Unusual Yb magnetic properties in YbMn6Ge1.8−xSn4.2Gax and YbMn6Ge1.8Sn4.2−yGay

• Some Ga derivatives of YbMn6Ge1.8Sn4.2 have been synthesized.
• The Yb sublattice magnetically orders at high temperature (up to ∼170 K).
• The Yb atoms are likely in an intermediate valent state.
• The magnetic properties are strongly changed upon Ga substitution.

We investigate the magnetic properties of the new HfFe6Ge6-type (P6/mmm) YbMn6Ge1.8−xSn4.2Gax (x=0.05, 0.4 and 0.9) and YbMn6Ge1.8Sn4.2−yGay (y=0.05, 0.5 and 1.0) alloys from DC magnetization and powder neutron diffraction experiments. The electronic structure modifications induced by the Ga substitution are found to promote the ferromagnetic order of the Mn sublattice, to enhance the Yb ordering temperature (up to TYb∼170 K) and to strongly increase the low-temperature coercive field (up to µ0Hc∼1 T). By contrast, the changes in the Yb magnetic moment magnitude upon Ga substitution are analyzed to be mainly driven by chemical pressure effects. The reduced Yb magnetic moment (1.0 µB