Bohumil David, Naděžda Pizúrová, Oldřich Schneeweiss, E. Santava, Vít Kudrle, Ondřej Jašek
gamma-Fe2O3 Nanopowders Synthesized in Microwave Plasma and Extraordinarily Strong Temperature Influence on Their Mossbauer Spectra
Journal of Nanoscience and Nanotechnology 12 (2012) 9277-9285
The article reports on two nanopowders synthesized in microwave plasma: the first sample was synthesized in a torch discharge at 1 bar and the second sample was synthesized in low-pressure plasma at 40 mbar. Morphology, composition and properties of the powders were studied by TEM, XRD, Raman and Mossbauer spectroscopies, and magnetic measurements. In the XRD patterns of the samples only g-Fe2O3 was identified (mean crystallite size dxrd was 24 nm for the first sample and 13 nm for the second sample). Based on the Mossbauer spectra measured at 5 K, the presence of other iron oxide phases was excluded in both samples. Unusually strong temperature dependence of the Lamb-Mossbauer factor was observed: Isa(5 K)/IsA(293 K) = 6 in the case of the first sample and ISA(5 K)/IsA(293 K) = 22 for the second sample (ISA denotes integral spectrum area). This effect is explained as the consequence of the reduced agglomeration of electrically charged nanoparticles in the plasma, i.e., particles can either move at 293 K (when they are free) or tilt (if they are a part of a chain). Superparamagnetic phase was not observed in the room-temperature Mossbauer spectra of both samples.
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