Nanoscale measurement of giant saturation magnetization in α"-Fe16N2 by electron energy-loss magnetic chiral dichroism

Metastable alpha ''-Fe16N2 thin films were reported to have a giant saturation magnetization of above 2200 emu/cm(3) 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 alpha ''-Fe16N2 phase, as it is often mixed with the parent alpha'-Fe8N 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 alpha ''-Fe16N2 (containing ordered N atoms) and alpha'-Fe8N (containing disordered N atoms). The ratio of saturation magnetization in alpha ''-Fe16N2 to that in alpha'-Fe8N is determined to be 1.31 +/- 0.10 from quantitative EMCD measurements and dynamical diffraction calculations, confirming the giant saturation magnetization of alpha ''-Fe16N2. Crystallographic information is also obtained about the two phases, which are mixed on the nanoscale.