Abstract
Based on measurements of soft-x-ray magnetic scattering and symmetry considerations, we demonstrate that the magnetoelectric effect in TbMn2O5 arises from an internal field determined by S→q→×S→-q→ with S→q→ being the magnetization at modulation vector q→, whereas the magnetoelastic effect in the exchange energy governs the response to external electric fields. Our results set fundamental symmetry constraints on the microscopic mechanism of multiferroicity in frustrated magnets. © 2007 The American Physical Society.