Incorporation of Sc into the structure of barium-hexaferrite nanoplatelets and its extraordinary finite-size effect on the magnetic properties

Abstract In this investigation we analyze an unprecedented difference in the behavior of nanoparticles when compared to the corresponding bulk. We have found that a chemical substitution can have the opposite effect on the magnetic properties of nanoparticles compared to the bulk, as revealed for the first time in the case of Sc-substituted barium-hexaferrite nanoplatelets. Even though the Sc substitution is known to greatly decrease the saturation magnetization, MS, of the bulk barium hexaferrite, it showed the opposite effect for nanoplatelets. The MS values of the nanoplatelets (in average 50 nm wide and approximately 3 nm thick) increased to over 38 Am2/kg, compared to ∼16 Am2/kg for unsubstituted nanoplatelets of comparable average size. The Sc incorporation was investigated with a combination of atomic-resolution imaging and elemental mappings in a scanning-transmission electron microscope. As in the bulk, the Sc3+ ions showed a clear preference for incorporation into an R block of the hexaferrite SRS∗R∗ structure for the nanoplatelets (R and S represent a hexagonal (BaFe6O11)2- and a cubic (Fe6O8)2+ structural block, respectively). A clear difference between the nano and the bulk observed for the first time was in the partial substitution of the Sc3+ for the Ba2+ in the nanoplatelets; however, this cannot explain the large increase in MS. Ab-initio calculations suggest that the opposite effect of the Sc substitution in the nanoplatelets to that in the bulk can be ascribed to specific, two-dimensional magnetic ordering in the platelets.