Bimodal distribution of blocking temperature for exchange-bias ferromagnetic/antiferromagnetic bilayers: a granular Monte Carlo study with less stable magnetic regions spread over the interface

In exchange bias based devices, ferromagnetic/antiferromagnetic disordered interfacial spins which exhibit low freezing temperatures contribute to the alteration of the exchange bias properties. In particular, the blocking temperature distributions earlier observed experimentally show two contributions: the common high-temperature peak due to the antiferromagnetic grain volume distribution and a less usual low-temperature contribution presumably attributed to disordered interfacial spins. Here, in order to test this assumption Monte Carlo simulations based on a granular level model are used to calculate blocking temperature distribution. Small magnetic grains with weaker anisotropy and interfacial coupling are introduced at the ferromagnetic/antiferromagnetic interface to account for the disordered interfacial spins. As a result, the bimodal character of blocking temperature distributions is reproduced and varying the amount of these smaller magnetic grains tunes the low-temperature contribution of the distribution. These simulations therefore validate the assumption that less stable magnetic regions spread over the ferromagnetic/antiferromagnetic interface may originate the bimodal character of blocking temperature distributions, as earlier brought forward by experimentalists to explain their results. In addition, because of the heterogeneities of the interface in the presence of less stable small grains, our simulations show that such bilayers cannot be described using a simple model of uniform rotation of the ferromagnetic layer interacting with an average interfacial coupling.