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

Abstract 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 (A 2 BB′O 6 ). In this study, we explore these interactions between the fundamental excitations such as phonons and magnons in square-lattice Sr 2 CuTeO 6 , Sr 2 CuWO 6 , and Ba 2 CuWO 6 isostructural double perovskites that exhibit both short-ranged ( T max ) as well as long-ranged Néel antiferromagnetic ( T N ) transitions. Our Raman measurements at variable temperatures reveal an intriguing broad peak (identified as 2-magnon (2M)) surviving beyond T max for 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 −1 of Sr 2 CuTeO 6 and ∼168 cm −1 of Sr 2 CuWO 6 , 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.