Valence and magnetic transitions in YbMn2Si2$minus;xGex

Neutron diffraction and differential scanning calorimetry measurements (DSC) on YbMn2Si2-xGex provide clear evidence for a critical region around x(c)similar to 1.6. Below x(c)similar to 1.6 the compounds exhibit an axial antiferromagnetic structure while at higher Ge concentration the structure transforms to planar antiferromagnetism for T > similar to 200 K with canted antiferromagnetism prevailing at lower temperatures. The structural changes are consistent with a valence change between the trivalent behaviour of YbMn2Si2 and the divalent-like behaviour of YbMn2Ge2. (C) 2003 Elsevier B. V. All rights reserved.