Terahertz spectroscopy of strongly frustrated helical magnet Fe₃PO₄O₃

The hexagonal lattice in Fe₃PO₄O₃ is made up of three triangularly coordinated Fe units that make the magnetic interactions strongly geometrically frustrated. Along the c-axis, layers of these triangular units arrange in a larger, triangular lattice pattern, making the magnetic interactions also frustrated. Even with this level of frustration, magnetic order occurs below T_N ~ 165 K in the form of a long-wavelength helical pattern. We use time-domain terahertz (THz) spectroscopy to study the magnetic excitations in powder samples of this material. We find an overall softening of the magnetic spectrum around T_N, but with no well-defined sharp resonant modes. However, below ~40 K we find a well defined resonant mode centered around ~ 900 GHz suggesting the existence of an additional transition below this temperature. We will discuss the possible explanations of this behavior within linear spin-wave theory.