Ionically-Driven Synthesis and Exchange Bias in Mn$_{4}$N/MnN$_{x}$ Heterostructures

Abstract: Ferrimagnets have received renewed attention as a promising platform for spintronic applications. Of particular interest is the Mn4N from the ${\epsilon}$-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 Mn${4}$N thin film by sputtering Mn onto ${\eta}$-phase Mn${3}$N${2}$ seed layers on Si substrates. As the deposited Mn thickness varies, nitrogen ion migration across the Mn${3}$N${2}$/Mn layers leads to a continuous evolution of the layers to Mn${3}$N${2}$/Mn${2}$N/Mn${4}$N, Mn${2}$N/Mn${4}$N, and eventually Mn${4}$N alone. The ferrimagnetic Mn${4}$N 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 Mn${4}$N and antiferromagnetic Mn${3}$N${2}$ and Mn$_{2}$N. These results demonstrate a promising all-nitride magneto-ionic platform with remarkable tunability for device applications.