Superparamagnetism in AFM Cr2O3 nanoparticles

In this work we report the size effects on the magnetic properties of AFM Cr2O3 nanoparticles. From transmission electron microscopy we determined that the system presents high crystallinity and narrow lognormal size distribution centred at 〈ϕ〉 = 7.8 nm with σ = 0.3. The magnetic properties of the nanoparticles were studied by magnetization and electron paramagnetic resonance (EPR) experiments. By EPR spectroscopy we established that the AFM order temperature, TN, shifted to ∼270 K when the size is reduced (TN(Bulk) ∼ 308 K). From the zero-field-cooling and the field-cooling magnetization curves we determined the blocking temperature TB = 28 K. Below TB the system presents exchange bias effect. We discuss the results by using recent models in terms of the internal magnetic structures of the nanoparticles.