Detection of para–antiferromagnetic transition in Bi2Fe4O9 powders by means of microwave absorption measurements

• The changes in lineshape of the EPR spectra in Bi2Fe4O9 powders are studied.
• The onset of the para–antiferromagnetic transition is detected.
• A weak ferromagnetism is also observed in this material.
• MAMMAS and LFMA techniques are used to give a further knowledge on the bismuth ferrite.

An electron paramagnetic resonance (EPR) study of Bi2Fe4O9 powders is carried out in X-band (8.8–9.8 GHz) and the 200–350 K temperature range. For all the temperatures, the EPR spectra show a single broad line attributable to Fe3+ (S=5/2) ions. The onset of the para–antiferromagnetic transition has been determined from the temperature dependence of the parameters deduced from EPR spectra: the peak-to-peak linewidth (ΔHpp) and the resonant field (Hres); a weak ferromagnetism is also observed at low temperature and it is attributed to canting of Fe3+ ion sublattices in the antiferromagnetic matrix. The magnetically modulated microwave absorption spectroscopy (MAMMAS) and the low-field microwave absorption (LFMA) are used to give further information on this material. These techniques give evidence of the magnetic transition, suggesting a weak ferromagnetism at low temperature.