  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 quasiparticle (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.
