Abstract
Multiferroics, showing simultaneous ordering of electrical and magnetic degrees of freedom, are remarkable materials as seen from both the academic and technological points of view1,2. A prominent mechanism of multiferroicity is the spin-driven ferroelectricity, often found in frustrated antiferromagnets with helical spin order1,3,4,5. There, as for conventional ferroelectrics, the electrical dipoles arise from an off-centre displacement of ions. However, recently a different mechanism, namely purely electronic ferroelectricity, where charge order breaks inversion symmetry, has attracted considerable interest6. Here we provide evidence for ferroelectricity, accompanied by antiferromagnetic spin order, in a two-dimensional organic charge-transfer salt, thus representing a new class of multiferroics. We propose a charge-order-driven mechanism leading to electronic ferroelectricity in this material. Quite unexpectedly for electronic ferroelectrics, dipolar and spin order arise nearly simultaneously. This can be ascribed to the loss of spin frustration induced by the ferroelectric ordering. Hence, here the spin order is driven by the ferroelectricity, in marked contrast to the spin-driven ferroelectricity in helical magnets.
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Acknowledgements
We thank H-A. Krug von Nidda and H. Jeschke for helpful discussions. This work was supported by the Deutsche Forschungsgemeinschaft through the Transregional Collaborative Research Centers TRR 80 and TRR 49. Work at Argonne was supported by the US Department of Energy Office of Science, operated under contract no. DE-AC02-06CH11357.
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M.L., A.L., P.L. and J.M. conceived and supervised the project. J.A.S. grew the high-quality single crystals. B.H., J.M. and R.R. prepared the samples for the experiments. S.K., P.L. and F.S. performed the dielectric measurements and analysed the data. P.L. together with M.L. and J.M., wrote the paper with contributions from C.H., A.L., J.A.S. and M.d.S. All authors discussed the results and commented on the manuscript.
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Lunkenheimer, P., Müller, J., Krohns, S. et al. Multiferroicity in an organic charge-transfer salt that is suggestive of electric-dipole-driven magnetism. Nature Mater 11, 755–758 (2012). https://doi.org/10.1038/nmat3400
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DOI: https://doi.org/10.1038/nmat3400
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