Role of the tuning parameter at magnetic quantum phase transitions

Heavy-fermion systems, with their competition between Kondo and RKKY interactions, offer a rich variety of materials that may 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 CeCu 6− x Au x and CePd 1− x Ni x Al. While in the former we compare pressure and concentration tuning of the magnetic structure, the latter has the additional feature of geometric frustration due to the distorted kagomé sublattice of Ce atoms in the basal plane. We further present the system CeAu 2 Ge 2 where minor structural differences between crystals grown from Sn or Au-Ge flux lead to pronounced differences in the magnetic structure, with several field-induced phases in samples grown from Au-Ge flux. Finally, non-isoelectronic substitution of Ti by V is studied in CeTi 1− x V x Ge 3 where CeTiGe 3 is a ferromagnet, thus allowing the study of ferromagnetic quantum criticality, a rare case for heavy-fermion systems.