Monte Carlo study of the exchange bias and the training effect in nanoparticles with core/shell morphology



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 (Hex) and a strong coercive field (Hc). 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 Hc(T) and Hex(T) curves depends on the type and the strength of the shell anisotropy. Using the MC simulations, we have also studied the effect of interparticle dipolar interactions 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. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)