Reduced crystal symmetry as the origin of the ferroelectric polarization within the incommensurate magnetic phase of TbMn2O5

Abstract

The precise crystal symmetry, and hence the emergence of the electric polarization, still remains an open question in multiferroic $R$Mn${}_{2}$O${}_{5}$ ($R$=rare earth, Bi, Y). While previous diffraction studies have indicated the centrosymmetric space group $P\phantom{\rule{0}{0ex}}b\phantom{\rule{0}{0ex}}a\phantom{\rule{0}{0ex}}m$, an atomic displacement allowing for electric polarization would require a noncentrosymmetric symmetry. Here, single-crystal neutron diffraction experiments on TbMn${}_{2}$O${}_{5}$ provide direct evidence of a reduced crystallographic symmetry with the polar space group $P1211$ already above the multiferroic phase transition, indicating that a symmetric ${\mathbf{S}}_{\mathbf{i}}\ensuremath{\cdot}{\mathbf{S}}_{\mathbf{j}}$ spin coupling, i.e., magnetostriction is the dominant mechanism in the commensurate magnetic phase.