A model for the modifications of the exchange bias (EB) field 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 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 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.

Calculations are presented for the distribution of stress and strain in thin films and substrates from inhomogeneous plane stresses developed in the film during deposition. The results can be applied to the ’’bending-plate method’’ of determining the unknown stresses in a film, in which the deformation of the substrate surface is measured. Thereby the local values of the stress tensor within the film plane (averaged through the film thickness) and the center of the stress distribution normal to the film plane can be obtained in principle. A more convenient evaluation based on linear approximations is applicable when the film thickness is small. Restrictions are discussed which result from neglecting the conditions of compatibility for the components of the strain tensor.

Long-term and temperature stability of the exchange coupling are important conditions for the practical application of spin-valve devices. Here we present magnetic measurements on the properties of sputtered NiO/NiFe bilayers. After deposition a temperature dependent magnetic self-alignment process was observed where the exchange bias field H eb increased according to a ln(t) law. The temperature dependence of this aftereffect indicates that a thermally activated process takes place which can be explained by a two energy-level relaxation model. In addition the long-term stability of the exchange coupling during a forced antiparallel alignment of the NiFe layer was investigated. A significant improvement of the coupling stability can be reached by cooling the samples through the Neél temperature in a magnetic field. Finally, the temperature dependence of H eb was determined for differently treated samples.

No abstract

No abstract