Direct imaging of cross-sectional magnetization reversal in an exchange-biased CoFeB/IrMn bilayer

The exchange coupling between ferromagnetic (FM) and antiferromagnetic materials has been intensely studied for fundamental physics and technological applications in various devices. However, the experimental reported magnitudes of exchange coupling are often smaller than that predicted theoretically, and for which the formation of springlike spin structure in the FM layer has been suggested as the cause. However, investigating the spin structure around the interface of exchange-coupled systems is challenging. Here we report the direct imaging of the cross-sectional magnetization reversal and spin structure at the interface of a model exchange-biased CoFeB/IrMn bilayer by a Lorentz transmission electron microscope with electron holography techniques. Through imaging of in situ magnetization reversal and spin structure at the remanent state, the springlike spin structure (either Bloch-wall-like or N\'eel-wall-like) in the CoFeB layer has been deduced within subnanometer region of the interface. This result puts a strong constraint on the theories of exchange coupling in inhomogeneous magnetic systems.