Valence and magnetic transitions in YbMn 2 Si 2-x Ge x

Abstract

A series of YbMn2Si2-xGex compounds (x=0, 0.4, 1.0, 1.4, 2.0) with the tetragonal ThCr2Si2-type structure was investigated using neutron powder diffraction. The behaviour of YbMn2Si2-xGex with x<∼1.4 is consistent with magnetic structures similar to that of YbMn2Si2 – predominant collinear antiferromagnetism of the Mn sublattice at high temperatures, with antiferromagnetic ordering of the Yb ions at ∼1.5 K. The results also reveal a change in the Yb valence state from YbMn2Si2 to YbMn2Ge2 with the transition taking place above a critical concentration, xc∼1.4. YbMn2Si2 is trivalent, whereas analysis of the unit cell volumes leads to a Yb valence of about 2.35 in YbMn2Ge2.