Magnetization reversal of the ferromagnetic layer in IrMn/CoFe bilayers

The magnetization reversal of the ferromagnetic layer in IrMn/CoFe exchange-coupled bilayer films with different antiferromagnetic (AFM) layer thicknesses (d(AFM)) has been investigated using Lorentz microscopy and bulk magnetometry. These films exhibit very complex magnetization processes and the reversal mechanism is dependent on d(AFM). Holding the film at negative saturation of the ferromagnetic layer for up to 87 h results in no change in the reversal mechanism or coercivity, but in a decrease in the exchange field. We believe that two energy barrier distributions with different time constants coexist. Thermally activated reversal of the antiferromagnetic layer with a large time constant results in an increasing shift of the entire hysteresis loop towards zero field with increased period of time spent at negative saturation, because of a reduction in the overall unidirectional anisotropy in the films. Thermal activation with a small time constant contributes to loop broadening, an asymmetry in reversal, and training effects. As d(AFM) decreases, the energy barriers for thermally activated reversal of the antiferromagnetic layer decrease so the changes in the antiferromagnetic layer become more significant, resulting in a greater effect on the reversal of the ferromagnetic layer. (C) 2002 American Institute of Physics.