We report the synthesis of monoclinic ${\mathrm{Fe}}_{6.10\pm \delta }{\mathrm{Ge}}_5$ single crystals, as well as their magnetic, transport and thermodynamic properties, sharing similar key features within the range of $\delta \approx$ 0.25. The susceptibility and magnetization results reveal two distinct antiferromagnetic transitions at $T_{\mathrm{N}1}$ = 340 K and $T_{\mathrm{N}2}$ = 121 K, respectively, with the easy axis to be along the crystalline $c$ direction, as implied both by the anisotropy in the susceptibility and by a spin-flop transition. Insulating behavior with a narrow band gap of $\sim 1\mathrm{meV}$ is observed in resistivity measurement in the intermediate temperature range $T_{\mathrm{N}2}<T<T_{\mathrm{N}1}$, whereas metalliclike behavior manifests at lower temperatures $T<T_{\mathrm{N}2}$. These observations indicate that ${\mathrm{Fe}}_{6.10\pm \delta }{\mathrm{Ge}}_5$ exhibits rich magnetic phases, with charge carrier scattering highly sensitive to spin configurations in different magnetic phases, calling for further high-resolution experiments on its local atomic environment and magnetic structure.

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• Magnetism
• Thermal properties
• Transport phenomena
• Antiferromagnets