When organic radicals form assemblies, the molecules suffer either a ferromagnetic or an antiferromagnetic intermoelcular interaction. Although most of organic radicals exhibit the antiferromagnetic ones, there has been a dramatic development in the research of organic ferromagnetism based on the ferromagnetic intermolecular interaction. An important character of the organic radical crystals is the low-dimensionality, reflecting the anisotropic π orbitals. Such crystals have provided excellent models of one-dimensional magnets of Heisenberg spin. It is notable that they occasionally exhibit unusual phase transitions caused by the fact that their structures easily allow lattice modification along the stacking direction. In this report we describe the magneto-structural correlation in the iodide salts of p-N alkylpyridinium nitronylnitroxides (alkyl = methyl, ethyl, propyl, and butyl) which show an interesting change in magnetism from antiferromagnetic to ferromagnetic with the extension of the N-alkyl chain. We also report an unusual first order phase transition in a thiazyl radical, TTTA. The transition occurs between a paramagnetic high-temperature phase and a diamagnetic low-temperature phase with a surprisingly wide hysteresis loop in the vicinity of room temperature.
