We have employed the Monte Carlo (MC) simulation method to study at an atomic level the exchange bias effect, the vertical shift and the training effect observed in nanoparticles consisting of a ferromagnetic (FM) core and an antiferromagnetic (AFM) shell. Heisenberg exchange interaction is assumed between the spins in the ferromagnetic core, in the AFM shell and along the FM/AFM interface. We have demonstrated that the interface exchange interaction together with a strong interface anisotropy results in an exchange bias field (H-ex) and a strong coercive field (H-c). Our simulations show that while the interface is responsible for the exchange bias, in the vertical shift we have a contribution from the whole shell of the nanoparticles. In the training effect we have a contribution from the interface, and we can also have a contribution from the core and the shell , depending on the particle size. The shape of the H-c(T) and H-ex(T) curves depends oil the type and the,strength of the shell anisotropy. Using the MC simulations, we haven also studied the effect of interparticle dipolar in the exchange bias properties of an assembly of FM core/AFM shell nanoparticles for different nanoparticle densities. We show that these interactions result in a decrease of the coercive field and an increase of the exchange bias field with the concentration of the assembly. (c) 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

We have used the Monte Carlo simulation technique to investigate the mechanism that gives the exchange bias effects on composite magnetic nanoparticles with Ferromagnetic (FM) core/Antiferromagnetic (AFM) shell morphology. Our study shows that:. (a) the exchange bias field depends mainly on the structure of the interface and less on its size,. (b) when the shell size becomes bigger than the core size the exchange bias field decreases,. (c) the size of the vertical shift depends on the total number of uncompensated spins in the nanoparticle and. (d) the training effect has its origin to the rearrangement of the spins at the FM/AFM interface. Our results are in good agreement with experimental findings.

We have studied Co/CoO nanoclusters with a core-shell morphology produced by deposition from a gas aggregation source onto substrates in UHV and by theoretical modeling using the Monte Carlo simulation technique. The simulations give shifted hysteresis loops, in agreement with the experimental findings and result to a reversal in the size dependence the exchange bias field at low temperatures. (C) 2004 WLLEY-VCH Verlag GmbH & Co. KGaA, Weinheim.