J. I.; Burch, T. J.; Litrenta, T.; Niculescu, V.; Raj, K. Phys. ReV. B 1976, 13, 4060. (b) Herfort, J.; Scho¨nherr, H.-P.; Jenichen, B. J. Appl. Phys. 2008, 103, 07B506. (c) Lecocq, Y.; Lecocq, P.; Michel, A. C. R. Acad. Sci. 1964, 258, 5655. (8) Varadwaj, K. S. K.; Seo, K.; In, J.; Mohanty, P.; Park, J.; Kim, B. J. Am. Chem. Soc. 2007, 129, 8594. (9) Mandl, B.; Stangl, J.; Martensson, T.; Mikkelsen, A.; Eriksson, J.; Karlsson, L. S.; Bauer, G.; Samuelson, L.; Seifert, W. Nano Lett. 2006, 6, 1817. (10) Srivastava, P. C.; Tripathi, J. K. J. Phys. D: Appl. Phys. 2006, 39, 1465. (11) Zhang, D.; Liu, Z.; Han, S.; Li, C.; Lei, B.; Stewart, M. P.; Tour, J. M.; Zhou, C. Nano Lett. 2004, 4, 2151. (12) Sawatzky, E. IEEE Trans. Magn. 1971, 7, 374. (13) Hadjipanayis, C. G.; Bonder, M. J.; Balakrishnan, S.; Wang, X.; Mao, H.; Hadjipanayis, G. C. Small 2008, 4, 1925. (14) Ferrari, M. Nat. ReV. Cancer 2005, 5, 161. (15) Hauck, T. S.; Jennings, T. L.; Yatsenko, T.; Kumaradas, J. C.; Chan, W. C. W. AdV. Mater. 2008, 20, 3832. (16) Yu, M. K.; Jeong, Y. Y.; Park, J.; Park, S.; Kim, J. W.; Min, J. J.; Kim, K.; Jon, S. Angew. Chem., Int. Ed. 2008, 47, 5362. (17) (a) Kang, K.; Choi, J.; Nam, J. H.; Lee, S. C.; Kim, K. J.; Lee, S.-W.; Chang, J. H. J. Phys. Chem. B 2009, 113, 536. (b) Law, W.-C.; Yong, K.-T.; Roy, I.; Xu, G.; Ding, H.; Bergey, E. J.; Zeng, H.; Prasad, P. N. J Phys. Chem. C 2008, 112, 7972. (18) (a) Ouyang, L.; Thrall, E. S.; Deshmukh, M. M.; Park, H. AdV. Mater. 2006, 18, 1437. (b) Schmitt, A. L.; Bierman, M. J.; Schmeisser, D.; Himpsel, F. J.; Jin, S. Nano Lett. 2006, 6, 1617.
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