On the origin of ion bombardment induced exchange bias modifications in polycrystalline layers

A model for the modifications of the exchange bias (EB) field Heb-vector of antiferromagnet (AF)/ferromagnet (F) bilayers with a polycrystalline or multidomain AF layer induced by ion bombardment (IB) in an external magnetic field is proposed. The model is based on a known two-energy level model for an antiferromagnetic grain or domain in contact with a ferromagnet where two free energy minima are separated by an energy barrier. The model explains the as yet unexplained increase of Heb-vector upon IB on the basis of the grain/domain size and magnetic anisotropy constants distributions in the antiferromagnetic layer after its deposition and on the basis of a twofold effect of the IB on the antiferromagnetic grains/domains: (1) IB acts like local hyperthermal heating leading to an almost immediate increase of the sample's EB. (2) Defects induced by IB in the antiferromagnetic grains/domains lead to a decrease of the energy barrier between the two minima, resulting in a slow additional increase of Heb-vector with time (with temperature T as a parameter) after the bombardment. The model is tested by experiments on the time dependence of the EB and coercive fields after the IB of NiO/NiFe bilayers