Magnetic and ferroelectric properties of layered geometrically frustrated cluster compound FeTe2O5Br were investigated with single-crystal neutron diffraction and dielectric measurements. Incommensurate amplitude modulated magnetic order with the wave vector q=(\half, 0.463, 0) develops below TN=10.6(2)K. Simultaneously, a ferroelectric order with the spontaneous polarization perpendicular to q and to Fe3+ magnetic moments emerges. The observed ferroelectricity and extraordinary linear scaling of the ferroelectric and magnetic order parameter are provoked by the striction of the intercluster Fe-O-Te-O-Fe bridges leading to the shift of Te4+ ions and polarization of their lone-pair electrons.
@article{arxiv.0906.2336,
title = {Spin amplitude modulation driven magnetoelectic coupling in the new multiferroic FeTe$_2$O$_5$Br},
author = {M. Pregelj and O. Zaharko and A. Zorko and Z. Kutnjak and P. Jeglic and P. J. Brown and M. Jagodic and Z. Jaglicic and H. Berger and D. Arcon},
journal= {arXiv preprint arXiv:0906.2336},
year = {2009}
}