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When the structure reveals the multiferroic properties : towards energy-saving hard drives

We focus our attention on materials - called multiferroics - presenting simultaneously ferroelectric and magnetic orders coupled to one another. This magneto-electric coupling enables magnetic control via electric field, substantially reducing the energy cost for writing on magnetic storage device such as MRAM. This technological development is however conditioned to its existence at room temperature. Our fine study at 300K of the atomic structure of RMn2O5 multiferroic oxides reveals the absence of the inversion symmetry. This has a direct consequence : the existence of an electrical polarization already at room temperature…

Multiferroics are materials that couple the magnetic properties with the electric properties : this is called magnetoelectric coupling. From this fundamental property ensues a technological interest : it becomes possible to manipulate a magnetic state such as bits stored in a hard disk by means of an electric field. The use of an electric field instead of a magnetic field allows to increase considerably the speed of writing while reducing the energy consumption. Our researches thus focused on compounds presenting a strong magnetoelectric coupling to understand its origin. The manganite family of general formula RMn2O5 (R being a rare earth) presents this property at low-temperature. Until now, one thought that the strength of the coupling was connected to the fact that the electric polarization was induced by the magnetic ordering appearing at low temperature, and that they thus had a common origin.

Figure : Reciprocal space representation from X ray diffraction experiment on single crystal of DyMn2O5. This reconstruction show small intensities for odd values of K. Their presence is the signature of another atomic structure than the expected one, enabling the inversion symmetry breaking and thus the presence of room temperature ferroelectricity.

Our systematic study on several members of this family (for various rare earths) showed that the electric polarization was already present at room temperature. Indeed, our X rays diffraction experiments made in the Laboratoire de Physique des Solides as well as in the synchrotron SOLEIL showed that the crystalline structure of these compounds do not possess a center of inversion at room temperature. The ferroelectricity induced by the magnetism is not thus the reason of this strong magnetoelectric coupling. This questions the current theoretical models for the benefit of microscopic mechanisms where the magnetic order would come to increase the pre-existent electric polarization.

Contact : Victor Balédent

Reference :
V. Balédent, S. Chattopadhyay, P. Fertey, M. B. Lepetit, M. Greenblatt, B. Wanklyn, F. O. Saouma, J. I. Jang, and P. Foury-Leylekian
Evidence for Room Temperature Electric Polarization in RMn2O5 Multiferroics
Phys. Rev. Lett 114, 117601 (2015).