Antiferromagnetism in : nature of the A phase

We report on extensive neutron diffraction on single crystalline CeCu 2 ( Si 1 - x Ge x ) 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 CeCu 2 Si 2 . For all Ge concentrations x ⩽ 1 an incommensurate magnetic order with a propagation vector close to τ = ( 0.25 0.25 0.5 ) is observed. The magnetic order is determined by the nesting properties of the Fermi surface.