Particular behaviour of the EPR Gd3+ probe in the incommensurate phase of ThBr4

A detailed analysis of the spin Hamiltonian parameters of the EPR Gd3+ probe in the incommensurate phase of ThBr4 is presented. These parameters which are modulated depend, at first and second order, in the order parameter amplitude which characterizes the phase transition. We show that only the Hamiltonian depending linearly of the order parameter accounts for the linear and quadratic splitting of the EPR lines near the phase transition. Then by using a superposition model, which accounts for the spin Hamiltonian parameters in the high temperature phase, we show the Gd3+ EPR probe is sensitive to the well known displacement field in the (001) plane but also in the (010) plane. (author)