Size dependence of the magnetic order in antiferromagnetic Cr2O3 nanoparticles

DINA TOBIA; ELIN WINKLER; ROBERTO ZYSLER; MARA GRANADA; HORACIO TROIANI

Karlsruhe, Alemania

Conferencia; International Conference on Magnetism; 2009

The magnetic properties are determined by the effective exchange and anisotropy fields which are strongly modified by the surface to volume ratio. In this work we study the magnetic behaviour of antiferromagnetic Cr2O3 nanoparticles systems with sizes between 8 and 70 nm. The samples characterization was performed by x-ray diffraction and transmission electron microscopy, the magnetic properties were investigated by magnetization and electron paramagnetic resonance experiments. We have found a significant decrease of the Néel temperature (TN) when the size is reduced (TN-bulk = 307.8 K). As a function of the particle size the effective anisotropy decreases from the bulk value, Kbulk= 2.5 10^5 erg/cm^3, shows a minimum at ~30 nm (Keff = 3.8 10^3 erg/cm^3) and increases for smaller sizes. We analyze the size dependence of Keff in terms of the magnetocrystalline and surface anisotropy contributions. We introduce a phenomenological model that considers the size evolution of the contributions from the magnetically ordered core and the disordered particle surface.