Synthesis of Sandwiched Composite Nanomagnets by Epitaxial Growth of Fe<sub>3</sub>O<sub>4 </sub>Layers on Srfe<sub>12</sub>o<sub>19</sub> Nanoplates in High-Boiling Organic Solvent

Herein, we demonstrate the synthesis of sandwiched composite nanomagnets, which consist of hard magnetic hexaferrite cores and magnetite outer layers. The hexaferrite plate-like nanoparticles with average dimensions of 16.0 nm × 4.9 nm were prepared by glass crystallization method and were covered by spinel-type iron oxide via thermal decomposition of iron acetylacetonate in hexadecane solution. The hexaferrite nanoplates act as seeds for the epitaxial growth of the magnetite, which results in uniform continuous outer layers on both sides. The thickness of the layers can be adjusted by controlling the concentration of metal ions. In this way, layers with average thickness of 3.9 and 5.0 nm were obtained. Due to an atomically smooth interface the magnetic composites demonstrate the exchange coupling effect acting as single phases during remagnetization. The developed approach can be applied to any spinel-type material with matching lattice parameters and opens the way to expanding the performance of hexaferrite nanomagnets due to a combination of various functional properties.