Tuning heavy fermion systems into quantum criticality by magnetic field

We discuss a series of thermodynamic, magnetic, and electrical transport experiments on the two heavy fermion compounds CeNi₂Ge₂ and YbRh₂Si₂ in which magnetic fields, B, are used to tune the systems from a non-Fermi liquid (NFL) into a field-induced FL state. Upon approaching the quantum-critical points from the FL side by reducing B we analyze the heavy quasi-particle (QP) mass and QP-QP scattering cross sections. For CeNi₂Ge₂ the observed behavior agrees well with the predictions of the spin-density wave (SDW) scenario for three-dimensional (3D) critical spin-fluctuations. By contrast, the observed singularity in YbRh₂Si₂ cannot be explained by the itinerant SDW theory for neither 3D nor 2D critical spinfluctuations. Furthermore, we investigate the magnetization M(B) at high magnetic fields. For CeNi₂Ge₂ a metamagnetic transition is observed at 43 T, whereas for YbRh₂Si₂ a kink-like anomaly occurs at 10 T in M vs B (applied along the easy basal plane) above which the heavy fermion state is completely suppressed.