Magnetic properties of antiferromagnetic Cr2O3 nanoparticles and exchange interaction in Co / Cr2O3 nanostructures

D. TOBIA; E. WINKLER; R. D. ZYSLER; M. GRANADA; H. E. TROIANI; EMILIO DE BIASI

Rosario, Argentina

Congreso; 10th Inter-American Congress on Electron Microscopy; 2009

CIASEM

When the size of a magnetic system is reduced to nanometric scale, the magnetic anisotropy energy responsible for the stabilization of the magnetic moment along one direction becomes comparable to the thermal energy (superparamagnetic limit). The magnetic effective anisotropy energy can be enhanced through the exchange coupling at the ferromagnetic (FM) / antiferromagnetic (AFM) interface. In this work we present a study of the magnetic properties of ~1.5nm Co nanoparticles (NP’s) immersed in diamagnetic Al2O3 and antiferromagnetic Cr2O3 matrices. The crystalline and morphological characterizations were performed by X-ray diffraction and transmission electron microscopy (TEM) and the magnetic properties were studied by dc-magnetization measurements. In the case of Co NP’s in Al2O3 matrix, the NP’s are superparamagnetic above the blocking temperature <TB>=14K and the system exhibits small coercive fields below TB. On the other hand, when the Co NP’s are immersed in Cr2O3 they present large irreversibility up to T=300K and important shifts in the hysteresis loops (exchange bias field). The temperature evolution of the exchange bias field and the TEM images show that the simple chemical route used to synthesize these nanostructures provides well defined FM/AFM interfaces which enhanced the exchange coupling and increased the magnetic stability of the systems.