Polycrystalline exchange-biased bilayers: Magnetically effective versus structural

The magnetic characteristics of polycrystalline exchange-biased antiferromagnet/ferromagnet bilayers are determined by a complex interplay of parameters, describing structural and magnetic properties of the material system, including, in particular, the grain volume distribution of the antiferromagnet. An ideal characterization of such systems would be a nondestructive determination of the relevant parameters for each individual grain. This is in most cases not feasible, since typical characterization methods average over larger areas. Here, we show that it is, however, possible to determine averaged microscopic parameters from averaged macroscopic magnetic quantities measured by vectorial Kerr magnetometry in comparison to an elaborate model. In particular, we estimate the magnetically effective antiferromagnetic grain size distribution, being essential for the interface exchange coupling to the ferromagnetic layer. We found that the distribution of magnetically active grain sizes differs from the structural one, indicating that the antiferromagnetic order, relevant for the exchange bias, extends only over a part of the grains' structural volumes.