Ultrasonic investigation of the elastic properties of the approximant GdCd6

We have investigated complementarily the magneto-elastic coupling in the approximant GdCd6 by measuring the temperature and magnetic field dependence of the longitudinal elastic constant C11. A pronounced elastic softening toward the phase transition temperature Ts = 156 K was observed in the temperature dependence of the C11, suggesting the presence of a structural phase transition. Furthermore, anomalous elastic behavior was observed in the temperature dependence of the C11 below 20 K, closely related to the already known consecutive magnetic phase transitions. The H-T magnetic phase diagram deduced from elastic anomalies appearing at the different phase transitions are consistent with our previous data of the other independent elastic constants CE = (C11 - C12)/2 and C44 for a cubic crystal. We argue the origin of the structural phase transition and low temperature elastic property including a possible origin of such a rich variety of magnetic phases.