MAGNETIC PROPERTIES OF ANTIFERROMAGNETIC Cr2O3 NANOPARTICLES AND EXCHANGE INTERACTION IN Co / Cr2O3 NANOSTRUCTURES

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

Rosario, Argentina

Congreso; 10th Inter-American Congress of Electron Microscopy 2009 CIASEM 2009; 2009

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. In the case of Co NP’s in Al2O3 matrix, the NP’s are superparamagnetic above the blocking temperature <TB>=14K. 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. The magnetic anisotropy (K) of the AFM compound is one of the fundamentals parameters that control the coupling at the interface. In this context we study the evolution of the magnetic anisotropy with the size of the Cr2O3 nanoparticles systems, from bulk to nanoparticles of ~8nm. We analyze the dependence of K on the magnetocrystalline and surface contributions.