Ferromagnetic and antiferromagnetic phases of 4-(4-iodobenzylideneamino)-2,2,6,6-tetramethylpiperidin-1-yloxyl

Temperature dependences of d.c. magnetic susceptibilities and magnetizations were measured for two different crystallographic phases of 4-(4-iodobenzylideneamino)-2,2,6,6-tetramethylpiperidin-1-yloxyl. Intermolecular ferromagnetic (Weiss temperature theta = +0.71 K) and antiferromagnetic (theta = - 0.23 K) interactions were found for alpha- and beta-phases, respectively, in the magnetic measurements above 1.8 K; the former phase showed a bulk ferromagnetic transition at 0.28 K. Based on the X-ray crystallographic data, intermolecular magnetic interactions are discussed. In the alpha-phase crystal, NO-radical sites form two-dimensional zigzag sheets. The ferromagnetic interaction within the sheet can be interpreted in terms of a beta-hydrogen mechanism, ON(up arrow)-C-alpha(down arrow)-C-beta (up arrow)-H-beta(down arrow)...(up arrow)ON<, in which the magnetic interaction is transmitted by beta-hydrogens. A dimer structure was found in the beta-phase crystal, and direct exchange and/or dipole interactions between N-O radicals seem to be the main origin of the antiferromagnetic interaction.