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.

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Magnetocaloric Gd 5 Si 2 Ge 2 -based alloys exhibit a well-documented giant magnetocaloric effect at around room temperature. However, as candidate materials for real-world refrigeration devices, they suffer from a number of problems, one of which is large hysteresis losses while cycling in and out of the magnetic field. It has, however, been reported that a small amount of iron reduces these hysteresis losses very significantly, so increasing the total potential cooling capacity of the material. We have investigated the magnetic properties as well as the macro-and microstructural changes for a wide range of iron substitutions for silicon, according to the formula Gd 5 Si 2 x Fe x Ge 2 , where x was varied between 0 and 1. The samples were prepared with an arc-melter and homogenized in an inert atmosphere, after which they were analyzed with a nano-resolution field-emission-gun scanning electron microscope (FEG SEM) and an X-ray diffractometer. The magnetic measurements were made with a cryogenic vibrating-sample magnetometer. The substitution of silicon for iron presented us with some unusual macrostructures and a consistent change in the microstructures of the samples. While most of the iron forms part of the grain-boundary phases, it also results in the formation of a new Gd 5 (SiGe) 3 main phase. We also observed that the addition of iron lowered the Curie temperature of the alloy, but only had a minimal effect on reducing the hysteresis losses, suggesting that this previously observed reduction in the hysteresis losses only occurs with very small substitutions of iron.Index Terms-Gd 5 Si 2 Ge 2 -type alloys, magnetic hysteresis, magnetocaloric effect, microstructure.

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