Signatures of magnon–phonon coupling in frustrated double perovskite square lattices

Low-dimensional frustrated magnetic square networks feature a variety of unconventional phases with novel emergent excitations. Often these excitations are intertwined and manifest into intriguing phenomena, an area that has remained largely unexplored in square-lattice systems, especially, double perovskites (A₂BB'O₆). In this study, we explore these interactions between the fundamental excitations such as phonons and magnons in square-lattice Sr₂CuTeO₆, Sr₂CuWO₆, and Ba₂CuWO₆isostructural double perovskites that exhibit both short-ranged (<i>T</i>_max) as well as long-ranged Néel antiferromagnetic (<i>T</i>_N) transitions. Our Raman measurements at variable temperatures reveal an intriguing broad peak (identified as 2-magnon (2M)) surviving beyond<i>T</i>_maxfor W-based compositions contrary to the Te-based system, suggesting a key role of diamagnetic B'-site cation on their magnetism. The thermal response of 2M intriguingly shows signatures of correlation with phonons and control over their anharmonicity, depicting magnon-phonon interaction. Further, a few phonons exhibit anomalies across the magnetic transitions implying the presence of spin-phonon coupling. In particular, the phonon modes at ∼194 cm^-1of Sr₂CuTeO₆and ∼168 cm^-1of Sr₂CuWO₆, that show a strong correlation with the 2M, exhibit the strongest spin-phonon coupling suggesting their roles in mediating magnon-phonon interactions in these systems.