Magnetic annealing of the ion-beam sputtered IrMn/CoFeB bilayers – positive exchange bias and coercivity behaviour

Abstract

The effect of optimum dilution of antiferromagnetic (AF)/ferromagnetic (FM) interface necessary for observance of positive exchange bias in ion-beam sputtered Si/Ir₂₂Mn₇₈ (t _AF = 12, 18, 24 nm)/Co₂₀Fe₆₀B₂₀(t _FM = 6,9,15 nm) exchange coupled bilayers is investigated by magnetic annealing at 380, 420 and 460 °C for 1 h at 5 × 10^-6 Torr in presence of 500 Oe magnetic field. While the coercivity of the exchange coupled FM layer decreases with the increase in annealing temperature irrespective of the value of t _AF or t _FM, the hysteresis loops however shift by ≈+ 10 Oe whenever the coercivity drops in the 10–15 Oe range. This is consistent with the phase diagram of exchange bias field and coercivity derived from Meiklejohn and Bean model. The X-ray diffraction and X-ray reflectivity measurements confirmed that the texture, grain size and interface roughness of IrMn/CoFeB bilayers are thickness dependent and are correlated to the observed magnetic response of the bilayers. The results establish that optimum dilution of the IrMn/CoFeB interface by thermally diffused Mn-spins is necessary in inducing the effective coupling between the IrMn domains and diluted CoFeB layer. It is further shown that the annealing temperature required for the optimum dilution of the CoFeB interface critically depends on the thickness of the layers.