Multiferroic materials in which magnetism and ferroelectricity coexist are a time-honored subject of study, opening the route for the next generation of electronic devices. Understanding the pronounced interplay between these properties is a current challenge, especially for the realization of multiferroic devices. One of the largest reversible magnetoelectric couplings has been evidenced in TbMn${}_2$O${}_5$, the reference compound of the so-called $R$Mn${}_2$O${}_5$ multiferroics. The origin of this strong coupling has been intensively studied from the static properties. Here we report an inelastic neutron scattering investigation of the dynamic properties in this famous compound. Our work sheds light on the electromagnon, a hybrid mode theoretically predicted in multiferroics, which couples magnetic excitations with polar lattice degrees of freedom. We were able to determine in TbMn${}_2$O${}_5$ the exchange constants of a model Heisenberg Hamiltonian and to identify the spin excitation that transforms in the electromagnon on passing from the ferroelectric and magnetically collinear phase to the low temperature incommensurate and weakly ferroelectric one. Our results point to the unusual feature of the dynamic magnetoelectric coupling in this series of multiferroics.

• Received 1 February 2013

DOI:https://doi.org/10.1103/PhysRevB.87.140301