Reversal Mechanism of Exchange-Biased CoFeB/IrMn Bilayers Observed by Lorentz Electron Microscopy

The magnetization reversal mechanism of exchange-biased thin layers with different antiferromagnetic (AFM) layer thicknesses has been investigated using Lorentz transmission electron microscopy. The polycrystalline IrMn and amorphous CoFeB bilayers exhibit unidirectional anisotropy, which was induced by field annealing. Lorentz analyses revealed that the magnetic moments rotate away from the unidirectional axis before reversal, when the magnetic field was applied collinear to the unidirectional anisotropy direction. No asymmetry of the reversal process was found in these layers according to the vibrating sample magnetometry and electron microscopy observation. Small ( <10 mum in diameter) 360deg domain-wall loops act as nucleation sites for reversal and disappear during the reversal. A continuous rotation of magnetic moments was observed when the magnetic field was applied perpendicular to the unidirectional axis. Minor intermixing at the interface and structural defects such as interface roughness, grain boundaries, and stacking faults were identified as possible sources of magnetic frustration and uncompensated spins in the IrMn layer that contribute to exchange bias.