  We investigated the influence of the Ta buffer layer on the thermal stability
of polycrystalline Ta/ MnN/ CoFeB exchange bias systems, showing high exchange
bias of about 1800 Oe at room temperature. The thermal stability of those
trilayer systems is limited by nitrogen diffusion that occurs during annealing
processes. Most of the nitrogen diffuses into the Ta buffer layer, which is
necessary for good crystal growth of MnN and thus a crucial component of the
exchange bias system. In order to improve the thermal stability, we prepared
exchange bias stacks where we varied the Ta thickness to look for an optimum
value that guarantees stable and high exchange over a broad temperature range.
Our findings show that thin layers of 2-5 nm Ta indeed support stable exchange
bias up to annealing temperatures of more than $550^{\circ}$C. Furthermore, we
found that the introduction of a TaN$_{\text{x}}$ layer between MnN and Ta,
acting as a barrier, can prevent nitrogen diffusion. However, our results show
that those measures, even though being beneficial in terms of thermal
stability, often lead to decreased crystallinity and thus lower the exchange
bias.
