Lattice dynamics and magnetic exchange interactions in GeCo2O4, a spinel with S = 1/2 pyrochlore lattice

Prativa Pramanik; Sobhit Singh; Mouli Roy Chowdhury; Sayandeep Ghosh; Vasant Sathe; Karin M. Rabe; David Vanderbilt; Mohindar S. Seehra; Subhash Thota

doi:10.1103/PhysRevB.104.014433

paper

Lattice dynamics and magnetic exchange interactions in GeCo2O4, a spinel with S = 1/2 pyrochlore lattice

arXiv:2012.13310 · doi:10.1103/PhysRevB.104.014433

Abstract

GeCo $_{2}$ O $_{4}$ is a unique system in the family of cobalt spinels ACo $_{2}$ O $_{4}$ (A= Sn, Ti, Ru, Mn, Al, Zn, Fe, etc.) in which magnetic Co ions stabilize on the pyrochlore lattice exhibiting a large degree of orbital frustration. Due to the complexity of the low-temperature antiferromagnetic (AFM) ordering and long-range magnetic exchange interactions, the lattice dynamics and magnetic structure of GeCo $_{2}$ O $_{4}$ spinel has remained puzzling. To address this issue, here we present theoretical and experimental investigations of the highly frustrated magnetic structure, and the infrared (IR) and Raman-active phonon modes in the spinel GeCo $_{2}$ O $_{4}$ , which exhibits an AFM ordering below the Néel temperature $T_{N}$ ~21 K, followed by a cubic ( $F d 3 ˉ m$ ) to tetragonal ( $I 4_{1} / am d$ ) structural phase transition at $T_{S}$ ~16 K. Our density-functional theory (DFT+U) calculations reveal that one needs to consider magnetic-exchange interactions up to the third nearest neighbors to get an accurate description of the low-temperature AFM order in GeCo $_{2}$ O $_{4}$ . At room temperature three distinct IR-active modes ( $T_{1 u}$ ) are observed at frequencies 680, 413, and 325 cm $^{- 1}$ along with four Raman-active modes $A_{1 g}$ , $T_{2 g} (1)$ , $T_{2 g} (2)$ , and $E_{g}$ at frequencies 760, 647, 550, and 308 cm $^{- 1}$ , respectively, which match reasonably well with our DFT+U calculated values. All the IR-active and Raman-active phonon modes exhibit signatures of moderate spin-phonon coupling. The temperature dependence of various parameters, such as the shift, width, and intensity, of the Raman-active modes, is also discussed. Noticeable changes around $T_{N}$ and $T_{S}$ are observed in the Raman line parameters of the $E_{g}$ and $T_{2 g}$ modes, which are associated with the modulation of the Co-O bonds in CoO $_{6}$ octahedra during the excitations of these modes.

9 figures and 11 pages

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