Domain wall asymmetries in Ni81Fe19/NiO: proof of variable anisotropies in exchange bias systems

Abstract

Multiple changes in the internal structure of magnetic domain walls due to alterations of the interfacial coupling across the ferromagnetic/antiferromagnetic interface are reported for Ni81Fe19/NiO exchange coupled films. Depending on the antiferromagnetically induced anisotropy, three different types of domain walls are observed. Cross-tie domain wall structures of decreased vortex to anti-vortex spacing develop with the addition of a thin antiferromagnetic layer. For exchange biased samples strong asymmetries in domain wall structure occur for the ascending and descending branch of the magnetization loop. For the descending branch a symmetric 180° Néel wall develops, whereas a folded cross-tie domain wall structure forms during magnetization reversal along the ascending loop branch. The novel type of ‘zig-zagged’ cross-tie wall is characterized by cross-ties reaching differently into the surrounding domain areas. The wall alterations indicate the existence of bi-modal coupling strengths in exchange coupled systems, which is in accordance with models of exchange bias that assume pinned and unpinned spins at the ferromagnetic/antiferromagnetic interface.