Iron-manganese alloys are of fundamental interest in their amorphous form due to their re-entrant spin glass behavior [1], and in the crystalline form as the host system for shape-memory devices [2]. The most common routes to amorphous alloys are melt spinning, mechanical alloying, and rf sputtering. However, in the last decade, a chemical synthesis route to amorphous alloys has been utilized to prepare a variety of nanoscale materials, including alloys of FeB [3], FeCoB [4], FeNiB [5], FeCuB [6], FeWB [7] and FeNiPB [8]. The reaction involves the reduction of an aqueous solution of metal salts by aqueous sodium borohydride. Control of the product composition may be achieved by changing the molar ratios of the precursors; and other properties, including particle size and the degree of amorphicity or crystallinity, may be varied by changing the reagent concentration, the reaction pH, or the addition rate of the reagents. In this letter, we present the ®rst results of a study of the preparation of amorphous FexMnyB alloys by chemical reduction. The synthesis of FexMnyB alloys was performed using argon-purged solvents via the reduction of a solution (200 ml) containing iron (II) sulphate heptahydrate (8.92 g, 32.0 mmol) and manganese (II) sulphate tetrahydrate (0.89 g, 4.0 mmol) by the dropwise addition of aqueous sodium borohydride (1.13 g, 30.0 mmol, 200 ml), yielding a black precipitate accompanied by the evolution of hydrogen gas: 3BH4 2M2 2H2O! 2MB BO2 2H 7H2 (1) During the process, the pH was maintained at 5.5 by the addition of aqueous sodium hydroxide. Control of the pH of the reaction is critical for minimizing impurities in the alloy. After performing a series of reactions at different pH, we found that a pH of 5.5 gave the best results with products that contained minimal impurities. Following precipitation, the samples were washed with water (7 1) to remove any unreacted starting materials, and then stored in an argon glove box to help limit oxidation. A compositional analysis of the samples was performed using electron microprobe analysis (EMA) to determine the iron to manganese ratio, and atomic absorption spectroscopy (AAS) was done to determine the iron to boron ratio. An EMA line scan (1 im spots at 6 im intervals) showed little deviation in the iron to manganese ratio (8.1:1), which agreed with the 8:1 ratio used in the reaction. The EMA data are similar to that of a FeNiB alloy, prepared by us, that is known from the work of earlier researchers to contain a single amorphous phase when prepared by chemical reduction [5]. Combining the EMA and AAS data gives Fe57 Mn7B36 as the overall composition for the product. The morphology of the samples was investigated by scanning electron microscopy, which revealed roughly spherical grains of ca. 200 nm diameter. They agglomerated into larger particles without regular shape, ranging from 2 im to 30 im in size. The structural properties of the product was probed using X-ray powder diffraction, Fe MoEssbauer spectroscopy, and differential scanning calorimetry. The X-ray diffraction pattern (Fig. 1) shows two broad peaks at 208 and 348, characteristic of an iron-based amorphous alloy [9]. A detailed inspection of the X-ray diffraction data indicates minor crystalline peaks corresponding to Fe2O3. Small quantities of iron (III) oxide impurities are often seen in chemically prepared amorphous alloys. Nanophase amorphous alloys are always oxygensensitive, in some cases violently, and invariably require surface oxygen passivation. The Fe MoEssbauer spectrum (Fig. 2) consists predominately of a broad magnetic sextet, typical of an amorphous material, which accounts for 97:5 1:0% of the total spectra area. This component
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Nov 30, 2000
A.V Markin + 9
