Spin amplitude modulation driven magnetoelectic coupling in the new multiferroic FeTe${}_2$O${}_5$Br

Abstract

Magnetic and ferroelectric properties of layered geometrically frustrated cluster compound FeTe${}_2$O${}_5$Br were investigated with single-crystal neutron diffraction and dielectric measurements. Incommensurate amplitude modulated magnetic order with the wave vector $\mathbf{q}$=($\half$, 0.463, 0) develops below $T_N=10.6(2)\mathrm{K}$. Simultaneously, a ferroelectric order with the spontaneous polarization perpendicular to $\mathbf{q}$ and to Fe${}^{3+}$ 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 Te${}^{4+}$ ions and polarization of their lone-pair electrons.

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