Enhanced exchange bias in MnN/CoFe bilayers after high-temperature annealing

We report an exchange bias of more than $2700\,$Oe at room temperature in MnN/CoFe bilayers after high-temperature annealing. We studied the dependence of exchange bias on the annealing temperature for different MnN thicknesses in detail and found that samples with $t_{\text{MnN}}>32\,$nm show an increase of exchange bias for annealing temperatures higher than $T_{\text{A}}=400\,^{\circ}$C. Maximum exchange bias values exceeding $2000\,$Oe with reasonably small coercive fields around $600\,$Oe are achieved for $t_{\text{MnN}}= 42, 48\,$nm. The median blocking temperature of those systems is determined to be $180\,^{\circ}$C after initial annealing at $T_{\text{A}}=525\,^{\circ}$C. X-ray diffraction measurements and Auger depth profiling show that the large increase of exchange bias after high-temperature annealing is accompanied by strong nitrogen diffusion into the Ta buffer layer of the stacks.