An important issue in exchange-biased ferromagnetic-antiferromagnetic (F/AF) systems is the intrinsic asymmetric behaviour of the magnetization reversal (MR) [1]. MR via rotation processes is more relevant in one branch of the hysteresis loop. This is reflected in the magnetization loops of the parallel component (M||) by more rounded transitions, and/or in the loops of the perpendicular component (MT) by relevant larger values. Experimental data do not, however, agree on the pathways dominating the MR in the two field branches. For some of the systems, rounded M|| transitions and/or larger MT values are found in the descending branch, where the field is applied parallel to the bias direction. For other systems the opposite behaviour was found.
We have studied the influence of the anisotropies on the MR in F/IrMn bilayers and throw light on the aforementioned discrepancy on the asymmetric reversal phenomena. Our findings show that the competition between the uniaxial (F) and unidirectional (interface) anisotropies and the interfacial spin frustration play the main roles that explain such behaviour.
Layer, thickness and angle dependent measurements have been performed with high resolution vectorial kerr magnetometry. For Co/IrMn bilayers, there is an angular range around the easy axis where larger MT values and rounded M|| transitions are always found in the descending branch of the hysteresis. For FeNi/IrMn bilayers, however, these features can be found in both branches, depending on the sign of the applied magnetic field angle with respect to the easy axis [2]. The data are well reproduce with a Stoner-Wohlfarth model by considering collinear and, surprisingly, non collinear anisotropies for Co/IrMn and FeNi/IrMn, respectively. Numerical simulations reveal that the presence of interface roughness induces a randomness of the interfacial coupling at the F/AF interface which, depending on the anisotropy ratio, can results either in magnetic disorder in the AF layer (large F anisotropy) or in a spin reorientation of the F layer (small F anisotropy).
[1] J. Camarero et al., Phys. Rev. Lett. 95, 057204 (2005).
[2] E. Jimenez et al., submitted to Phys. Rev. Lett. |