Chemical Pressure Stabilization of the Cubic B20 Structure in Skyrmion Hosting Fe1-xCoxGe 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 (similar to 280 K). FeGe and related metal monogermanide compounds, such as CoGe and MnGe, have several structural polymorphs and typically require high pressure (similar to 4 GPa) and high temperature (similar to 1000 degrees C) to synthesize in the cubic B20 structure. Here, we report that the cubic B20 phase of both FeGe and alloys of Fe1-xCoxGe could in fact be formed without the application of high pressure by simply reacting elemental powders at modest temperatures (550 degrees C). Furthermore, the incorporation of Co into Fe1-xCoxGe (0.05 <= x <= 0.1) stabilizes the cubic B20 structure up to 650 degrees C, which we propose is caused by chemical pressure induced by the incorporation of Co into the lattice. Interestingly, chemical vapor reactions using Fe1-xCoxGe alloys as precursors yield plentiful growth of large (0.1 to 1 mm) single crystals of pure FeGe. Magnetic susceptibility measurements of the Fe0.95Co0.05 Ge alloy show evidence of a skyrmion phase not previously reported in the Fe1-xCoxGe system.