Exchange Bias

Figure 1.14: Exchange bias and coercive field of a CoFe layer in dependence on the MnIr thickness. From [124]

The exchange bias was discovered in 1956 by MEIKLEJOHN and BEAN [89,90] as a new type of magnetic anisotropy. They found an unidirectional pinning of a ferromagnetic layer by an adjacent antiferromagnetic layer. When the ferromagnet in contact with the antiferromagnet is cooled from above the
Neél temperature in an outer magnetic field, there is a shift from zero along the field axis in opposite direction of the applied field. This unidirectional shift is called exchange bias and it means that there is a preferred magnetisation direction for the ferromagnetic layer.

The exchange bias depends strongly on the thicknesses of the ferromagnetic and the antiferromagnetic layer. Figure 1.14 shows the dependence on the antiferromagnetic layer thickness for MnIr in contact with a CoFe layer as the ferromagnet. There clearly is a maximum exchange bias at a MnIr thickness of 7.5nm, which is typical for MnIr (see e.g. [2]). Such an exchange bias can also be impressed into an antiferromagnet-ferromagnet system by sputtering the thin layers within a magnetic mask, as it is done in this thesis (see chapter 6).