Exchange bias in epitaxial CoO Co bilayers with different crystallographic symmetries

The magnetic anisotropies of epitaxially grown exchange bias CoO Co bilayers with (111), (110), and (100) crystallographic orientations have been investigated by Brillouin light scattering (BLS) by means of spin waves. Experiments were carried out at 293 and 140 K, i.e., above and below, respectively, the Néel temperature of CoO. The unidirectional, twofold, and fourfold anisotropy constants, bulk and surface, were obtained by fitting the angular dependence of the spin wave frequencies. For all samples, we observed an isotropic up-shift of the spin wave frequencies. We conclude that all observed magnetic anisotropies are related to the crystallographic symmetry of the ferromagnetic and antiferromagnetic layers, depending on the lattice mismatch between layers, the sequence of layers upon deposition, and the quality of the ferromagnetic/antiferromagnetic interface.