Influence of local structural distortions on magnetism and spin-phonon coupling of multiferroic spinel chalcogenide

摘要

The direct Cr-Cr and Cr-Se-Cr exchange interactions and the associated spin-phonon coupling are crucial for the much envisaged multiferroicity in CdCr_2Se_4 spinel chalcogenide. The static chemical disorder created by the Sn~(4+) substitution at the Cd~(2+) site in CdCr_2Se_4 affects the local crystal structure around Cr-ions, and hence its magnetism. Although the spinel structure is maintained, the trends in lattice parameters are not a linear function of increasing Sn concentration. The magnetic properties are patterned after the trend in lattice parameters but in the opposite direction. Hence, a detailed investigation into the changing local crystal structure of Cd_(1-x)Sn_xCr_2Se_4 for (0 < x < 0.1) using EXAFS and XRD was carried out. Variation in the Cr-Se and Cr-Cr bonding is traced across the changing Sn-content. Raman spectroscopy probes the thermal excitations coupled with spin waves across the magnetic ordering temperature. The synchrotron x-ray diffraction results further complement the inference drawn from Raman studies. Our results highlight the significance of an intricate balance between spin and lattice degrees of freedom in controlling the magnetoelasticity and, hence, the spin-phonon coupling in CdCr_2Se_4.