Investigation of Nitrogen Ionic Motion and The Magneto-ionic Effect

Magneto-ionics have emerged as an effective approach to control magnetic properties through ionic motion, which is controllable by electric fields. This approach has the potential to electrically manipulate exchange bias (EB), which has fundamental applications in many spin-valve type spintronic devices. The sample structure proposed for PNR studies on the POLREF instrument is Ta/Mn4N/Mn3N2, which contains an antiferromagnetic (AF) Mn3N2 layer and ferrimagnetic (FiM) Mn4N layer. Through coupling between Mn4N and Mn3N2, a significant EB (1367 Oe) is observed when the structure is field cooled to 5K, and by driving N ions with an electric field, the EB can be enhanced by 13%(~184 Oe). X-ray diffraction and electron microscopy have identified Mn3N2, Mn2N, and Mn4N phases in the structure, but they, along with X-ray reflectivity, have failed to identify N content changes after electric field application. Unlike X-ray scattering length densities (SLDs), neutron SLDs have a wide variation between these different MnNx phases, which is contributed to by the large positive SLD of N and the negative SLD of Mn. Previous PNR investigations by our group have demonstrated that this allows small N variations in MnN to be resolved, below what is detectable by other techniques. Our goal is to resolve similar changes in the proposed structure, with the additional aim of using polarized neutrons to determine the depth-dependent distribution of the various MnNx phases in the structure.