Magnesium Oxide

We demonstrate high-throughput evaluation of the half-metallicity of Co₂MnSi Heusler alloys by spin-integrated hard X-ray photoelectron spectroscopy (HAXPES) of composition-spread films performed with high-brilliance synchrotron radiation at NanoTerasu, which identifies the optimum composition showing the best half-metallicity. Co_75-x Mn _x Si₂₅ composition-spread thin films for x = 10-40% with a thickness of 30 nm are fabricated on MgO(100) substrates using combinatorial sputtering technique. The L2₁-ordering and (001)-oriented epitaxial growth of Co₂MnSi are confirmed by X-ray diffraction for x = 18-40%. The valence band HAXPES spectra exhibit a systematic compositional dependence and the smallest photoemission intensity at the Fermi level (E _F) for a slightly Mn-rich composition of x = 27%. The density of states (DOS) for L2₁-ordered Co₂MnSi with different Mn compositions obtained from first-principles calculation also show the smallest total DOS at E _F for x = 27% because of the formation of a clear half-metallic gap in the minority spin channel and the less localized d-states in the majority spin channel, indicating the best half-metallic nature of this composition. Our experimental results demonstrate that high-throughput evaluation of half-metallicity is possible even with spin-integrated HAXPES by capturing systematic changes in the electronic structures through the measurements on the composition-spread film. Moreover, the anisotropic magnetoresistance (AMR) of the composition-spread film is measured for electric current directions along the [110] and [100] of Co₂MnSi. Previous studies indicated that a larger negative AMR ratio is a signature of a higher spin polarization. The largest negative AMR ratio is observed for x = 27% for both current directions, which also supports the best half-metallicity for this off-stoichiometric composition.

Ensuring the safety of researchers by protecting them from exposure to toxic gases in laboratories is of paramount importance. This study investigated the effectiveness of using high-surface-area MgO to remove HCl under atmospheric conditions. Two types of MgO were synthesized through the thermal decomposition 1-1-1, Tennodai, Tsukuba, of Mg(OH)₂ and MgC₂O₄·2 H₂O. HCl diluted with air passed through both MgO samples, and the amounts of HCl removed and morphological changes in the samples were compared. No significant differences in surface area or crystallinity were observed with the decomposition temperatures. X-ray diffraction analysis showed that the sample prepared from MgC₂O₄·2 H₂O reacted with HCl immediately upon introducing HCl gas. In contrast, the sample obtained from Mg(OH)₂ exhibited only MgO peaks, even 30 min after the introduction of HCl gas. Microscopic analysis revealed that the samples derived from Mg(OH)₂ showed no significant changes in shape after the reaction, whereas the MgO prepared from MgC₂O₄·2 H₂O exhibited substantial changes in overall shape. No correlation was observed between the surface area and the amount of HCl removed. When MgO is prepared from MgC₂O₄·2 H₂O, the reaction occurs in the bulk material, whereas when MgO is prepared from Mg(OH)₂, the reaction hardly progresses after HCl adsorbs onto the MgO surface. The order of magnitude of HCl removal was consistent with the base catalytic activity of the decomposition of diacetone alcohol to acetone. These results suggest that, compared with MgO obtained from Mg(OH)₂, MgO derived from MgC₂O₄·2 H₂O generates more active sites, resulting in the reaction with HCl from surface to progress into bulk.