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Figure 9. Differential scanning calorimetric curve of amorphous Co50Pt50 alloy showing glass transition, crystallization, and phase transition. Samples sintered at 900 °C appear to undergo a disorder (a) ^ order (s) disorder (a) transformation (300 °C ^ 650 "C ^ 900 °C), resulting in the fcc phase in vivo. Reprinted with permission from [54], S. S. Manoharan and M. L. Rao (private communication).

samples sintered at 900 " C appear to undergo a disorder (a) ^ order (s) ^ disorder (a) transformation (300 "C ^ 650 "C ^ 900 "C), resulting in the fcc phase in vivo. The cooling curve in the DSC suggests the phase transformation to be reversible. Upon further heating the sample, the endothermic peak due to crystallization is absent, while the fcc to fct phase transformation does reappear. As a result, heat-treated samples yield only the high-temperature fcc phase, which is a disordered alloy; see Figure 10. In a system such as this, the influence of the magnetic field on the electronic transport has shown interesting properties. First, the magnetoresistance ratio (which is defined as p0 — pH/po) of the order of 3% at relatively low fields is observed at 4 K and at 300 K; see Figure 11(a). This is the first observation of negative magnetoresistance in amorphous alloys. Second, with varying compositions of the Pt concentration, the MR ratio oscillates, showing a dependency on the short-range order, Figure 11(b). The same authors also reported the synthesis of Co-Pd alloys using high-frequency ultrasound using organometallic Co-carbonyl and PdCl2; by varying the molar concentrations of the solutions used, a series of alloys, Co100—xPdx (0 < x < 100) have been prepared. Interestingly, all of these nanosized alloys show an interesting conduction mechanism (Fig. 12) [55].

Cobalt-chromium multilayer thin-film systems have been studied in detail due to its potential for application as a read head memory devices. Alloys of amorphous Co-Cr alloys show an interesting phase transition from a pure hcp to fct to a pure fct phase with increasing Cr concentration. Similar magnetoresistive features as the Co-Pt series have been observed in these amorphous alloys [56]. Interesting soft magnetic behavior has been explored in the amorphous form

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Figure 10. X-ray diffraction patterns of Co-Pt nanoalloys showing a disordered fcc phase for the 900" C heat-treated samples. All the "as-prepared" samples are X-ray amorphous. Reprinted with permission from [54], S. Sundar Manoharan (private communication).

of Fe-Pt alloys, which otherwise is a commercially employed hard bulk magnet [57].

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