Composition and temperature dependence of the Yb valence in YbMn6Ge6-xSnx studied by RIXS

We investigate the composition and temperature (10-450 K) dependence of the Yb valence in YbMn6Ge6-xSnx (x = 0.0, 3.8, 4.2, 4.4, and 5.5) using resonant inelastic x-ray scattering (RIXS). The observed change in the Yb valence with composition (from v similar to 3 for x = 0 to v similar to 2.7 for x = 5.5) is likely driven by negative chemical pressure effects due to the Sn for Ge substitution. While Yb is nonmagnetic in YbMn6Ge0.5Sn5.5, intermediate valent Yb magnetically orders at unexpectedly high temperature (up to 90 K) in the alloys with x = 3.8, 4.2, and 4.4. The three latter alloys further exhibit an increase in the Yb valence upon cooling, which is opposite to the usual behavior. Interactions with the magnetically ordered Mn sublattice are invoked to account for these unprecedented phenomena through a simplified model based on an Anderson Hamiltonian with a Zeeman term mimicking the Mn-Yb exchange interactions. We further show that the Yb magnetic behavior in this series can be interpreted based on Doniach's picture. Compared with standard intermediate valent materials where Yb is embedded in a nonmagnetic matrix, the strong Mn-Yb exchange interaction enhances the intermediate valent Yb magnetic ordering temperature and allows for extending the stability domain of the Yb magnetic order towards lower Yb valence.