In compounds with normal rare earths at the same crystallographic site
, mixed magnetic phases with two distinct rare-earth magnetic states,
namely, a zero moment one and another with a high moment, can be stabi
lized at low temperature not only in zero applied field but also in di
fferent field-induced magnetic phases of a metamagnetic process. These
characteristics are observed in tetragonal TbRu2Ge2 from neutron diff
raction and magnetization measurements on a single crystal. It is show
n that such magnetic structures may originate from a moment instabilit
y due to the crystal field in the presence of frustrated exchange inte
ractions. Numerical simulations using the periodic-field model allowed
us to satisfactorily account for the observed properties and emphasiz
e the crucial role played by the crystal field.