Strong impact of all-in all-out magnetic order on magnetic and lattice dynamics in pyrochlore iridates studied by Raman spectroscopy*

We perform Raman spectroscopy on pyrochlore Eu2Ir2O7 which undergoes a paramagnetic-to-antiferromagnetic transition, as well as paramagnetic counterpart Pr2Ir2O7.
The first polarization analysis combined with density functional theory allows us to assign all observed peaks to six optical phonons without any ambiguity.
An additional peak clearly shows up below the transition temperature TN at 210 cm-1 which is reasonably close to the single-magnon excitation energy estimated from the resonant inelastic x-ray scattering.
Simultaneously, the Eg phonon, involving Ir-O-Ir bond bending motion, exhibits a remarkable softening by 35 cm-1 that is by one order of magnitude larger than those in 3d magnets.
These findings imply that the magnon-phonon coupling is unconventionally strong in this system, and intriguingly, the combination of the antiferromagnetic super-exchange and Dzyaloshinskii-Moriya interaction can play a vital role in the all-in all-out magnetic order, providing an important insight into the exotic phase transition in $5d$ transition metal oxides.