Microwave absorption behavior in Cr2O3 nanopowders

We have investigated the microwave power absorption at X-band (8.8-9.8 GHz) in Cr2O3 nanopowders, for the 294-400 K temperature range. For all temperatures, two different kinds of microwave absorptions are observed: the electron magnetic resonance (EMR) at high magnetic field and the low-field microwave absorption (LFMA) around zero field. EMR spectrum can be separated in two contributions associated with two different resonant absorption modes, where this result is interpreted as the combination of two different magnetic phases; corresponding to the core (mode A) and the surface (mode B) of the Cr2O3 nanoparticles. Also, we have detected the onset of the antiferro-paramagnetic transition in resonant mode A, which is characteristic of Cr2O3 samples; being confirmed the magnetic transition by means of the magnetically modulated microwave absorption spectroscopy (MAMMAS). Additionally, the temperature behaviors of the resonant mode B and the LFMA signal are associated with a ferromagnetic state on the surface of the Cr2O3 nanoparticles.