Static and Dynamic Characterisation of Exchange-Biased Mutlilayers for Magnetic Sensor Applications

Abstract

The coupling of ferromagnetic and antiferromagnetic materials through exchange-bias has been one of the key interactions in producing magnetic sensing and memory technology since soon after the discovery of the GMR and TMR effects. Much is known of this phenomenon, however it still provides excellent grounds for research, both in exploration of new exchange-bias capable antiferromagnetic materials, as well as the effects it has on ferromagnetic dynamics. Presented in this thesis is exploratory work on a candidate antiferromagnetic material, CuMn. Whilst theoretical calculations indicate an L12 ordered CuMn3 would exhibit the highly desirable non-collinear T3 magnetic moment configuration, no such ordering was detected in a suite of samples produced by sputtering under an exhaustive set of preparation conditions. A prototypical system of CoFe\PtMn annealed at different times and temperatures, with the results interpreted in the context of a grainvolume distribution model, where no change in the exchange-bias field or coercivity is observed across 270 C to 340 C, between 4 and 10 hours. The effects of exchange-bias on ferromagnetic resonance linewidth were also investigated, with a weak dependence on the strength of the bias field discovered in exchange-biased IrMn\NiFe bilayers. This dependence appears to be distinct from another, much larger contribution, which is responsible for an approximately ten-fold increase in the linewidth in the bilayers when compared to a control sample consisting of just Ta\NiFe\Ta. The role of the exchangebias anisotropy is modelled in the context of the intrinsic `dragging effect', with a similar dichotomous dependence produced in calculated linewidth data. Finally, perpendicularly magnetised films of [Co/Pt]n are investigated both with and without an exchange-biasing layer of IrMn. The linewidth in the samples without the biasing layer is determined to be predominantly due to dragging effect, however there exists an additional contribution which is unaccounted for. Similar measurements in the exchange-biased samples produce the same signature, and a quantitative effective circuit model is used to fit the data, suggestive of the role of spin current damping.