Exchange bias in systems based on glassy ultrafine IrMn layers

The study of the exchange bias (EB) interaction between ferromagnetic (FM) and antiferromagnetic (AFM) phases plays a crucial role both from the theoretical and from the technological point of view. In fact, although the EB effect is already exploited in emerging technologies, e.g. in non-volatile magnetoresistive magnetic random access memories, some key aspects of the underlying physics are still not completely understood [1]. Recent studies on polycrystalline FM/AFM nanostructures have addressed the issue of establishing the dependence of EB on size confinement, and agree that a comparison between the nanostructure size and an AFM characteristic length of magnetic correlation is to be taken into account [2,3]. The appearance of a magnetic correlation length can be related to the presence of AFM regions showing a magnetic glassy behaviour [3,4], and in this context, we proposed a model for the magnetic structure of the AFM phase based on the existence of a thin structurally and magnetically disordered region in the AFM layer interposed between the FM phase and the bulk of the AFM layer; the latter is supposed to consist of nanograins either magnetically independent from each other or weakly interacting. This model is based on the experimental evidence we found in bilayer systems constituted of Ni80Fe20 (NiFe) as FM phase and Ir25Mn75 (IrMn) as AFM phase. In this research work, we focus on the IrMn/NiFe system and purposely address the magnetothermal properties of the magnetically disordered IrMn region on its own. To this aim, we produce IrMn/NiFe systems constituted of a very thin IrMn phase, both in form of continuous films and nanodots, so to neglect the contribution of the bulk of the AFM layer to EB and highlight the role of the glassy IrMn region to both the exchange coupling mechanism and size confinement effects. In detail, we report on the magnetic properties of the Cu(3 nm)/IrMn(3 nm)/Py(3 nm) system deposited on a Si substrate by electron beam lithography and lift-off using dc sputtering deposition in presence of a static magnetic field Hdep; we investigate both the continuous reference film and a square array of circular dots with a diameter of ~140 nm, and centre-to-centre distance of ~200 nm.