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. © 2002 American Institute of Physics.