Antiferromagnetic thickness and temperature dependence of the exchange bias properties of Co/IrMn nanodots and continuous films: A Monte Carlo study
Résumé
Motivated by the challenge of understanding the complex influence of the antiferromagnetic (AF) thickness and the temperature on exchange bias (EB) properties, and by the necessity of miniaturization of devices, we investigate EB properties of Co/IrMn nanodots and of continuous films by using kinetic Monte Carlo simulations. To that purpose, we use a granular model, which takes into account disordered interfacial phases in the AF layer and, in the case of nanodots, disordered phases at the edges in the AF layer. Our results show that the AF thickness dependence of the exchange field H E (measured at room temperature) in both nanodots and continuous films exhibits a maximum in agreement with experimental results. We explain these results in terms of superparamagnetic and blocked grains in the AF layer at room temperature and also not polarized AF grains during the initial field-cooling. The simulated values of H E in nanodots are smaller than that in continuous films for small AF thicknesses and larger for larger ones due to the contribution of the disordered phases at the edges in the AF layer. Also, we investigate the temperature and AF thickness effects on H E and on the coercive field H C. We found that H E slightly decreases at low temperatures due to the disordered interfacial phases. Importantly, at the maximum blocking temperature of the AF grains, H E vanishes and H C exhibits a maximum. Our numerical results are successfully compared to experimental data on Co/IrMn bilayers for various IrMn thicknesses and all temperatures. In addition, our results indicate that H E is smaller in nanodots at low measurement temperature due to the presence of disordered phases at the edges. Concerning H C , our data show that it can be either larger or smaller in nanodots depending on the measurement temperature.
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