Role of the tuning parameter at magnetic quantum phase transitions

Heavy-fermion systems, with their competition betweenKondo and RKKY interactions, offer a rich variety of materials thatmay be driven to a magnetic quantum phase transition. Quite often,a quantum critical point can be approached by chemical substitution,notably of isoelectric ligands of Ce, as in CeCu6−xAux andCePd1−xNixAl. While in the former we compare pressure and concentrationtuning of the magnetic structure, the latter has the additionalfeature of geometric frustration due to the distorted kagom´e sublatticeof Ce atoms in the basal plane. We further present the systemCeAu2Ge2 where minor structural differences between crystals grownfrom Sn or Au-Ge flux lead to pronounced differences in the magneticstructure, with several field-induced phases in samples grown fromAu-Ge flux. Finally, non-isoelectronic substitution of Ti by V is studiedin CeTi1−xVxGe3 where CeTiGe3 is a ferromagnet, thus allowingthe study of ferromagnetic quantum criticality, a rare case for heavyfermionsystems