Nonlinear effects in a series oscillator circuit containing a ferroele
ctric capacitor are investigated as a function of the temperature and
the amplitude of the applied field. A relation is established between
the circuit parameters and the resonance shift in samples of a pure tr
iglycine sulfate crystal and in samples containing chromium impurity i
ons. The observed effects are explained on the basis of a feature in t
he domain structure of the samples and the influence of point defects
on it. (C) 1997 American Institute of Physics.