Size dependence of exchange bias field in Co nanoparticles embedded in Cr2O3

D. TOBIA; E. WINKLER; R.D. ZYSLER; H.E. TROIANI; D. FIORANI

Estambul, Turquía

Congreso; International Conference on Nanoscale Magnetism(ICNM-2007); 2007

The exchange biasing was studied in the system composed by Co clusters embedded in Cr2O3 antiferromagnetic nanoparticles. The Co nanoparticles are crystalline in fcc phase with a size of ~1.5 nm. The matrix size and shape change from elongated particles of 50 nm to round particles of 200 nm, depending on the thermal treatment (see figure 1). We investigated the dependence on the exchange field (HE) as a function of the size and shape of the antiferromagnetic matrix by magnetization measurements. The samples were prepared from chemical precipitation followed by calcination in an Ar2/H2 atmosphere at 1300 K. The system presents a shift of the hysteresis cycle when it is cooled applying a magnetic field through the Cr2O3 antiferromagnetic transition temperature TN = 308 K. The HE value depends on the cooling field and the temperature. The maximum HE was observed cooling the sample with an applied field of 2000 Oe. We found a clear dependence of the HE on the size and shape of the matrix that decreases from 75 Oe to 6 Oe when the matrix surface/volume ratio decreases. We analyzed the results taking into account the influence of the exchange interaction at the Co/Cr2O3 interface, the matrix crystallinity and surface anisotropy.2O3 antiferromagnetic nanoparticles. The Co nanoparticles are crystalline in fcc phase with a size of ~1.5 nm. The matrix size and shape change from elongated particles of 50 nm to round particles of 200 nm, depending on the thermal treatment (see figure 1). We investigated the dependence on the exchange field (HE) as a function of the size and shape of the antiferromagnetic matrix by magnetization measurements. The samples were prepared from chemical precipitation followed by calcination in an Ar2/H2 atmosphere at 1300 K. The system presents a shift of the hysteresis cycle when it is cooled applying a magnetic field through the Cr2O3 antiferromagnetic transition temperature TN = 308 K. The HE value depends on the cooling field and the temperature. The maximum HE was observed cooling the sample with an applied field of 2000 Oe. We found a clear dependence of the HE on the size and shape of the matrix that decreases from 75 Oe to 6 Oe when the matrix surface/volume ratio decreases. We analyzed the results taking into account the influence of the exchange interaction at the Co/Cr2O3 interface, the matrix crystallinity and surface anisotropy.