From heavy fermion antiferromagnetism to localized ferromagnetism: competition of two ground states in CeRu2Ge2 on cooling

Resistivity, specific heat and neutron scattering measurements performed on single crystals of CeRu2Ge2 are reported. The intrinsic behaviour of this compound is to exhibit a double transition from a paramagnetic to an antife rromagnetic and then a ferromagnetic state on lowering the temperature with T-N = 8.3 K and respectively T-C = 7.5 K. For the first time the antiferro magnetic order is characterized by neutron scattering. The propagation vect or of the sinewave modulated structure is k = (0.31, 0, 0) as in the Si dop ed compounds. This phase is found to be easily weakened by disorder, but an antiferromagnetic moment as large as 1.5 mu(B) develops between T-N and T- C for the best crystal.