Exchange bias in disordered granular systems

The exchange bias properties of a granular system composed of Fe nanoparticles (mean size ~6 nm) embedded in a structurally and magnetically disordered oxide have been investigated. The exchange bias field, resulting from the exchange coupling between the ferromagnetic particles and the frozen spin-glass-like oxide, strongly depends on the magneto-thermal history and ageing of the sample. Such dependence is a consequence of the magnetically disordered nature of the oxide phase and it is explained by considering that different frozen spin configurations of the oxide are selected on varying the temperature and the field-cooling process and that they evolve with the application time of the cooling field. Monte Carlo simulations on core/shell nanoparticles (ferromagnetic core/disordered ferrimagnetic shell) well reproduce the ageing effect.