Site Preference and Possible Coexistence of Antiferromagnetic Order and Magnetic Frustration in (Co_1–xMg_x)₁₀Ge₃O₁₆(0 ≤ x ≤ 30%)

Geometrically frustrated magnetism has attracted tremendous attention, while chemical doping has been utilized as an important tool to probe frustrated magnetism in various systems. Here, we perform a systematic study by doping nonmagnetic Mg²⁺ into a magnetically complicated system, Co₁₀Ge₃O₁₆, which contains three frustrated sublattices of Co²⁺, e.g., triangular Co1, Kagome Co2, and Co3 sublattices. By growing crystals for (Co_1–xMg_x)₁₀Ge₃O₁₆ (0 < x ≤ 30%), we observed obvious site preference of Mg²⁺ on Co1 and Co3 sites over the Co2 site. Powder X-ray diffraction (XRD) patterns confirm the high purity of the samples and indicate a systematic peak shift, consistent with the loading compositions. Although previously investigated, the magnetic structure and expected magnetic frustration in this system have not been fully uncovered. Our temperature-dependent magnetic susceptibility measurements suggest that the high-temperature magnetostructural phase transition with antiferromagnetic ordering and a low-temperature broad peak are suppressed with Mg²⁺ doping, while two new magnetic features emerge at a high Mg²⁺ level. Moreover, the structural phase transition from the high-temperature R3̅m to the low-temperature C2/m space group is absent at the antiferromagnetic ordering temperature, as confirmed by single-crystal XRD. By analyzing the heat capacity and neutron powder diffraction results of the highest doped sample, (Co_0.7Mg_0.3)₁₀Ge₃O₁₆, we speculate that the Co1 site is responsible for the long-range antiferromagnetic ordering, while the other two sites are short-range correlated in addition to a Mg²⁺-induced spin-glass state. This study provides more insights into the complex magnetism in Co₁₀Ge₃O₁₆ by using the nonmagnetic Mg²⁺ as a probe. However, detailed magnetic structure requires further efforts for growing large single crystals.