Multiferroics, showing simultaneous ordering of electrical and magnetic degrees of freedom, are remarkable materials as seen from both the academic and technological points of view 1,2 . A prominent mechanism of multiferroicity is the spin-driven ferroelectricity, often found in frustrated antiferromagnets with helical spin order 1,3,4,5 . There, similar to 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 interest 6 . Here we provide evidence for this exotic type of ferroelectricity, accompanied by antiferromagnetic spin order, in a two-dimensional organic charge-transfer salt, thus representing a new class of multiferroics. 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.In the present work, we have investigated single crystalline -(BEDT-TTF) 2 Cu[N(CN) 2 ]Cl (-Cl), where BEDT-TTF stands for bis(ethylenedithio)-tetrathiafulvalene (often abbreviated as ET). Two crystals with different geometries and contact materials were investigated (see methods section). In these compounds, dimers of ET molecules form an anisotropic triangular lattice with a halffilled dimer band, where the strong on-dimer Coulomb interaction U drives the system to a Mott insulating state 7,8 . In addition, the importance of intra-dimer degrees of freedom and inter-site interactions V have been pointed out 9,10,11 . -Cl consists of alternating conducting ET layers and insulating anion sheets (see Supplementary Information (SI), Fig. S1). Within the ET layers, adjacent molecules form dimers on which a single electron hole is located. Below T N  27 K, intralayer antiferromagnetic and interlayer ferromagnetic ordering of hole spins occur, followed by weak ferromagnetic canting below 23 K (refs. 12,13). -Cl becomes superconducting below 12.8 K, when applying pressures of 300 bar 14 . Figure 1 shows the temperature dependence of the conductivity ' of crystal 1, measured at 2.1 Hz, providing a good estimate of the dc conductivity  dc , see SI. Aside of the wellknown overall semiconducting characteristics of  dc (T) 14,15 , we find a jump-like decrease by two decades at around 27 K, about the same temperature where long-range antiferromagnetic ordering is reported 7,8 . A corresponding jump was also found in sample 2 (see SI Figure 2 shows the dielectric constant '(T) of sample 1 for various frequencies. Pronounced peaks reaching absolute values up to several hundreds are revealed. While the peak positions are nearly frequency independent, their amplitudes become strongly suppressed with increasing frequency. The overall behaviour is typical for order-di...

We report on structural, magnetic, dielectric, and thermodynamic properties of Eu 1−x Y x MnO 3 for Y doping levels 0 ഛ x Ͻ 1. This system resembles the multiferroic perovskite manganites RMnO 3 ͑with R = Gd, Dy, Tb͒ but without the interference of magnetic contributions of the 4f ions. In addition, it offers the possibility to continuously tune the influence of the A-site ionic radii. For small concentrations x ഛ 0.1 we find a canted antiferromagnetic and paraelectric ground state. For higher concentrations x ജ 0.3 ferroelectric polarization coexists with the features of a presumably spiral magnetic phase analogous to the observations in TbMnO 3 . In the intermediate concentration range around x Ϸ 0.2 a multiferroic scenario is realized combining weak ferroelectricity and weak ferromagnetism, presumably due to a conelike magnetic structure.

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We present a detailed study of complex dielectric constant and ferroelectric polarization in multiferroic LiCuVO 4 as function of temperature and external magnetic field. In zero external magnetic field, spiral spin order with an ab helix and a propagation vector along the crystallographic b direction is established, which induces ferroelectric order with spontaneous polarization parallel to a. The direction of the helix can be reoriented by an external magnetic field and allows switching of the spontaneous polarization. We find a strong dependence of the absolute value of the polarization for different orientations of the spiral plane. Above 7.5 T, LiCuVO 4 reveals collinear spin order and remains paraelectric for all field directions. Thus this system is ideally suited to check the symmetry relations for spiral magnets as predicted theoretically. The strong coupling of ferroelectric and magnetic order is documented and the complex (B,T) phase diagram is fully explored.

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