Fig. 4: HR-TEM and EELS compositional mappings.

a HAADF-STEM image of as-grown FeCoN nanodot. Red and orange rectangles indicate two areas where EELS compositional mapping was performed. EELS compositional maps, comprising the entire as-grown nanodot (Area 1, b) and closer examination of nanodot’s interior (Area 2, c), show uniform distribution of Fe, Co and N throughout both regions. d HAADF-STEM image of FeCoN nanodot treated with –25 V. EELS compositional maps performed in two areas (e, f) indicated in (d) show that upon negative voltage treatment Fe/Co ions and N^3– ions migrate in opposite directions, thereby creating a distinct planar migration front indicated by black dashed lines in (f). This planar front divides the nanodots into two sub-layers with distinct magnetic properties: ferromagnetic FeCo/(Fe,Co)₄N (i.e., nitrogen-depleted) at the bottom part and paramagnetic FeCoN (i.e., nitrogen-rich) in the upper part of the nanodot. g, h Normalized EELS compositional line profiles of as-grown and treated FeCoN dots, respectively, collected starting from the top of the nanodot. HR-TEM images of nanodot’s interior before (i) and after (j) voltage treatment (at –25 V), including the FFT of the areas marked by orange squares. Reflections marked by yellow and cyan circles in (i) correspond to the interplanar distances of 2.26 Å, 2.60 Å, respectively, and stem from (200) and (111) planes of \({{\rm{F}}}\bar{4}3{{\rm{m}}}\) (Fe,Co)N. After voltage treatment (j), new interplanar distances of 1.10 Å (green) and 2.15 Å (pink) emerge. Interplanar distance of 1.10 Å can be ascribed to (311) planes of face-centered cubic (fcc) \({{\rm{Fm}}}\bar{3}{{\rm{m}}}\) Co, (211) planes of body-centered cubic (bcc) \({{\rm{Im}}}\bar{3}{{\rm{m}}}\) Fe, or to (211) planes of simple cubic \({{\rm{Pm}}}\bar{3}{{\rm{m}}}\) FeCo alloy. Similarly, interplanar distance 2.15 Å can be related to (111) planes of fcc \({{\rm{Fm}}}\bar{3}{{\rm{m}}}\) Co, (110) planes of bcc \({{\rm{Im}}}\bar{3}{{\rm{m}}}\) Fe, (110) planes of \({{\rm{Pm}}}\bar{3}{{\rm{m}}}\) FeCo or (111) planes of \({{\rm{Pm}}}\bar{3}{{\rm{m}}}\) (Fe,Co)₄N.