Nano body-centered cubic CoFe 2 alloy precursor for cobalt ferrite via sonoreduction process
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A stable nano body-centered cubic (bcc) CoFe2 alloy was isolated by employing a combination of N2H4.H2O reduction and sonochemical route under basic conditions. This alloy has proved to be a potential precursor for CoFe2O4 production. X-ray diffraction and transmission electron microscopy confirms the formation of a bcc phase CoFe2 alloy with particle size 10). Thermogravimetric analysis (TGA) of the stable CoFe2 alloy powder measured in a stream of air shows a steady increase in weight up to 450 °C, as shown in Fig. 2(b). The weight gain is about 21.5% against a theoretical value of 27.4%, suggesting that a partial surface oxidation of CoFe2 could be possible during the synthesis, although not evident from the powder XRD pattern. The alloy converts into corresponding CoFe2O4 between 200–500 °C [see Fig. 2(a)]. The differential scanning calorimetry (DSC) curve 1850
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FIG. 1. (a) TEM image showing the morphology of the as-prepared CoFe2 alloy showing weak agglomeration. Note the Moiré fringes are evident in these nano alloy particles. (b) d spacing of the alloy is 0.203 nm.
for the as-prepared alloy in air is shown in the inset of Fig. 2(b). The DSC curve shows a weak exothermic peak at 260 °C, which could be associated with the phase transformation of the bcc to body-centered tetragonal (bct) phase (since we do not see any weight loss/gain in the TGA curve), and the exothermic peak around 400 °C is associated with the oxidation of the alloy, suggesting that the alloy converts to the CoFe2O4 phase due to the high reactivity of the nano-sized CoFe2 particles. Further, we show in Fig. 2(b) the Fourier transform infrared spectra (FTIR) for the CoFe2O4 sample, which shows characteristic high frequency infrared (IR) bands corresponding to the Fe–O vibration arising from the tetrahedral sites. In Fig. 3, we show the progressive evolution of magnetization starting from the CoFe2 alloy to the
J. Mater. Res., Vol. 23, No. 7, Jul 2008 Downloaded: 13 Mar 2015
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M. Mohan et al.: Nano body-centered cubic CoFe2 alloy precursor for cobalt ferrite via sonoreduction process
FIG. 3. (a) Hysteresis loops of the as-prepared CoFe2 alloy and CoFe2O4 recorded at 300 K shows the relative room temperature magnetization of the alloy and the ferrite. (b) Hysteresis loops of asprepared CoFe2 alloy at 100, 200, and 300 K; note the increase in coercivity as we cool the sample from 300 K.
serves as evidence to the existence of the stable alloy phase, whose magnetization value is nearly three times that of the ferrite. Further, the fact that better saturation is achieved for the alloy at much lower fields,
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