The effect of single crystalline substrates and ion-beam bombardment on exchange bias in nanocrystalline NiO/Ni₈₀Fe₂₀ bilayers

Methods to modify the magnetic coercivity and exchange bias field of nanocrystalline antiferromagnetic/ferromagnetic NiO/Ni₈₀Fe ₂₀ thin films were investigated for bilayers grown using ion-assisted deposition onto different single crystalline substrates. An enhanced coercivity was found at 298 K for the films deposited on single crystalline MgO (100) and Al₂O₃ (11-20) substrates. After field cooling the films to 50 K, the NiO/NiFe bilayer grown on Al₂O₃ (11-20) exhibited the largest exchange bias (-25 Oe). The second part of the study investigated ion-beam modification of the ferromagnetic surface prior to the deposition of the NiO layer. A range of ion-beam bombardment energies (V _EH) were used to modify in situ the NiFe surface during the deposition of NiO/NiFe/SiO₂ films. Cross-sectional transmission electron microscopy showed a systematic reduction in the thickness of the NiFe layers with increasing Ar⁺ bombardment energies attributed to etching of the surface. In addition, the bombardment procedure modified the magnetic exchange bias of the composite structure in both the as-prepared and field-cooled state.