Possible room-temperature signatures of unconventional 4f-electron quantum criticality in YbMn6Ge6−xSnx

We investigate the Sn composition dependence of the Yb valence and local magnetization in YbMn₆Ge_6−xSn_x (4.25 􏰁≤x≤ 5.80) using x-ray absorption spectroscopy (XANES) and x-ray magnetic circular dichroism at the Yb L₃ edge. In these materials, where Mn is ferromagnetically ordered, we observe a decrease of the Yb valence upon reducing the chemical pressure by Sn doping and a suppression of the Yb magnetic moment for strongly hybridized  4f states (ν ∼ 2.77). In the vicinity of the Yb magnetic instability (x_c ∼ 5.23), a remarkably sharp peak is observed in the composition dependence of the valence and magnetic moment at low temperature, which is reminiscent of enhanced valence and magnetic susceptibilities predicted by recent theories on unconventional quantum criticality. This anomaly is also present in the room-temperature XANES signal, suggesting that rare- earth intermetallics with a magnetized 3d sublattice form a distinct group of heavy-fermion metals where the high energy of the exchange interactions can bring quantum criticality up to high temperature.