FePt nanowires DOI: 10.1002/smll.200500328 Ferromagnetic FePt Nanowires: Solvothermal Reduction Synthesis and Characterization** Yanglong Hou, Hiroshi Kondoh, Renchao Che, Masaki Takeguchi, and Toshiaki Ohta* Recently, one-dimensional (1D) nanostructures such as wires, rods, belts, and tubes have attracted much interest as building blocks for future electronics. [1–3] It is generally ac- cepted that 1D nanostructures are promising candidates to investigate the dependence of electrical and thermal trans- port or mechanical properties on dimensionality and quan- tum effects. They are also expected to play an important role as interconnections and functional units in fabricating electronic, optoelectronic, biological, and bioelectromagnet- ic devices with nanoscale dimensions. Many 1D nanoscale materials have been prepared by advanced lithographic techniques, [4,5] such as electron-beam writing, proximal probe patterning, and electrospinning. In contrast, unconventional methods based on chemical syn- thesis might provide an alternative and intriguing strategy for generating 1D nanostructures. [6] These bottom-up chemi- cally derived materials may present advantages over top- down lithographically patterned materials, such as better in- trinsic properties, [7] higher degrees of structural or surface perfection, [8] lower cost, higher flexibility of substrates used for the assembly of these materials, and the potential for high-volume production. [9,10] On the other hand, magnetic nanoscale materials have also attracted much attention in recent years. [9,11–13] In par- ticular, chemically synthesized FePt nanoparticles have become the focus of intensive research because of the chem- ical stability and potential applications in high-density data storage and high performance permanent magnets. [14,15] Generally, the particles are synthesized via thermal decom- position of iron pentacarbonyl ([Fe(CO) 5 ]), reduction of platinum acetylacetonate (Pt(acac) 2 ), and co-reduction of iron salt and Pt(acac) 2 . [15a,b,16] Both structural and physical properties of magnetic nanoparticles depend not only on the size and composition of the particles but also on their dimensions. [1,17] Understanding of the magnetic properties with respect to size and dimension is essential for mapping the scaling limit of future high-density magnetic storage technology. A number of efforts have been made to prepare 1D magnetic nanowires. [18–21] However, there have been few reports on FePt 1D nanostructures so far. [22] Due to their flexibility, 1D nanoscale materials have been widely pre- pared by solvothermal routes. [23–28] For example, the prepa- ration of CoPt nanowires was reported by Dai and co-work- ers by means of a solvothermal reaction. [21] Here, we have developed a facile, solvothermal reduction approach to pre- pare FePt nanowires. The morphologies and magnetic prop- erties of the sample at different annealing temperatures have been characterized. The morphology and chemical composition of the prod- ucts were characterized by transmission electron microscopy (TEM) and energy-dispersive X-ray spectroscopy (EDS). A typical TEM image, as shown in Figure 1a, indicates that the sample consists of  2-mm-long nanowires with diame- ters of 30–50 nm. They have a chemically disordered face- centered cubic (fcc) phase (A1, Fm3 ¯ m), the lattice parame- ter, a, of which is 0.386 nm, as shown in the selected-area electron diffraction pattern (Figure 1a, inset). An EDS mea- surement carried out during the TEM observation indicated that the atomic ratio of Fe to Pt is 52:48. A high-resolution TEM image (HRTEM, Figure 1b) shows that the nanowires consist of isolated nanoparticles self-assembled into wires, and that the as-synthesized nanowires are polycrystalline (Supporting Information, Figure SI-1). Thermal treatment induces structural change from the chemically disordered A1-FePt phase to the chemically ordered fct FePt phase (L1 0 , P4/mmm). The HRTEM image of the sample (Figure 1c) annealed at 500 8C for 30 min indicates a very high crystallinity, while that of the sample annealed at 600 8C for 30 min, shown in Figure 1d, reveals a lattice spacing of 0.22 nm, characteristic of the (111) planes in the chemically ordered face-centered tetragonal (fct) FePt phase (JCPDS file 43-1359). The corre- sponding electron diffraction patterns are shown in the upper insets of Figure 1a,c, and d. Comparison of these electron diffraction patterns (see, for example, some differ- ent reflection rings, such as {110}, {120}, {112}, etc.) indicates the occurrence of the phase change from chemically disor- dered A1 phase to chemically ordered L1 0 phase. The X-ray diffraction (XRD) patterns also exhibit the change in internal particle crystal structure with annealing temperature. The sample at the as-synthesized condition shows a chemically disordered structure (Figure 2a). The chemically ordered structures were obtained by annealing, as indicated by the (111) peak shift and the evolution of the (001) and (110) peaks (see Figure 2c). It is generally accepted that solvent properties, such as polarity, redox potential, viscosity, and coordination ability, strongly affect the solubility and transport behavior of the precursors involved in such heterogeneous liquid–solid reac- tions. In the present case, the selection of solvents was the key to the formation of FePt nanowires. Ethylenediamine (en) has proven to be an ideal solvent for controlling the [*] Dr.Y. Hou, Dr. H. Kondoh, Prof. T. Ohta Department of Chemistry, School of Science The University of Tokyo, Tokyo 113-0033 (Japan) Fax:(+ 81)3-3812-1896 E-mail: ohta@chem.s.u-tokyo.ac.jp Dr. R. Che, Dr. M. Takeguchi National Institute for Materials Science Tsukuba, Ibaraki-ken 305-0003 (Japan) [**] This work was supported in part by the Japan Society of the Pro- motion of Science (JSPS), the 21st Century COE Program, and the Nanotechnology Support Project from the Ministry of Education, Culture, Sports, Science, and Technology (MEXT) of Japan. Supporting information for this article is available on the WWW under http://www.small-journal.com or from the author. small 2006,2,No.2,235–238 # 2006 Wiley-VCH Verlag GmbH&Co. KGaA, D-69451 Weinheim 235