Chemical Pressure Stabilization of the Cubic B20 Structure in Skyrmion Hosting Fe_1–<i>x</i>Co_<i>x</i>Ge Alloys

Iron monogermanide (FeGe) with the noncentrosymmetric cubic B20 structure is a well-known helimagnet and a magnetic skyrmion host with a relatively high ordering temperature (∼280 K). FeGe and related metal monogermanide compounds, such as CoGe and MnGe, have several structural polymorphs and typically require high pressure (∼4 GPa) and high temperature (∼1000 °C) to synthesize in the cubic B20 structure. Here, we report that the cubic B20 phase of both FeGe and alloys of Fe_1–<i>x</i>Co_<i>x</i>Ge could in fact be formed without the application of high pressure by simply reacting elemental powders at modest temperatures (550 °C). Furthermore, the incorporation of Co into Fe_1–<i>x</i>Co_<i>x</i>Ge (0.05 ≤ <i>x</i> ≤ 0.1) stabilizes the cubic B20 structure up to 650 °C, which we propose is caused by chemical pressure induced by the incorporation of Co into the lattice. Interestingly, chemical vapor transport reactions using the Fe_1–<i>x</i>Co_<i>x</i>Ge alloys as precursors yield plentiful growth of large (0.1 to 1 mm) single crystals of pure FeGe. Magnetic susceptibility measurements of the Fe_0.95Co_0.05Ge alloy show evidence of a skyrmion phase not previously reported in the Fe_1–<i>x</i>Co_<i>x</i>Ge system

Effect of Cu content on interfacial reactions between Sn(Cu) alloys and Ni/Ti thin-film metallization

[[abstract]]The effect of Cu content in Sn(Cu) alloys on the interfacial reaction between Ni thin film and Sn(Cu) alloys has been investigated. We have found that the variation of Cu content has a strong influence on the spalling of the Ni thin film. With small Cu additives in the Sn, spalling was deferred to longer reflowing time. When the Cu content increased to about 1.0 wt.%, a layer of Cu-Sn compound formed on the Ni thin film, and no spalling was observed after 20-min reflowing. The possible mechanism of spalling deferring is proposed. A Cu flux from the solder to the interface compensated the ripening flux of the semispherical compound grains; therefore, spalling was retarded. The driving force of the Cu flux was attributed to the reduction of Cu solubility caused by the presence of Ni at the interface of the Ni thin film. The Cu flux from solder to the interface is calculated to be in the same order with the ripening flux of the Cu6Sn5 compound grains, which confirms the proposed mechanism of spalling deferring. For the Sn(Cu) alloys having Cu content over 1.0 wt.%, the Cu-Sn compound layer grew so fast that the surface of the interfacial compound layer was free of Ni. There was no Cu flux to compensate the ripening flux; therefore, the ripening flux dominated, and spalling occurred after a short reflowing time.[[notice]]補正完