Magnetic Excitations in Multiferroic GdMn$_2$O$_5$.

Orthorhombic $R$Mn$_2$O$_5$ compounds recently attracted attention due to a complex frustrated ground state and unconventional excitations. $YMn_2O_5$ with non-magnetic rare earth (RE) shows incommensurate spiral state with spins in the neighbouring chains aligned at 90-degrees to each other [1]. RE ions with unquenched angular momentum enable the control of polarization by magnetic field in the multiferroic materials, such as $TbMn_2O_5$ [2]. Strongly $GdMn_2O_5$, with magnetic rare earth in S=7/2, L=0 state, exhibits a spiral state below 40K, that concedes to a commensurate state below $\sim30K$. The latter hosts large magnetically-induced polarization of 3600 $\mu C/m^2$ induced via Heisenberg exchange striction mechanism. This polarization changes by 5000 $\mu C/m^2$ under the external magnetic field [3]. Here we corroborate THz magnetoabsorption data with the microscopic modelling. The magnetic excitations are calculated using model Hamiltonian with parameters extracted from ab-initio simulations. \\ \\ [1] J.-H. Kim et al., Phys. Rev. Lett. {\bf 107}, 097401 (2011).\\ [2] N. Hur et al., Nature {\bf 429}, 392 (2004).\\ [3] N. Lee et al., Phys. Rev. Lett. {\bf 110}, 137203 (2013).