  We demonstrate from detailed ac-susceptibility and calorimetry studies under
hydrostatic pressure that YbCu2Si2 probably orders ferromagnetically at high
pressure. The (p,H,T) phase diagram, shows that the transition temperature
increases with pressure, but also with an applied magnetic field. We suggest
that many ytterbium systems may show a trend towards ferromagnetism and we
discuss the possible reasons for this. We also examine the implications,
including the potential of YbCu2Si2 and other Yb compounds for further studies
of the rich physical properties that may occur near a ferromagnetic critical
point.
