The magnetoelectric coupling, a phenomenon inducing magnetic (electric) polarization by application of an external electric (magnetic) field and first conjectured by Curie in 1894, is observed in most of the multiferroics and used for many applications in various fields such as data storage or sensing. However, its microscopic origin is a long-standing controversy in the scientific community. An intense revival of interest developed in the beginning of the 21st century due to the emergence of multiferroic frustrated magnets in which the ferroelectricity is magnetically induced and which present an inherent strong magnetoelectric coupling. The Dzyaloshinskii-Moriya interaction (DMI) well accounts for such ferroelectricity in systems with a noncollinear magnetic order such as the ${\mathrm{RMnO}}_3$ manganites. The DMI effect is, however, inadequate for systems presenting ferroelectricity induced by quasicollinear spin arrangements such as the prominent ${\mathrm{RMn}}_2{\mathrm{O}}_5$ manganites. Among different microscopic mechanisms proposed to resolve this incompatibility, the exchange-striction model stands as the most invoked candidate. In this scenario, the polar atomic displacements originate from the release of a frustration caused by the magnetic order. Despite its theoretical description 15 years ago, this mechanism had yet to be unambiguously validated experimentally. The breakthrough finally comes from ${\mathrm{SmMn}}_2{\mathrm{O}}_5$ presenting a unique magnetic order revealed by powder neutron diffraction. The unique orientation of its magnetic moment establishes the missing element that definitely validates the exchange striction as the effective mechanism for the spin-induced ferroelectricity in this series. More generally, this is a proof of concept that validates this model on actual systems, facilitating the development of a new generation of multiferroics with unrivaled magnetoelectric properties.

• Received 5 October 2016

DOI:https://doi.org/10.1103/PhysRevB.95.184112