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Abstract

Most magnetic materials which are used in technologicalapplications are metals, alloys or simple compounds, all basedon inorganic chemistry. The search to find organicmaterials which show magnetism has three principal motivations.First, organic materials are extremely chemically tunablebecause of the flexibility of carbon chemistry; there is anobvious advantage in choosing a route already favoured bybiological systems. Second, organic materials can possessinteresting optical properties, thus allowing the possibility ofdevices which implement new functionality. Third, there is thepleasure of achieving something which was once thoughtimpossible: making ferromagnetism in materials containing only sand p electrons. Heisenberg's theory offerromagnetism [1], which introduced the concept ofexchange, had explained the observation that all ferromagnetsknown at that time had d and f electrons. Conventionalwisdom has it that, for example, carbon (containing only s and pelectrons) does not have a spontaneous magnetic moment in any ofits allotropes.Organic ferromagnetism was first achieved using organic radicalscalled nitronyl nitroxides. Many organic radicals exist whichhave unpaired spins, but few are chemically stable enough toassemble into crystalline structures. Even when that ispossible, aligning these spins ferromagnetically is usuallydifficult to achieve. Therefore, the discovery of long-rangeferromagnetism in one of the crystal phases of para-nitrophenyl nitronyl nitroxide (C13H16N3O4)was particularly exciting, eventhough the transition temperature was a disappointingly low0.65 K [2] (though a similar material was soon foundwhich more than doubled this record [3]). Weakferromagnetism in a dithiadiazolyl radical(C8N3F4S2) has been found below 36 K [4](rising to 65 K under pressure [5]). The orderingtemperature can be raised to well above room temperature bypreparing `hybrid materials' known as molecular ferromagnets inwhich organic groups are combined with transition metalions [6,7]. Here the organic groups are themselvesnot magnetic but are used to mediate the magnetism betweentransition metal ions.A further strategy is to use carbon `buckyballs' to makeorganic materials with novel properties. CertainC60-containing compounds can exhibitsuperconductivity [8], and weak magnetism below16 K was found in TDAE-C60 (TDAE is tetrakis(dimethylamino)ethylene)[9], although thenature of the ordering provoked some controversy; this was foundto be due to the existence of two distinct phases ofTDAE-60 which are associated with different relativeorientations of the C60 molecules [10]. Untilrecently, C60 itself was not thought to be a source ofmagnetism, although superconductivity below 52 K has beeninduced in a C60 film using hole injection in afield-effect transistor geometry~[11] (or below 117 Kin lattice-expanded C60 films [12]). Inpristine solid C60, the buckyballs are arranged in aface-centred cubic lattice and held together by van der Waalsbonding. The application of pressure allows the production of anumber of different crystalline, polymeric and amorphous phasesof C60 [13], one of which is a two-dimensionalpolymerized rhombohedral phase containing layers of covalentlybonded C60 molecules [14]. This phase recently showed a signature of spontaneous magnetization up to about500 K [15], raising the possibility that an allotropeof carbon might have a room temperature spontaneousmagnetization after all.In this issue of Journal of Physics: Condensed Matter,Wood et al [16] describe a structural andmagnetic study of this intriguing two-dimensional polymerizedrhombohedral phase of C60, although prepared underslightly different conditions. Reports of weak magnetization inorganic materials have often proved to be wrong, so the newconfirmation of the earlier work [15] is extremelyencouraging. More importantly, using electron microscopy andx-ray diffraction Wood et al show that the buckyballsare undamaged in the most magnetic phase. With higherpreparation temperatures (or higher preparation pressures) thecages can collapse and an sp2 amorphous phase is produced [14]; this amorphous phase could contain unpaired defects which would explain the magnetic properties.However, by demonstrating that the C60 molecules are notcollapsed in the most magnetic phase, Wood et al haveruled out this explanation of the magnetic properties and theirwork consequently strongly supports a model ascribing themagnetism to radical centres formed by the dangling bonds whichare left following the breaking of the intermolecular bridgingbonds. The mechanism for the magnetic coupling between theseradical centres is not yet elucidated and the magnetization isvery small and far from being uniformly distributed throughoutthe sample. Nevertheless, this paper demonstrates thatperforming careful structural studies of samples prepared atdifferent temperatures and pressures is the right way forward tounderstand this effect. The lesson from TDAE-C60 is thatthe precise orientations of the buckyballs can be of crucialimportance in determining the magnetic ground state,demonstrating that combined structural and magnetic studies aredefinitely needed. The letter of Wood et al reminds usthat organic materials continue to be full of surprises.