Exchange anisotropy in ferromagnetic/antiferromagnetic bilayers

Interfacial exchange coupling between thin films of ferromagnetic and antiferromagnetic materials was investigated. This direct exchange coupling results in magnetic anisotropy for magnetization of the ferromagnetic film called exchange anisotropy. In this thesis, exchange anisotropy was measured in Fe/MnF₂ bilayers with epitaxial antiferromagnetic MnF₂ layer and polycrystalline Co/CoO bilayers. Exchange anisotropy in Fe/MnF ₂ bilayers was found to consist of unidirectional, uniaxial, threefold and fourfold symmetry components. The threefold anisotropy leads to an asymmetry of the magnetization reversal mechanisms for the two branches of the hysteresis loop of the system. The fourfold anisotropy component results in enhancement of coercivity of the bilayer.; A theoretical model explaining the origin of the threefold and the fourfold exchange anisotropy components as arising from a partial domain wall in the antiferromagnetic material was developed. Application of this theoretical model to the experimental data analysis allows one to separately determine the fraction of uncompensated interfacial spins in the antiferromagnet and the exchange coupling energy between the ferromagnetic magnetization and the antiferromagnetic interfacial spins.; The angular dependence of the exchange anisotropy energy in polycrystalline Co/CoO bilayers was found to be described by a combination of unidirectional and uniaxial components and a small rotatable anisotropy component. A model that gives correct order of magnitude for the exchange anisotropy components using only the bulk properties of the constituent layers was developed. Using ac susceptibility measurements, the origins of the coercivity enhancement in Co/CoO bilayers were determined.