γ-Fe₂O₃ Nanopowders Synthesized in Microwave Plasma and Extraordinarily Strong Temperature Influence on Their Mössbauer Spectra

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 Mössbauer spectroscopies, and magnetic measurements. In the XRD patterns of the samples only γ-Fe₂O₃ was identified (mean crystallite size d _XRD was 24 nm for the first sample and 13 nm for the second sample). Based on the Mössbauer spectra measured at 5 K, the presence of other iron oxide phases was excluded in both samples. Unusually strong temperature dependence of the Lamb-Mössbauer factor was observed: I _SA (5 K)/I_SA (293 K)=6 in the case of the first sample and I_SA (5 K)/I_SA (293 K)=22 for the second sample (I_SA 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 Mössbauer spectra of both samples.