Coexistence of ferromagnetism and metallic conductivity in a molecule-based layered compound

Crystal engineering—the planning and construction of crystalline supramolecular architectures from modular building blocks—permits the rational design of functional molecular materials that exhibit technologically useful behaviour such as conductivity and superconductivity1, ferromagnetism2 and nonlinear optical properties3. Because the presence of two cooperative properties in the same crystal lattice might result in new physical phenomena and novel applications, a particularly attractive goal is the design of molecular materials with two properties that are difficult or impossible to combine in a conventional inorganic solid with a continuous lattice. A promising strategy for creating this type of ‘bi-functionality’ targets hybrid organic/inorganic crystals comprising two functional sub-lattices exhibiting distinct properties. In this way, the organic π-electron donor bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) and its derivatives, which form the basis of most known molecular conductors and superconductors1, have been combined with molecular magnetic anions, yielding predominantly materials with conventional semiconducting or conducting properties4,5, but also systems that are both superconducting and paramagnetic6,7. But interesting bulk magnetic properties fail to develop, owing to the discrete nature of the inorganic anions. Another strategy for achieving cooperative magnetism involves insertion of functional bulky cations into a polymeric magnetic anion, such as the bimetallic oxalato complex [MnIICrIII(C2O4)3]-, but only insoluble powders have been obtained in most cases8,9,10,11,12. Here we report the synthesis of single crystals formed by infinite sheets of this magnetic coordination polymer interleaved with layers of conducting BEDT-TTF cations, and show that this molecule-based compound displays ferromagnetism and metallic conductivity.