Influence of interfacial disorder and temperature on magnetization reversal in exchange-coupled bilayers

Polarized neutron reflectometry is used to measure the thermal response of the net-magnetization vector of polycrystalline ferromagnetic (F) Fe films exchange coupled to twinned (110) MnF₂ antiferromagnetic (AF) layers. We observe a strong correlation between the temperature dependencies of the net sample magnetization perpendicular to the applied field at coercivity and exchange bias. For cooling field and measurement conditions involving magnetization reversal via rotation, we find a range of temperature dependencies. For the smoothest F-AF interface, the temperature dependence of exchange bias compares well to a S=5/2 Brillouin function - an observation predicted by some theoretical models. This temperature dependence is expected for the sublattice magnetization and the square root of the anisotropy constant √K₁ of bulk MnF₂. In contrast, for a rough F-AF interface the magnetization reversal process (and exchange bias) showed little temperature dependence up to temperatures approaching the AF Néel point - a clear consequence of increasing interfacial disorder in a F-AF epitaxial system.