High resolution Kerr microscopy study of exchange bias phenomena in FePt/Fe exchange spring magnets

Magnetization and magnetic microstructure of top soft magnetic layer (Fe), which is exchange spring coupled to bottom hard magnetic layer ($L1_0$ FePt) is studied using high resolution Kerr microscopy. When the sample (FePt/Fe) is at remanent condition of hard magnetic layer, considerable shifting of Fe layer hysteresis loop from centre i.e., exchange bias phenomena is observed. It is observed that one can tune the magnitude of exchange bias shift by reaching the remanent state from different saturating fields ($H_{SAT}$) and also by varying the angle between measuring field and $H_{SAT}$. The M-H loops and domain images of top soft Fe layer demonstrates unambiguously that soft magnetic layer at remanent state in such exchange coupled system is having unidirectional anisotropy. An analogy is drawn and the observations are explained in terms of the mostly accepted models of exchange bias phenomena exhibited by bilayers consisting of ferromagnetic(FM) and anti-ferromagnetic (AFM) layers, when the AFM layer is field cooled across $N\acute{e}el$ transition temperature.