A ferromagnetic transition at 1.48 K in an organic nitroxide

IN one of the pioneering studies of magnetism at the microscopic level, Heisenberg1 concluded that ferromagnetism could not exist in compounds consisting only of light elements. A few well defined2, purely organic materials (consisting only of the elements carbon, hydrogen, oxygen and nitrogen) have subsequently been found3–5 to exhibit evidence of ferromagnetic interactions at low tem-peratures, with a small number of these showing a transition to a true ferromagnetic state. For example, the p-nitrophenyl nitronyl nitroxide radical has a Curie (transition) temperature of 0.60 K (ref. 6), and a possible ferromagnetic transition has been identified7,8 in a C60-based ionic complex. More recently, it was reported9 that the magnetization and magnetic susceptibility behaviour of a crystalline nitroxide biradical, N,N′-dioxy-l,3,5,7-tetramethyl-2,6-diazaadamantane, are indicative of ferromagnetic interactions. Here we report the existence of a ferromagnetic transition in this system, with a Curie temperature of 1.48 ± 0.02 K. As yet, this is the highest transition temperature found for a purely organic, non-ionic material.