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=50.3em0exK, increases with increasing the time tw spent at T=500.3em0exK, after applying the cooling field. During tw, 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.
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Fiorani et al. (2006) studied this question.
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