Switching of domains and domain walls in Fe50Mn50/Ni81Fe19 bilayers with non-180° ferromagnetic domains

Abstract

We have studied FeMn/Ni81Fe19 bilayers using vectorial vibrating-sample magnetometry (VSM) and the magnetooptic-indicator-film (MOIF) technique and observed stationary antiferromagnetic (AF) domain walls throughout the hysteresis process. The FeMn/Ni81Fe19 bilayer was demagnetized at 450 K and then cooled to 300 K in a zero field. The resultant hysteresis loop at 300 K clearly shows two loops, one shifted to the left of the origin and the other, to the right. Moreover, one loop is slanted and the other is square, indicating that the two sets of ferromagnetic (FM) domains have noncollinear magnetizations. At the zero applied field, the MOIF images clearly reveal the FM domains and domain walls. These domain walls, however, are hybrid domain walls connecting both FM and AF domain walls. With the application of a magnetic field, the FM domain walls can be swept. But, regardless of the applied field, including the case where the FM layer has been made into a single domain, domain walls at the original locations are still visible. Of the two sets of the FM domains with different magnetic-anisotropy axes, one set becomes reversed by rotation during magnetization reversal, while the other set becomes reversed by domain-wall nucleations and propagations.