Defect-induced modulation of the exchange anisotropy by swift heavy ion irradiation in Ni81Fe19/Ir7Mn93 bilayers

This study investigates the influence of swift heavy ion irradiation on exchange anisotropy (H_EA) and coercivity (H_C) in a set of top-pinned SiO_x/Cu/Ni₈₁Fe₁₉/Ir₇Mn₉₃/Ta bilayers fabricated by ion beam sputtering at room temperature (RT) in presence of an in-plane in situ static field of 1 kOe. Subsequently, the bilayers were subjected to a magnetic annealing process at 300 °C for an hour in the presence of a 3.5 kOe magnetic field. By systematically increasing the Au ion fluences from pristine to 3.3 × 10¹¹ ion/cm², the positive exchange anisotropy (PEA) and negative exchange anisotropy (NEA) were found to be enhanced by 10 Oe and ∼178 Oe, respectively. However, once the ion fluences surpassed the necessary threshold of 3.3 × 10¹¹ ion/cm² required for defect creation/pinning centers in the antiferromagnetic (AF) layer, a reduction in both $H_{\mathrm{EA}}$ and $H_C$ was observed due to the interfacial mixing. The enhancement in PEA and NEA is attributed to the creation of defects or hyperthermal heating in the AF layer as a consequence of ion irradiation. These experimental results align with the framework of the diluted antiferromagnetic model. The persistent training effect, which is observed even after irradiation, confirms the existence of a highly metastable interface between the NiFe/IrMn layer. Thus, ion irradiation emerges as a powerful tool for precisely tailoring the H_EA and H_C of the bilayers by systematically controlling the ion fluence.