Magnetic Interface Structures in Exchange Bias Systems

Systematic measurements were performed on sputtered polycrystalline multilayer thin films containing CoFe (5 nm)/IrMn (10 nm). A number of phenomena were described and explained for the first time. The York Model of Exchange Bias was used to interpret the effect of the interface between the ferromagnetic (F)/antiferromagnetic (AF) layers.

An ultra-thin interfacial layer of Mn with a thickness between 0 and 0.6 nm was added between the F and AF layers to alter the interfacial composition. The films were analysed by X-ray diffraction (XRD) and transmission electron microscope (TEM) imaging. Their magnetic behaviour was measured using temperature-controlled vibrating sample magnetometry (VSM).

A marked irreversibility in the loop shift Hex after setting was observed together with large increases in the coercivity Hc (×4) and athermal training ∆Hc1. These latter effects are attributed to spin cluster freezing which also increased Hex by up to 50%. The cluster freezing was observed to occur over a wide range of temperatures (> 200 K) and to below 5 K. This is consistent with diffusion of Mn at the F/AF interface which varied with the thickness of the Mn interfacial layer.

An asymmetry of the saturation magnetisation ∆ms between the set and reversed directions was measured in detail. This was found to be consistent with the F alignment of a single Mn atomic layer at the surface of the IrMn. ∆ms was observed to exhibit a peak at < 10 K. At present this observation is unexplained.