Novel Ba-hexaferrite structural variations stabilized on the nanoscale as building blocks for epitaxial bi-magnetic hard/soft sandwiched maghemite/hexaferrite/maghemite nanoplatelets with out-of-plane easy axis and enhanced magnetization

Abstract

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 (BaFeO) R block and a cubic (FeO) 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 BaFeO 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, M, of just over 30 emu g. Given the cubic S block termination of the platelets, layers of maghemite, γ-FeO, (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 M of more than 50 emu g while essentially maintaining the out-of-plane easy axis. The enhanced M could pave the way for their use in diverse platelet-based magnetic applications.

M-SrFe 12 O 19 and ferrihydrite-like ultrathin nanoplatelets as building blocks for permanent magnets: HAADF-STEM study and magnetic properties

“…In case of edgewise platelets it has been possible to identify the SRS stacking along the c axis showing that the metal atom stacking is symmetrical starting from a central layer that contains the Sr atoms. This situation is different from what observed B. Belec et al in a previous study on BaFe 12 O 19 -like ultrathin particles [21]. The platelets in that case were identified mainly as SRSRS stacking i.e.…”

“…symmetrical from a S block. The SRSRSRS stacking was also observed but as minor product [21]. It suggests that the nucleation step of the ultrathin platelets depends on the composition of the starting mixture.…”

Topical Review: Progress and Prospects of Hard Hexaferrites for Permanent Magnet Applications

“…After the deposition of M layers, the M/BFO/M composite nanoplatelets retained the single-phase hysteresis loops without any constriction or kink, indicating rigid exchange coupling of the two magnetically-different phases [174,206,207]. Figure 18 shows the hysteresis loops for the BFO core nanoplatelets and the composite nanoplatelets with different thicknesses of the M layers [70]. A slightly constricted loop was only observed for the composites with the thickest M layer (CNP2.5).…”

“…In addition, the samples of this study are magnetically oriented, which allow accurately establishing a remarkable 22% increase in energy product with respect to the pure SFO specimen: from (BH) max = 21.9 kJ m −3 to (BH) max = 26.1 kJ m −3 . The research group (Department for Materials Synthesis) from Jožef Stefan Institute, Slovenia, used a simple water-based method for synthesis of the hard-soft exchange coupled nanoparticles composed of soft-magnetic spinel ferrite (maghemite-M) layers deposited on hard-magnetic BFO platelet cores [70,71]. Even the composite nanoplatelets were developed for advanced medical applications; they can also be relevant as building blocks for production of permanent magnets.…”

“…Details of the unique structure of composite nanoplatelets were revealed based on direct atomic-resolution high-angle annular dark field (HAADF) imaging with a probe aberration corrected STEM [70]. The M was deposited onto the Sc-substituted BFO core nanoplatelets, with relatively narrow distribution of widths, 42 ± 12 nm.…”