© 2017 The Royal Society of Chemistry. Atomic-resolution scanning-transmission electron microscopy showed that barium hexaferrite (BHF) nanoplatelets display a distinct structure, which represents a novel structural variation of hexaferrites stabilized on the nanoscale. The structure can be presented in terms of two alternating structural blocks stacked across the nanoplatelet: a hexagonal (BaFe6O11)2- R block and a cubic (Fe6O8)2+ spinel S block. The structure of the BHF nanoplatelets comprises only two, or rarely three, R blocks and always terminates at the basal surfaces with the full S blocks. The structure of a vast majority of the nanoplatelets can be described with a SR∗S∗RS stacking order, corresponding to a BaFe15O23 composition. The nanoplatelets display a large, uniaxial magnetic anisotropy with the easy axis perpendicular to the platelet, which is a crucial property enabling different novel applications based on aligning the nanoplatelets with applied magnetic fields. However, the BHF nanoplatelets exhibit a modest saturation magnetization, MS, of just over 30 emu g-1. Given the cubic S block termination of the platelets, layers of maghemite, γ-Fe2O3, (M), with a cubic spinel structure, can be easily grown epitaxially on the surfaces of the platelets, forming a sandwiched M/BHF/M platelet structure. The exchange-coupled composite nanoplatelets exhibit a remarkably uniform structure, with an enhanced MS of more than 50 emu g-1 while essentially maintaining the out-of-plane easy axis. The enhanced MS could pave the way for their use in diverse platelet-based magnetic applications.