Glassy dynamics in the exchange bias properties of the iron/iron oxide nanogranular system

We have observed a glassy dynamics of the exchange bias properties in a granular system composed of Fe nanoparticles dispersed in a structurally and magnetically disordered (cluster glasslike) Fe oxide matrix. The exchange field, measured at T=5 K, increases with increasing the time t(w) spent at T=50 K, after applying the cooling field. During t(w), the oxide phase evolves towards a lower energy configuration, resulting in a stronger interface exchange coupling with the Fe particle moments. Monte Carlo simulations on core/shell nanoparticles reproduce such aging effect, provided that a shell random anisotropy is assumed.