Powders with superparamagnetic Fe₃C particles studied with Mössbauer spectrometry

Two nanopowders with superparamagnetic Fe₃C particles were synthesised by the method of laser-induced pyrolysis of gaseous precursors. Both were characterised by X-ray diffraction, Mössbauer spectrometry and standard magnetic measurements. The mean crystallite size of Fe₃C was 3 nm for the first sample and 10 nm for the second sample (Scherrer formula), i.e. it was lower than in our previously studied ferromagnetic Fe₃C-based sample. Fe₃C phase in both present samples exhibited by ~20 K reduced Curie temperature which is interpreted as a nanosize effect. After annealing of the samples at 1073 K for 30 minutes the Curie temperature of the Fe₃C phase in both samples matched its standard bulk value. Beside Fe₃C phase also Fe₃O₄ and carbon black were present in the synthesised samples.