Field cooling dependence of the anisotropy in exchange biased FeF2∕Co films

Abstract

The magnetization reversal in exchange biased FeF2/Co films was studied using polarized neutron reflectometry and magneto-optical imaging (MOI). After field cooling in a field H perpendicular to the c axis (the easy axis of FeF2) through the FeF2 Néel temperature, the magnetization curve measured with H parallel to the c axis showed a double loop, one with a positive exchange bias field (HE) and one with a negative HE. This behavior suggests that the antiferromagnet (AFM) is split into two types of domains with HE in opposite directions along the c axis. However, the observation of spin-flip reflectivity in the field region separating the two loops indicates that some rotation of the Co magnetization also occurred during reversal. Concurrently, small inhomogeneous domains with sizes much less than 30μm were observed using MOI. This implies that small AFM domains with opposing HE created during cooling and the resulting frustration of the interface exchange interaction with the ferromagnet are responsible for the perpendicular magnetization during reversal.