Probing the Low-Temperature Phase Transition in $\text{Er}_{x}\mathrm{Y}_{1-x}\text{FeO}_{3}$ by Terahertz Magnetospectroscopy

We have studied the terahertz response of the rare-earth orthoferrite Er
_x
Y
_1-x
FeO
₃
as a function of magnetic field, temperature, and yttrium composition ( x). Through theoretical modelling based on an extended Dicke Hamiltonian, we provide new insight into the role of cooperative, “superradiant” interactions between Er spins and Fe magnons in the low-temperature phase transition that this magnetic system exhibits.