Temperature dependence of the exchange bias properties of ferromagnetic/antiferromagnetic polycrystalline bilayers

Monte Carlo simulations using a granular level model of coupled ferromagnetic/antiferromagnetic (F/AF) layers that accounts for thermal effects on both F and AF grains are performed to calculate the temperature dependence of the exchange bias properties. The influence of the coupling between F grains and of the grain size dispersion is also investigated. The predicted thermal variation of the exchange bias and coercive fields are in good qualitative agreement with experiment. For monodispersive grains, it is shown that the exchange field vanishes below the blocking temperature of the AF grains, i.e. when the AF grains become sufficiently unstable, so that they switch but after the F grains. At the same temperature, the coercive field exhibits a maximum due to the coupling with the AF layer. It is also evidenced that a grain size distribution increases the stability of each layer, thus improving the exchange bias properties.