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

Abstract

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.

Fundamentals for magnetic patterning by ion bombardment of exchange bias layer systems

“…2a). As predicted within the model by the defect induced decrease of K AF,i V i * and a subsequent thermal relaxation H eb increases slowly with time after the IB towards increasing H eb fields, similar to the behaviour of the FeNi (5 nm)/ NiO (50 nm) bilayer system in [13]. This increase is getting smaller for higher ion doses since there the H eb decreasing effects are dominant.…”

“…Therefore due to the local hyperthermal heating an increase in the macroscopic EB must be observed, with the EB field directed in the direction of the applied magnetic field during IB. The enhancement of H eb by this mechanism is observable immediately after the IB as has been experimentally shown for a FeNi (5nm)/NiO (50nm) bilayer [13].…”

“…A tentative model [12,13] has been recently proposed to explain the IB induced modifications of H eb in AF/FM bilayers, based on a two energy level model described in [14] and developed and applied to EB systems with polycrystalline AF layers in [15,16] and references therein. In this model it is assumed that the AF consists of magnetically independent grains or domains (in the following briefly denoted as "domains"), like, e.g., in a polycrystalline AF layer.…”

“…Therefore the present paper will show experimental results on these three issues and tries to connect the experimental results with a recently proposed model for the ion bombardment induced magnetic modifications in exchange biased bilayers [12,13]. The model is briefly presented in Section 2 and predicts that the IB modified H eb shows a defined time dependence after the bombardment, which has been verified already as a proof of principle [12,13] for a FeNi (5 nm)/NiO (50 nm) bilayer. Therefore, in Section 3, it will be shown that the time dependence is also observed for other systems, verifying a broader applicability of the model.…”

Antiparallel interface coupling evidenced by negative rotatable anisotropy in IrMn/NiFe bilayers

“…Special care was taken to assure that the data are collected after time interval sufficient to avoid temporal changes of H eb (thermal drift effect 27 ). Also, the maximum measurement field was sufficiently high to avoid EB overestimation due to minor-loops effects.…”