Experimental evidence for the microscopic mechanism of the unusual spin-induced electric polarization in GdMn₂O₅

We report in this paper the temperature evolution of the magnetic structure of GdMn₂O₅, in the range 2–40 K, studied by neutron diffraction on an isotope-enriched powder. We detail a thorough analysis of the microscopic mechanisms needed to release the different magnetic frustrations that are at the origin of the polarization. In addition to the usual exchange-striction term, known to be at the origin of the polarization in this family, an additional exchange-striction effect between the Gd³⁺ and Mn³⁺ spins is found to be responsible for the very large polarization in the Gd compound.