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0 10 20 30 40 atomic percentage of Pt

0 10 20 30 40 atomic percentage of Pt

Figure 11. Negative magnetoresistance in Co-Pt alloys prepared using sonochemical decomposition of metal-organic precursors. Magnetoresistance ratio (which is defined as p0 — pH/po) on the order of 3% at relatively low fields is observed at 4.2 K and at 300 K. (b) Variation of magnetoresistance as a function of Pt concentration, with a maximum at 30%. With varying composition of Pt concentration, the MR ratio oscillates showing dependency on the short-range order. Reprinted with permission from [54], S. Sundar Manoharan (private communication).

Sonication of aqueous iron(II)acetate in the presence of ^-cyclodextrin is reported to yield magnetite nanorods [61]. The as-prepared magnetite nanorods are ferromagnetic, and their magnetization at room temperature is ~78 emu/g, as shown in Figure 13. The particle sizes measured from transmission electron micrographs are about 48/14 nm (L/W). A mechanism for the sonochemical formation of magnetite nanorods is discussed. Measurements of the magnetic irreversibility line and time-logarithmic decay of the magnetization are described for sonchemically synthesized three Fe2O3 samples composed of regular amorphous, acicular amorphous, and crystalline nanoparticles. The relaxation rate is the largest and the irreversibility temperature is the lowest for the regular amorphous nanoparticles. The crystalline material exhibits the lowest relaxation rate and the largest irreversibility temperature. Prozorov and co-workers [62] developed a phenomenological model to explain the details of the experimental results. The main new aspect of the model is the dependence of the barrier for magnetic relaxation on the instantaneous magnetization, and therefore on time. The time-dependent barrier yields a natural explanation for the time-logarithmic decay of the magnetization. Interactions between particles as well as shape and

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