We investigated the physical microstructure and the domain structure of a CoFeB layer 30 Å and 14 Å thick, embedded in a multilayer composed of MgO (50 Å)/CoFeB/MgO (15 Å), by transmission electron microscopy and Lorentz microscopy. Structurally both films appeared similar and polycrystalline in character. However, the magnetization reversal behaviour observed during Lorentz microscopy experiments was found to vary considerably with the CoFeB thickness. Indeed, in the 14 Å CoFeB layer we found an unusual asymmetric orientation of domain walls when varying the direction of the applied field. To assist in the interpretation of the experimental results, we carried out calculations using a modified Stoner–Wohlfarth model, including unidirectional anisotropy. A good agreement was found between measured and calculated domain wall orientations. To explain the presence of the unidirectional anisotropy, we propose the possibility of formation of an interfacial structure that can give rise to an exchange bias effect, which has been previously seen from other studies.
Bellini, E., McVitie, S., MacLaren, D. A., O’Shea, K., Chapman, J. N., Cao, J., … & Freitas, P. P. (2013). Unexpected exchange bias behaviour in CoFeB ultrathin films for MTJ sensors investigated by Lorentz microscopy. Journal of Physics D: Applied Physics, 46(30), 305001.