Fresnel Magnetic Imaging of Ultrasmall Skyrmion Lattices

Magnetic skyrmions with ultrasmall nanometric dimensions hold significant promise for next-generation high-density spintronic devices. Direct real-space imaging of these topological spin textures is critical for elucidating their emergent properties at the nanoscale. Here, Lorentz transmission electron microscopy studies of nanometric skyrmion lattices in B20-structured Mn_0.5Fe_0.5Ge crystals are presented using the Fresnel mode. According to conventional chiral discrimination methods relying on static bright-dark contrast, an abnormal periodic chiral-reversal phenomenon is demonstrated, retrieved through the transport of intensity equation analysis of defocus-dependent Fresnel images. Through systematic off-axis electron holography experiments and numerical simulations, these chiral misinterpretations are attributed to the sinusoidal modulation mechanism of the contrast transfer function that correlates with both defocus values and skyrmion dimensions. The findings establish quantitative limitations of conventional Fresnel contrast analysis for ultrasmall skyrmions while revealing fundamental insights into defocus-mediated phase-to-intensity conversion processes in nanoscale magnetic imaging.