Antiferromagnetism in CeCu2(Si1-xGex)2: nature of the A phase

We report on extensive neutron diffraction on single crystalline CeCu2(Si1-xGex)2 to determine the antiferromagnetic order in the regime with low Ge content, initially called A phase due to the unknown origin of the magnetic order. Starting from the x=0.45 alloy we followed the magnetic order to lower Ge concentrations and to pure CeCu2Si2. For all Ge concentrations x=<1 an incommensurate magnetic order with a propagation vector close to τ=(0.250.250.5) is observed. The magnetic order is determined by the nesting properties of the Fermi surface