1206.2755 (Tadataka Watanabe et al.)

Observation of elastic anomalies driven by coexisting dynamical spin
Jahn-Teller effect and molecular spin fluctuations in paramagnetic phase of
the frustrated MgCr$_2$O$_4$    [PDF]

Tadataka Watanabe, Shin-Ichi Ishikawa, Haruki Suzuki, Yusuke Kousaka, Keisuke Tomiyasu

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 molecular spin fluctuations. These results strongly suggest the coexistence of a dynamical spin Jahn-Teller effect and molecular spin fluctuations in the paramagnetic phase, which is compatible with the coexistence of magnetostructural order and molecular spin excitations in the antiferromagnetic phase.

View original: http://arxiv.org/abs/1206.2755