Role of the Ta buffer layer in Ta/MnN/CoFeB stacks for maximizing exchange bias

Abstract

Ta/MnN/CoFeB systems show high exchange bias of about 1800 Oe at room temperature; however, their thermal stability is limited by nitrogen diffusion that occurs during annealing processes [Quarterman et al., Phys. Rev. Mater. 3, 064413 (2019) and Dunz et al., AIP Adv. 8, 056304 (2018)]. In this study, we investigate the consequences of nitrogen diffusion in Ta/MnN/CoFeB exchange bias stacks in dependence on the Ta buffer layer thickness. Furthermore, we test the effects of introducing a TaNx layer between MnN and Ta as a diffusion barrier. Our findings show that the Ta buffer layer plays a decisive role in determining the exchange bias in the Ta/MnN/CoFeB system. It acts as a crystallographic seed layer for better growth of MnN and as a nitrogen sink during the annealing process. We show that both of these functions are crucial for the outcome of high exchange bias. Additionally, our results reveal that the measures decreasing nitrogen diffusion, even though being beneficial in terms of thermal stability, mostly lead to decreased crystallinity and thus weaker exchange bias.

Anisotropy in magnetic materials for sensors and actuators in soft robotic systems

“…Dunz et al investigated the influence of the interaction of a Ta/MnN/CoFeB system with a Ta buffer layer on the exchange bias in the system. 145 Thicker Ta layers led to a higher exchange bias field, because this approach improved the crystallinity of MnN and decreased N 2 diffusion during annealing. Thus, various approaches have been explored for 144 Copyright 2020, American Physical Society.…”

“…Dunz et al investigated the influence of the interaction of a Ta/MnN/CoFeB system with a Ta buffer layer on the exchange bias in the system. 145 Thicker Ta layers led to a higher exchange bias field, because this approach improved the crystallinity of MnN and decreased N 2 diffusion during annealing. Thus, various approaches have been explored for the roughness control of growing materials, including the thickness control of the buffer layer, 135 annealing processes to form oxide layers, 136,137 and use of reactive ion-etched substrate.…”

Recent Developments on MnN for Spintronic Applications

“…Over the last few years, it has become apparent that the Ta buffer layer is key to the performance of MnN-based exchange-bias systems, as it acts as a crystallographic seed layer for the MnN to grow on and a nitrogen sink during the annealing process. The effect of nitrogen diffusion in Ta/MnN/CoFeB stacks has been investigated as a function of the Ta layer thickness [21]. Samples with composition Ta(t Ta )/MnN(30 nm)/Co 40 Fe 40 B 20 (1.6 nm)/Ta (0.5 nm)/Ta 2 O 5 (2 nm) were prepared on thermally oxidized Si wafers.…”

“…This leads to a trade-off between thermal stability and large exchange bias. The effect of introducing a TaN x layer between the Ta seed and the MnN layers was also investigated in the same article [21]. Although the introduction of this extra layer can enhance the thermal stability of the bilayers, it has a negative effect on the magnitude of the loop shift.…”