Novel phase transition and metastable regions in the frustrated magnet CdCr₂O₄

A new magnetic phase transition, which we assign to the separation of two different spiral phases, has been observed by ultrasound studies below the Néel temperature (T_N = 7.8 K) in the frustrated antiferromagnet CdCr₂O₄. This transition renormalizes the velocity and amplitude of the transverse acoustic mode c_T whereas the longitudinal mode c_L is not affected. The specific heat does not show any significant change in the entropy at this transition. Furthermore, in an applied magnetic field, the mode c_T exhibits extended metastable magnetostructural states neighboring the one-half magnetization plateau in CdCr₂O₄. By applying an exchange-striction model we can quantitatively describe the field dependence of the sound velocity below and above the one-half magnetization plateau.