Surface anisotropy effect and interparticle interactions in NiO nanoparticles

E. WINKLER; R.D. ZYSLER; M. VASQUEZ MANSILLA; D. FIORANI; M. VASILAKAKI; K.N. TROHIDOU

Roma, Italia

Congreso; 6th International Conference on Fine Particle Magnetism (ICFPM07), New trends in nanoparticle magnetism; 2007

Magnetization and AC susceptibility (÷AC) experiments have been performed on NiO interacting and non-interacting nanoparticles. The studied systems consist in ~3 nm NiO nanoparticles in powder form and 0.55 % w/w NiO (NiO weight / total sample weight) dispersed in a polymer, with an average interparticle distance of 28 nm. The results show that the internal order of the particles is composed by an uncompensated antiferromagnetically ordered core with a magnetically disordered shell. The high temperature behaviour is dominated by the uncompensated moment of the core whose blocking is centred at TB ~ 60 K for the non-interacting system. The interparticle interactions lead to an increasing of the anisotropy energy shifting TB to higher temperature, resulting TB ~ 70 K for the powder sample. Moreover, the powder sample presents lower magnetization than the non-interacting particles indicating the demagnetizing character of the interparticle interactions. The low temperature behaviour is dominated by the surface spins that freeze in a cluster-glass-like state at T ~ 15 K and at T ~17 K for the dispersed and powder sample respectively. The freezing temperature is signalling by a peak showed in the magnetization vs. temperature and ÷AC(T) curves and an abrupt change in the temperature dependence of the viscosity, obtained from the relaxation measurements.÷AC) experiments have been performed on NiO interacting and non-interacting nanoparticles. The studied systems consist in ~3 nm NiO nanoparticles in powder form and 0.55 % w/w NiO (NiO weight / total sample weight) dispersed in a polymer, with an average interparticle distance of 28 nm. The results show that the internal order of the particles is composed by an uncompensated antiferromagnetically ordered core with a magnetically disordered shell. The high temperature behaviour is dominated by the uncompensated moment of the core whose blocking is centred at TB ~ 60 K for the non-interacting system. The interparticle interactions lead to an increasing of the anisotropy energy shifting TB to higher temperature, resulting TB ~ 70 K for the powder sample. Moreover, the powder sample presents lower magnetization than the non-interacting particles indicating the demagnetizing character of the interparticle interactions. The low temperature behaviour is dominated by the surface spins that freeze in a cluster-glass-like state at T ~ 15 K and at T ~17 K for the dispersed and powder sample respectively. The freezing temperature is signalling by a peak showed in the magnetization vs. temperature and ÷AC(T) curves and an abrupt change in the temperature dependence of the viscosity, obtained from the relaxation measurements.