Decrease of superparamagnetic fraction at room temperature in ultrafine CoFe2O4 particles by Ag doping

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Co1 − xAgxFe2O4 nanoparticles have been prepared by the combustion route. The average crystallite sizes for compositions with x = 0 and 0.2 are found to be 36 and 33 nm respectively from the XRD line broadening. Compared to the pure CoFe2O4, Ag-doping reduces the intrinsic magnetization values (M, Mr), but enhances coercivity (Hc). Mössbauer spectra show two sextets, indicating occupancies of tetrahedral and octahedral sites by Fe3+. Hyperfine fields of 505 and 477 kOe in pure CoFe2O4 have been found for octahedral and tetrahedral sites respectively at liquid nitrogen temperature. The hyperfine field decreases with Ag-doping which also corroborates the magnetization studies. EPR study confirms the room temperature ferromagnetic behavior for Co1 − xAgxFe2O4 (x = 0.2). The room temperature Mössbauer studies on x = 0.0 and 0.2 show the ferromagnetic sextets (95%) along with superparamagnetic doublet (5%). However, x = 0.6 sample shows the ferromagnetic sextets only at room temperature. Highly Ag doped samples could be useful for the fabrication of the high-density magnetic materials as well as magnetic drug delivery.

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