Intertwined spin and phonon correlations across the magnetic compensation temperature in magnetoelectric Li0.5Fe2.5−xCrxO4

Recent reports in the literature suggest that emerging magnetic phenomena, like magnetic compensation, magnetic reorientation, etc., in correlated complex oxide systems arise due to the coupling of electronic spin with phonons in the lattice. Here, we investigate the interlinkage of electronic spin with phonon dynamics across tetrahedral and octahedral magnetic sublattices in ${\mathrm{Li}}_0.5{\mathrm{Fe}}_{2.5\ensuremath{-}x}{\mathrm{Cr}}_x{\mathrm{O}}₄$ ($x=1.1$, 1.3, 1.5, and 1.6) that shows magnetic compensation. Temperature-dependent micro-Raman spectroscopic measurements exhibit the anomalies in phonon wave number vs temperature plots below the magnetic-ordering temperature. The estimated spin-phonon coupling strengths in all compounds over the entire temperature range of interest reveal a dual nature of magnetoelastic coupling at both sublattice sites in this system. The coupling strength for the tetrahedral sublattice site is found to be an order of magnitude higher in the regime between near-compensation and phase-transition temperatures than over the low-temperature regime, while the same nearly vanishes above the compensation temperature for the octahedral site. The study suggests the possible role of phonon in establishing the magnetic compensation phenomenon involving two magnetic sublattices. Additionally, the Fano line shape of the ${A}_{1\mathrm{g}}$ mode in the tetrahedral site, especially the variation of the Fano asymmetry parameter with temperature, carries the signature of a cross talk between spin and lattice degrees of freedom in this system. The anomaly in the variation of Raman spectral width of the same mode with temperature suggests a relatively strong electron-phonon coupling in these compounds. The study demonstrates a complex interplay between electronic spin dynamics, electrical and lattice degrees of freedom in two magnetic sublattices of ${\mathrm{Li}}_0.5{\mathrm{Fe}}_{2.5\ensuremath{-}x}{\mathrm{Cr}}_x{\mathrm{O}}₄$.