Observation of elastic anomalies driven by coexisting dynamical spin   Jahn-Teller effect and dynamical molecular spin state in paramagnetic phase   of the frustrated MgCr$_2$O$_4$$
  Ultrasound velocity measurements of magnesium chromite spinel MgCr$_2$O$_4$ reveal elastic anomalies in the paramagnetic phase that are characterized as due to geometrical frustration. The temperature dependence of the tetragonal shear modulus $(C_{11}-C_{12})/2$ exhibits huge Curie-type softening, which should be the precursor to spin Jahn-Teller distortion in the antiferromagnetic phase. The trigonal shear modulus $C_{44}$ exhibits nonmonotonic temperature dependence with a characteristic minimum at $\sim$50 K, indicating a coupling of the lattice to dynamical molecular spin state. These results strongly suggest the coexistence of dynamical spin Jahn-Teller effect and dynamical molecular spin state in the paramagnetic phase, which is compatible with the coexistence of magnetostructural order and dynamical molecular spin state in the antiferromagnetic phase. 
Tadataka Watanabe, Shin-Ichi Ishikawa, Haruki Suzuki, Yusuke Kousaka,   and Keisuke Tomiyasu
cond-mat.str-el cond-mat.mtrl-sci