Superparamagnetism of AFM Cr2O3 nanoparticles

DINA TOBIA; ELIN L. WINKLER; ROBERTO D. ZYSLER; MARA GRANADA; HORACIO E. TROIANI

Buenos Aires, Argentina

Simposio; 15th International Symposium on Metastable, Amorphous and Nanostructured Materials; 2008

The ultrafine particles have been an object of increasing interest in the past few years due to the new physical properties that they present when their size is reduced to nanometric scale. In the special case of the antiferromagnetic (AFM) nanoparticles a significant increase in the magnetic moment is observed due to the spin non-compensation at the surface as the particle size is reduced. In the present work we report the size effects on the magnetic properties of AFM Cr2O3 nanoparticles. We show that the parameters that characterize the AFM system are strongly modified when the size of the nanoparticles is reduced. The Cr2O3 nanoparticles were synthesized by calcination of the precursor Cr(OH)3 in air atmosphere at 773 K. The precursor was obtained by chemical route. The X-ray diffraction pattern of the powder corresponds to the R3c Cr2O3 phase, without any other crystalline phase. From the line broadening of the diffraction pattern we determined the average nanoparticle diameter of 18 nm. Transmission electron microscopy (TEM) images show that the particles present poor crystallinity and slightly ellipsoidal shape. The magnetic properties of the nanoparticles where studied by magnetization and electron spin resonance (ESR) experiments. By the ESR spectroscopy we determined that the AFM order temperature, TN, shifted to ~ 270 K (TN(Bulk) ~ 308 K) when the size is reduced. On the other hand, we still observed a paramagnetic (PM) signal below TN, which could be attributed to a high surface disorder associated to free surface spins that remain PM even below TN.The magnetization as a function of the temperature, M(T), exhibits an irreversibility between the zero-field-cooling (ZFC) and the field-cooling (FC) curves. The ZFC M(T) curve, measured with an applied field of 50 Oe, presents a maximum centred at T ~ 30 K,  characteristic of a change from superparamagnetic to blocked behaviour. The blocking temperature (TB) shifts to lower temperatures for increasing applied fields. The magnetization as a function of the applied field curves, M(H), exhibit large coercive fields (HC) at temperatures below the blocking temperature, whilst for T > 60 K the magnetization curves are reversible. We discuss the results from magnetization measurements by using recent models in terms of the internal magnetic structures of the nanoparticles.