Shape controlled synthesis of iron–cobalt alloy magnetic nanoparticles using soft template method Santosh K. Pal, D. Bahadur ⁎ Department of Metallurgical Engineering and Materials Science, Indian Institute of Technology Bombay, Powai, Mumbai 400076, India abstract article info Article history: Received 27 November 2009 Received in revised form 29 January 2010 Accepted 31 January 2010 Available online 4 February 2010 Keywords: Magnetic materials Metals and alloys FeCo Shape Soft template Fe x Co 1 -x alloy nanoparticles of spherical (x =0.25, 0.68, 0.85), hollow spherical (x =0.60) and sheet like (x =0.60) shapes were prepared at room temperature by reduction of iron chloride tetrahydrate and cobalt chloride hexahydrate with sodium borohydride, using N-Cetyl-N,N,N-trimethyl ammonium bromide (CTAB)/ water/hexanol system as soft template. The size and shapes of nanostructures were found to depend on the concentrations of CTAB and hexanol in water. Composition and shape dependence of magnetic properties of spherical, hollow spherical and sheet like Fe x Co 1 -x alloy nanostructures were discussed. The highest saturation magnetization of 235 emu/g with a coercivity of 160 Oe was obtained for spherical Fe 0.68 Co 0.32 nanoparticles. © 2010 Elsevier B.V. All rights reserved. 1. Introduction Iron–cobalt intermetallic compounds are known to exhibit the highest saturation magnetization among the commercial magnetic materials [1]. In addition, they exhibit high Curie temperature (∼ 900 °C), good permeability, good mechanical strength, and are ideally suited for applications requiring high flux density such as transformer cores and electrical generators [2] and applications requiring high magnetophoretic mobility [3]. Several physical and chemical synthetic techniques have been applied to synthesize FeCo alloy nanoparticles, including chemical vapor deposition, arc-discharge, laser-pyrolysis, polyol process, sono- chemical method, thermal decomposition, and chemical reduction of metal ions by borohydride derivatives [4–9]. But till now the synthesis is limited to spherical and irregular shapes except a few reports on dice [10], cube [11] and wire [12] shaped nanostructures. Soft template method, pioneered by Pileni et al. [13] who demonstrated that the shape of nanocrystals is related to that of the micro-structured fluid in which the chemical reaction takes place, is an important method for size and shape controlled synthesis of nanoparticles. To the best of our knowledge, there is no report on the shape control synthesis of FeCo alloy nanoparticles using CTAB/water/hexanol system as soft template. In this communication we present and discuss the results of size and shape controlled synthesis of iron–cobalt alloy nanoparticles by chemical reduction of metal salts in soft template solution of CTAB/water/hexanol. Further, their magnetic properties are compared according to the size, shape and composition of alloys. 2. Experimental Cobalt chloride hexahydrate (CoCl 2 ·6H 2 O (98%)), Iron chloride tetrahydrate (FeCl 2 ·4H 2 O (99+%)), and sodium borohydride (NaBH 4 (99%)) were purchased from Aldrich. Surfactant CTAB (99%), ethanol (99.9%) and hexanol were purchased from local vendor and were used as obtained without further purification. High purity argon gas (99.999+%) was used to prevent oxidation during the synthesis of nanoparticles. Spherical, hollow spherical and sheet like Fe x Co 1 -x alloy nanostruc- tures were prepared by mixing same volume of two microemulsion solutions of CTAB/water/hexanol, one containing the iron and cobalt salts and the other the reducing agent. The microemulsion compositions were selected according to the phase diagram [14] and prepared following the procedure described elsewhere [15], water/CTAB ratio (w) (with hexanol wt.%) of 10 (11.0) for spherical, 11 (1.4) for hollow spherical and 10 (6.5) and 4 (6.5 and 16.5) for sheet like nanostructures. Concentration of salt (CoCl 2 ·6H 2 O + FeCl 2 ·4H 2 O) solution was taken as 80 mM for spherical and 400 mM for hollow spherical and sheet like nanostructure with slightly(∼10%) larger molar ratio of iron to cobalt than the desired one, concentration of reducing agent was kept at 0.8 times the salt concentration. Salt microemulsion was transferred in four-necked round bottom flask and reducing agent microemulsion was dropped under vigorous magnetic stirring in argon atmosphere. On addition of reducing agent, solution changed to black indicating the formation of FeCo nanoparticles. After 1 h of reaction the solution was centrifuged and washed with distilled water and ethanol several times to remove residual impurities and surfactant. Structural characterizations were performed by means of X-ray diffraction (XRD) using a Philips, X-Pert powder diffractometer with Cu Materials Letters 64 (2010) 1127–1129 ⁎ Corresponding author. Fax: + 91 22 2572 3480. E-mail address: dhirenb@iitb.ac.in (D. Bahadur). 0167-577X/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.matlet.2010.01.086 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/matlet