Nanoscale measurement of giant saturation magnetization in α″-FeN by electron energy-loss magnetic chiral dichroism.

Metastable α″-FeN thin films were reported to have a giant saturation magnetization of above 2200 emu/cm in 1972 and have been considered as candidates for next-generation rare-earth-free permanent magnetic materials. However, their magnetic properties have not been confirmed unequivocally. As a result of the limited spatial resolution of most magnetic characterization techniques, it is challenging to measure the saturation magnetization of the α″-FeN phase, as it is often mixed with the parent α'-FeN phase in thin films. Here, we use electron energy-loss magnetic chiral dichroism (EMCD), aberration-corrected transmission electron microscopy, X-ray diffraction and macroscopic magnetic measurements to study α″-FeN (containing ordered N atoms) and α'-FeN (containing disordered N atoms). The ratio of saturation magnetization in α″-FeN to that in α'-FeN is determined to be 1.31 ± 0.10 from quantitative EMCD measurements and dynamical diffraction calculations, confirming the giant saturation magnetization of α″-FeN. Crystallographic information is also obtained about the two phases, which are mixed on the nanoscale.