Ionically driven synthesis and exchange bias in Mn₄N/MnN_x heterostructures

Ferrimagnets have received renewed attention as a promising platform for spintronic applications. Of particular interest is the Mn₄N from the ε-phase of the manganese nitride as an emergent rare-earth-free spintronic material due to its perpendicular magnetic anisotropy, small saturation magnetization, high thermal stability, and large domain wall velocity. We have achieved high-quality (001)-ordered Mn₄N thin film by sputtering Mn onto η-phase Mn₃N₂ seed layers on Si substrates. As the deposited Mn thickness varies, nitrogen ion migration across the Mn₃N₂/Mn layers leads to a continuous evolution of the layers to Mn₃N₂/Mn₂N/Mn₄N, Mn₂N/Mn₄N, and eventually Mn₄N alone. The ferrimagnetic Mn₄N, indeed, exhibits perpendicular magnetic anisotropy and forms via a nucleation-and-growth mechanism. The nitrogen ion migration is also manifested in a significant exchange bias, up to 0.3 T at 5 K, due to the interactions between ferrimagnetic Mn₄N and antiferromagnetic Mn₃N₂ and Mn₂N. These results demonstrate a promising all-nitride magneto-ionic platform with remarkable tunability for device applications.