On the origin of positive exchange bias and coercivity enhancement in proximity to the blocking temperature

Abstract

Magnetic properties of two oxidised Co thin films prepared by different sputtering techniques are analysed. Exchange bias (EB) blocking temperatures (TB) of the samples were found to be 50 K different due to difference in grain sizes of antiferromagnetic CoO. In proximity to TB the EB changes its sign from negative to positive. This positive EB behaviour is strikingly similar for the two samples with the exception of its position on the temperature scale. A computational model, taking into account temperature dependence of the antiferromagnet’s magnetocrystalline anisotropy and its sublattices’ magnetisation, reproduces positive EB in proximity to TB. This happens when a significant fraction of antiferromagnetic entities becomes involved in magnetisation reversal, and there is antiferromagnetic exchange coupling between those entities. The coercivity enhancement, accompanying positive EB, is due to anisotropy energies of the reversible antiferromagnetic entities contributing to the ferromagnetic layer’s energy barrier. The model suggests that in order for the two samples to have identical positive EB behaviour, the reversible antiferromagnetic entities should represent individual spins.