Yb-site deficiency and sample-dependent thermal and magnetic hysteresis in kagome antiferromagnet YbMn6Ge6

Abstract

YbMn6Ge6 is an antiferromagnet in which Yb3+ moments couple to the kagome Mn sublattice, stabilizing a conical incommensurate magnetic state at low temperature. However, previously reported magnetization and resistivity behaviors remain partially inconsistent. Here, we demonstrate pronounced sample-dependent thermal and magnetic hysteresis in YbMn6Ge6 single crystals grown by the Sn-flux method and show that these variations are consistent with Yb-site deficiency and possible spatial inhomogeneity. Both structural and chemical composition analyses indicate Yb vacancies in all crystals. The transition onset temperature and its width vary between batches with different inferred Yb occupancies: in resistivity measurements, crystals with higher Yb occupancy exhibit transitions at 68 K (cooling) and 80 K (heating), whereas samples with lower inferred Yb occupancy display a broadened transition window of 110-128 K. While these transitions are also reflected in magnetization measurements, measurements on crystals and smaller fragments suggest that the apparent onset is governed not simply by macroscopic sample size but also by a distribution of local transition temperatures. Weak and sample-dependent signatures, including the presence of thermal hysteresis, are also found in specific heat measurements, indicating that spatial averaging over inhomogeneous regions can smear thermodynamic signatures. At low temperature, we further observe strongly anisotropic field-induced metamagnetic hysteresis and its sensitivity to the sample-dependent defect landscape. Magnetoresistance measurements track the metamagnetic features, indicating a close correlation between magnetic order and charge transport. Overall, our results suggest that Yb-site deficiency and associated spatial inhomogeneity are important factors governing the sample-dependent hysteretic behavior and highlight the importance of the R-site chemistry in the isostructural RMn6Ge6 family.