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Role of the uncompensated interface spins in polycrystalline exchange-biased systems

This work reports on the evolution of the exchange-bias (EB) field and the shape of the magnetization curves with the thickness of the non-magnetic spacer layer (SL, either Cu or Al2O3) of polycrystalline Co/IrMn/SL/Co films as well as their modifications caused by different post-deposition annealing treatments. Conventional x-ray diffractometry, small-angle x-ray reflectivity and cross-section transmission electron microscopy were used for the structural characterization. The hysteresis loops traced on the as-made films without SL present a pattern of two oppositely displaced subloops; it was observed that the positively shifted subloop gradually vanishes with the increase in the SL thickness. Our study points out that the significant decrease in the EB field of the top-pinned Co layers after magnetic annealing could be attributed to relaxation of the bottom interfacial IrMn spin structure caused by the heating. This relaxation mechanism might be considered as an alternative to the commonly accepted IrMn and Co interdiffusion and defect creation at the interface. Models for the uncompensated spins' configurations at each of the ferromagnet (FM)/antiferromagnet (AF) and AF/FM interfaces are proposed in order to explain the modifications of the magnetic behaviour with the SL thickness and with the annealing.