Large exchange-bias in magneto-ionic Mn₄N/Mn₃N₂heterostructures

Nitrogen based magneto-ionics offers an alternative to that in the more traditional oxide-based systems [1-3]. Of particular interest is the manganese nitride ferrimagnet Mn₄N as an emergent rare-earth-free spintronic material due to its uniaxial anisotropy, small saturation magnetization (M_s), and high thermal stability. We have achieved high quality all-nitride Mn₄N/Mn₃N₂ thin film heterostructures via sputtering onto Si substrates. X-ray diffraction confirms the Mn₃N₂ and Mn₄N phase with (010) and (001) out-of-plane orientation, respectively. Magnetometry shows that the 20nm-Mn₄N layer exhibits a M_s of over 120 emu/cm³ at room temperature, the largest so far in Mn₄N films grown by sputter deposition. Hall measurements reveal a very large exchange bias field of up to 6 kOe at 5K after field cooling, resulted from nitrogen migration across the interface between the ferrimagnetic Mn₄N and the antiferromagnetic Mn₃N₂ layer. The exchange bias effect is further correlated with sample microstructures, studied by transmission electron microscopy. These results demonstrate a promising new platform for spintronic applications via nitrogen-based magneto-ionics.

[1]. J. de Rojas et al, Nat. Commun. 11, 5871 (2020).

[2]. D. A. Gilbert et al, Nat. Commun. 7, 11050 (2016).

[3]. P. D. Murray et al, ACS Appl. Mater. Interfaces 13, 38916 (2021).