Terahertz study of potential multiferroic materials Sr$_2$FeSi$_2$O$_7$ and BaFe$_{12}$O$_{19}$*

We present data on the complex dielectric function of two iron-based quantum magnets in the terahertz frequency range. We study the phase transitions in Sr$_2$FeSi$_2$O$_7$ at low temperature when it becomes a collinear antiferromagnet, in contrast with the canted antiferromagnetic state of iso-structural multiferroic Ba$_2$CoGe$_2$O$_7$. We compare the terahertz response of these two materials in light of the recent observation of electromagnon excitations in Ba$_2$CoGe$_2$O$_7$. We have also studied BaFe$_{12}$O$_{19}$, a hexagonal ferromagnet that is predicted to have an antiferroelectric state. We will report experiments at terahertz frequencies probing this predicted state.