Picometer atomic displacements behind ferroelectricity in the commensurate low-temperature phase in multiferroic YMn2O5

Abstract

The material system YMn 2 O 5 has several low temperatures phases, where magnetism and ferroelectricity occur. Especially, the origin of ferroelectricity in the commensurate phase is an open question. Literature agrees upon a magnetically driven principal mechanism from changes in the Mn spin conguration, which may be based either on magnetostriction due to symmetric exchange, the antisymmetric inverse Dzyaloshinskii-Moriya interaction or a combination of the two. These mechanisms are accompanied by specic atomic displacements of ions in the structure. The space group P bam (55) of the paraelectric phase does not allow the respective polar displacements and a renement of the charge structure in a lower symmetric phase has not been successful so far, mostly because conventional structure analysis lacks the sensitivity required to resolve the expected positional displacements. We applied the new Resonantly Suppressed Diraction (RSD) method, which is sensitive to minuscule structural changes in the sub-pmrange, in order to resolve potential ionic displacements within a polar space group and shed new light on this controversial discussion.We measured the energy dependent RSD spectra of carefully selected reections above and below the phase transition temperature T CM = 39 K. With the data above T CM , we rened the static and dynamic displacements of the paraelectric phase to receive an improved initial model for the structural characterization of the ferroelectric phase. Subsequently, we rened 50 static displacement parameters in the lower symmetric space group P b2 1 m, which allows polarization in b-direction and present the rst structure renement of the commensurate phase in YMn 2 O 5 .We found a signicant displacement of Mn ions and the O partial structure, which results in a calculated absolute spontaneous polarization P s = (1.3 ± 0.4) mC m −2 , in good agreement with a measured value P s = (0.88 ± 0.06) mC m −2 . With the presented investigation, we nally conrm that P s has an ionic contribution and is predominately induced by magnetostriction.