Exchange bias of Co nanoparticles embedded in Cr2O3 and Al2O3 matrices

The magnetic properties of similar to 1.5 nm Co nanoparticles embedded in a diamagnetic Al2O3 or anti ferromagnetic (AFM) Cr2O3 matrix were investigated. For Co nanoparticles in Al2O3 matrix, a typical behavior of weakly interacting nanoparticles is observed, characterized by a superparamagnetic regime and a progressive blocking of particle moments centered at < T-B > = 14 K. On the other hand, when the Co nanoparticles are immersed in a Cr2O3 matrix a very different magnetic behavior was found. The system shows large irreversibility in field-cooling/zero-field-cooling magnetization curves and much larger coercivity was observed even up to room temperature. Hysteresis loop shift is present when the system is field-cooled from a temperature above the Cr2O3 Neel temperature. We found that the exchange bias field follows a Brillouin type temperature dependence and goes to zero at T-N. These results evidence the enhancement of thermal stability of the Co nanoparticle moments, associated to the increase of anisotropy due to the interface exchange interaction between the ferromagnetic particles and the AFM Cr2O3 matrix. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3259425]