2, National Institute of Chemistry, Ljubljana, , Slovenia
Barium hexaferrite (BaFe12O19 - HF) nanoplatelets display a high uniaxial magnetocrystalline anisotropy with an easy axis perpendicular to the platelet. Due to this unique property, they show tremendous potential in innovative applications, especially in relation to their ability to be effectively aligned with an applied magnetic field. However, for some applications their saturation magnetization MS should be increased. As a hard-magnetic material, HF displays a rather modest MS, 72 Am2/kg in the bulk, while in the form of nanoplatelets the MS is significantly smaller, up to 35 Am2/kg. The MS of the nanoplatelets can be effectively increased by coating them with a shell of soft-magnetic iron oxide maghemite (γ-Fe2O3). If the two phases are magnetically exchange-coupled the formed composite nanoplatelet homogeneously magnetizes like a single-phase particle.
In this work the composite nanoplatelets were synthesized using the deposition of epitaxial maghemite (M) layers onto the HF nanoplatelet cores dispersed in an aqueous suspension. The layers were deposited by a heterogeneous nucleation and growth on the surfaces of the core nanoplatelets during the precipitation of Fe2+/Fe3+ ions. The composite nanoplatelets were characterized using an aberration-corrected scanning-transmission electron microscope (HAADF STEM) in combination with other methods (TEM, EDXS, XRD, Raman). The magnetic properties were thoroughly analyzed.
Atomic-resolution imaging revealed that the synthesized composite nanoplatelets display an incredibly uniform structure of the “sandwich”-type, with a HF core in between two M layers. As the core nanoplatelets adopt a distinct structure and composition, which are significantly different to the bulk, they can be considered as novel structural variations of hexaferrite stabilized on the nanoscale. The HF structure is characterized by two alternating structural blocks, i.e., a hexagonal “R” block ((BaFe6O11)2-) and a cubic “S” block ((Fe6O8)2+), stacked along the c-direction of their hexagonal structure (RSR*S*, * denotes the rotation of 180 °around the c-axis). The nanoplatelets contain only two, or rarely three, R blocks and always terminate with the full S block at the basal surfaces. The vast majority of the nanoplatelets showed the SR*S*RS stacking, corresponding to the composition of Ba2Fe30O46. Given the cubic S block termination of the platelets, layers of maghemite with a cubic spinel structure, can be easily grown epitaxially on the basal surfaces of the platelets. The epitaxial M layers deposited on the cores are of very uniform thickness.
Magnetic measurements proved the exchange coupling between the two magnetic materials. The exchange-coupled composite nanoplatelets exhibit a remarkably uniform structure, with an enhanced MS of more than 50 emu/g 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.