Magnetic phase transitions in organic radicals

From the measurements of magnetization and electron-spin-resonance spectra as a function of temperature, a magnetic phase transition at 145 K with a thermal hysteresis is found for an organic radical 5-carboxy-2-thienyl nitronyl nitroxide (2-CATNN). Ferromagnetic (FM) intermolecular interaction is observed in the high-temperature phase while antiferromagnetic (AFM) interaction is obtained for the low-temperature phase. The nature of the magnetic phase transition in 2-CATNN is compared with that observed in a radical galvinoxyl, which also shows a phase transition at 85 K between FM and AFM phases, and in a radical ion salt potassium chloranil exhibiting a phase transition at 260 K between two AFM phases with a large thermal hysteresis. (C) 2001 Elsevier Science Ltd. All rights reserved.