Thermal instabilities in exchange biased materials

The use of antiferromagnetic (AF) materials to pin a soft ferromagnet (FM) in a spin-valve, read head, or MRAM cell, has focussed attention on the exchange bias phenomenon. We report on a study of the thermal stability of exchange bias systems, IrMn/CoFe and FeMn/NiFe, with different thicknesses and grain sizes. For these materials a temperature has been determined at which thermal activation does not affect the state of the antiferromagnet, leading to reproducible data. Grain size studies have shown that the coercivity and exchange bias have different origins. Training effects studied over a range of temperatures from 4.2 to 400 K indicate a range of different contributions to exchange bias from the magnetic state of the antiferromagnet.