Nanoalloys

Sonochemical techniques can also be used to prepare nanostructured alloys. For example, Fe-Co alloys have been synthesized from the readily available Fe(CO)5 and Co(CO)3(NO) precursors. Simply by altering the ratio of solution concentrations of the precursors, one can control the composition of the Fe-Co alloys. Alloy compositions ranging from pure Fe to pure Co can easily be realized [50]. Shafi and co-workers [51] reported the preparation of nanosized amorphous alloy powders of Fe20Ni80, Fe40Ni60, and Fe60Ni40 sonochemical decomposition of solutions of volatile organic precursors such as Fe(CO)5 and Ni(CO)4 in decalin, under an argon pressure of 100-150 kPa at 273 K. The magnetic susceptibility of Fe40Ni60 and Fe60Ni40 indicates blocking temperatures of 35 K and a magnetic particle size of about 6 nm. Thermogravimetric measurements of Fe20Ni80 give Curie temperatures of 322 °C for amorphous and 550 °C for crystallized forms. Differential scanning calorimetry exhibits an endothermic transition at 335 °C from a combination of the magnetic phase transition and alloy crystallization. The Mossbauer spectrum of crystallized Fe20Ni80 shows a sextet pattern with a hyperfine field of 25.04 T. The same authors have reported the preparation of a Co-In alloy [52].

Nanosized amorphous alloy powders of Co20Ni80 and Co50 Ni50 have been prepared by the sonochemical decomposition of solutions of volatile organic precursors, Co(NO)(CO)3 and Ni(CO)4 in decalin, under an argon pressure of 100150 kPa, at 273 K. A transmission electron micrograph of the heated Co20Ni80 sample showed near uniform particles with sizes less than 10 nm. Magnetic measurements indicated that the as-prepared amorphous CoNi alloy particles were superparamagnetic. The observed magnetization, measured up to a field of 15 kG, of the annealed Co20Ni80 sample (54 emu • g-1) was significantly lower than that for the reported multidomain bulk particles (75 emu • g-1), reflecting the ultrafine nature of the sample. Thermogravimetric measurements of Co20Ni80 with a permanent magnet yielded a glass transition temperature of 338 °C for the amorphous form, and a Curie temperature of 565 °C for the crystallized form. The differential scanning calorimetry showed crystallization temperatures of 400 °C for Co20Ni80 and 365 °C for Co50Ni50 amorphous samples. The as-prepared amorphous Fe-Ni-Co alloy particles [53] were superparamagnetic, as indicated by the magnetic studies. The observed magnetization measured up to a field of 1.5 kG of the annealed Fe-Ni-Co samples (75-87 emu • g-1) was significantly lower than that for the reported multidomain bulk particles (175 emu • g-1), reflecting the ultrafine nature of our sample.

Rao and Manoharan [54] recently reported the effect of short-range order on the magnetic and electronic transport properties of a series of cobalt-platinum alloys. In an amorphous phase, the order parameter differs with different Pt concentrations. The crystallization temperature is accompanied by a structural phase transition (fcc-fct) at around 380 °C, as inferred from the DSC studies (Fig. 9). For the Co-Pt binary alloy, transition from the soft magnetic disorderd fcc phase to an ordered fct structure occurs around 675 °C, as evident from the DSC curve. However,

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