Fig. 2: Construction and microstructural characterization of the MOS.

a Schematic of the field-assisted self-assembly technique (FASA) to construct MOS. The initial melt temperature is precisely controlled to pre-generate ordered NdFeB cluster structures, and then a temperature gradient field is applied to create a gradient in undercooling. Secondly, the NdFeB cluster structures with low nucleation barriers can crystallize into Nd₂Fe₁₄B grains and maintain a high growth rate at low undercooling, while high undercooling induces extensive nucleation. Thirdly, the continuous growth of Nd₂Fe₁₄B grains causes the residual melt to be Fe-rich, resulting in the precipitation of α-Fe grains around the Nd₂Fe₁₄B grains. Fourthly, the further element enrichment of the residual melt leads to the formation of TiNb between the α-Fe and Nd₂Fe₁₄B grains. b–d Bright-field TEM images of the grain size gradient, with a sandwich core-shell structure at the surface layer. e Grain size statistics along the thickness (t) of the ribbon materials. Error bars represent the standard deviation from the independent measurements of at least 1000 grains. f XRD patterns of the MOS, which exhibit diffraction peaks of both α-Fe and Nd₂Fe₁₄B crystals. g Energy dispersive spectroscopy mapping in region (b) yielded by a scanning transmission electron microscopy (STEM-EDS). h HRTEM images of MOS. i, j Fast Fourier transform (FFT) images corresponding to the blue and red regions in (h). k The enlarged view of the orange region in (h), showing an amorphous characteristic. l STEM-EDS mapping of sandwich core-shell structure. m STEM-EDS composition line profile in (l).