Magnon excitation and spin-phonon coupling in 𝐴2⁢Ir2⁢O7 (𝐴=Gd, Dy, and Er)

Abstract

We present a comparative study of the temperature-dependent Raman scattering of pyrochlore iridates 𝐴2⁢Ir2⁢O7, where 𝐴=Gd, Dy, and Er, spanning the temperature range from 295 to 5 K. Our investigation covers the metal-insulator phase transition, coinciding with the paramagnetic-to-all-in/all-out (AIAO) spin ordering at the magnetic transition temperature 𝑇𝑁. The crystal symmetry remains unchanged across this magnetic transition. Below 𝑇𝑁, two additional modes (𝑁1 and 𝑁2) emerge, associated with single magnon excitations within the AIAO-ordered state. Analysis of magnon energies enables us to estimate the isotropic exchange (𝐽) and Dzyaloshinskii-Moriya interaction parameter (𝐷). The resulting 𝐷/𝐽 ratios are ∼0.15 for Gd2⁢Ir2⁢O7 and Dy2⁢Ir2⁢O7. In all the three systems, the Ir-O-Ir bond bending vibrations, 𝐴1⁢𝑔 and 𝐸𝑔, exhibit anomalous frequency softening in the magnetically ordered AIAO state, primarily originating from the spin-phonon interaction induced by the phonon modulation of the Dzyaloshinskii-Moriya spin-exchange interaction. The two stretching modes 𝑇2 2⁢𝑔 and 𝑇4 2⁢𝑔 experience significant hardening in the magnetic insulating phase. Additionally, substantial anomalous linewidth broadening of the 𝐸𝑔 mode is observed in all samples, indicating strong phonon renormalization below 𝑇𝑁. Our experimental findings establish experimental evidence of magnon excitations and strong spin-phonon coupling in the AIAO-ordered state for these iridates.