Multiferroicity in an organic charge-transfer salt: Electric-dipole driven magnetism

Multiferroics, showing simultaneous ordering of electrical and magnetic degrees of freedom, are remarkable materials not only from an academic but also from a technological point of view. In most multiferroics the ferroelectric polarization arises from an off-centre displacement of ions. However, recently a different mechanism, namely charge-order driven ferroelectricity has attracted considerable interest. Here, the arrangement of electrons (or holes), instead of ions, leads to the polarization of the sample.

relaxation process
Figure 1: Temperature dependence of the dielectric constant of κ-Cl, for various frequencies. The behavior is typical for order-disorder type ferroelectricity, setting in below about 25 K. [P. Lunkenheimer et al., Nature Mater. 11, 755 (2012)]


We found the simultaneous occurrence of magnetic ordering and ferroelectricity in a two-dimensional organic charge-transfer salt, namely κ-(BEDT-TTF)2Cu[N(CN)2]Cl (κ-Cl), where BEDT-TTF stands for bis(ethylenedithio)-tetrathiafulvalene (often abbreviated as ET) (Fig. 1). This represents the first example of multiferroicity within this interesting group of materials. In addition, it is one of the few examples of electronic ferroelectricity, where ferroelectric ordering is based only on electronic degrees of freedom. Moreover, and most interestingly, we found clear hints for a new mechanism of multiferroicity: In this material, ferroelectric ordering breaks geometric spin frustration, thus triggering magnetic order. Therefore, here the ferroelectric order drives magnetic order, just the opposite case to what is observed in the famous multiferroics with helical spin order (e.g., TbMnO3) where a spin-driven mechanism induces ferroelectricity.

relaxation times

Figure 2: Schematic drawing of the ac planes of (κ-Cl) for temperatures below and above TFE. Thick grey lines: ET molecules; red spheres: holes (the red shaded areas for T>TFE indicate their delocalization); orange arrows: dipolar moments arising below TFE. [P. Lunkenheimer et al., Nature Mater. 11, 755 (2012)]


The following model can explain the experimental observations: κ-Cl consists of alternating conducting ET layers and insulating anion sheets. Within the ET layers, adjacent molecules form dimers on which a single electron hole is located (Fig. 2). These dimers form an anisotropic triangular lattice. As the holes are delocalized on the dimers, their spins - on average - sit at the centres of the dimers, forming a triangular lattice (Fig. 2, right). Thus, here we have the prototypical situation of geometrical frustration and, indeed, in the related system κ-(ET)2Cu2(CN)3 a spin-liquid state is formed. There a dielectric investigation has revealed relaxor ferroelectricity, i.e. no long-range but short-range clusterlike FE order only. However, in the present system we find conventional FE order, ascribed to the collective off-center positioning of the holes within the dimers below a temperature Tc (Fig. 2, left). Then the hole positions (and thus those of the spins) no longer form a perfect triangular lattice. Therefore, the frustration should be broken and magnetic order can be expected to arise, which indeed is observed. This explains the simultaneous occurrence of magnetic and electrical order. Obviously, in this system the ferroelectric ordering drives the magnetic one. Thus, here we have not only a new class of MFs but also a new mechanism that leads to a close coupling of magnetism and ferroelectric ordering.

To learn more, see:

Original work:
Multiferroicity in an organic charge-transfer salt that is suggestive of electric-dipole driven magnetism
P. Lunkenheimer et al., Nature Mater. 11, 755 (2012)

Press release:
Neuer Weg zu simultanem Auftreten von Ferroelektrizität und Magnetismus in einem organischen Material