Spin correlations and spin-phonon coupling in frustrated Cr spinels
Dr. Joachim Deisenhofer (Universität Augsburg)

The antiferromagnetic oxide-spinel systems ACr2O4 (A = Cd, Mg, Zn) are prototypical examples for highly frustrated magnets, where the magnetic Cr3+ ions with spin S = 3/2 reside on the vertices of corner-sharing tetrahedral forming a pyrochlore lattice. An antiferromagnetic nearest-neighbour Heisenberg exchange on the pyrochlore lattice leads to inherent frustration and a multiply degenerate magnetic ground state. In real systems this degeneracy will be released at finite temperatures by coupling to other degrees of freedom and, thus, the ACr2O4 (A = Cd, Mg, Zn) undergo a magnetostructural transition with antiferromagnetic order at 7.8, 12.7, and 12.5 K, although their Curie-Weiss temperatures T_CW are −71, −346, and −390 K, respectively.
The coupling between spin and lattice degrees of freedom influences strongly the phononic spectrum which has been probed by infrared and Raman measurements. To estimate the spin-phonon coupling in the correlated paramagnetic phase for T_N<T<T<sub>CW, the optical data is compared to the spin correlations derived from the magnetic contribution to the specific heat and the magnetic susceptibility. Moreover, the peculiar spin configuration in these compounds consisting of hexagonal spin molecules both below and above T_N seems to leave its traces in electron spin resonance spectra and optical absorption features.