Chemical Pressure Stabilization of the Cubic B20 Structure in Skyrmion Hosting Fe_1–xCo_xGe 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– x Co x 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– x Co x Ge (0.05 ≤ x ≤ 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– x Co x 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 0.95 Co 0.05 Ge alloy show evidence of a skyrmion phase not previously reported in the Fe 1– x Co x Ge system.