A simple survey of the magnetic properties of new tetragonal RM2X2 com
pounds is given. Particular attention is paid to the interaction of Cr
ystal Field in the case of nonmagnetic singlet ground states. A number
of physical effects which result from this situation are discussed. I
n particular, the nature of this type of ground state is proved to be
indispensable for the existence of amplitude magnetic structures at 0
K and for the most amazing of mixed magnetic phases, in which Rare Ear
th ions with two magnetic states are involved. Evidence of this behavi
our through several examples in such compounds as PrNi2Si2, TbNi2Si2,
TbRu2Si2 and TbRu2Ge2 from the joint analysis of different magnetic pr
operties will be presented. (C) 1998 Elsevier Science S.A.