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Abstract:

Most materials in magnetic applications are based on inorganic materials. Recently, however, organic and molecular materials have begun to show increasing promise. Purely organic ferromagnets, based upon nitronyl nitroxide radicals, show long range magnetic order at very low temperatures in the region of 1K, while sulphur-based radicals show weak ferromagnetism at temperatures of up to 36K. It is also possible to prepare molecule-based magnets in which transition-metal ions are used to provide the magnetic moment, but organic groups mediate the interactions. This strategy has produced magnetic materials with a large variety of structures, including chains, layered systems, and three-dimensional networks, some of which show ordering at room temperature and some of which have very high coercivity. Even if long range magnetic order is not achieved, interesting materials displaying the spin crossover effect may be prepared and these can have useful applications. Further magnetic materials may be obtained by constructing charge-transfer salts, which can produce metallic molecular magnets. A very exciting recent development is the preparation of single molecule magnets, which are small magnetic clusters. These materials can show macroscopic quantum tunnelling of the magnetization and may have uses as memory devices or in quantum computation applications. These systems can be powerfully studied using various experimental methods, including magnetometry, neutron scattering, muon-spin rotation and synchrotron radiation techniques.

Abstract:

Magnetism in organic diradical ion salts prepared from the diradicals, pyridine-2,6-diylbis(nitronyl nitroxide) [(2,6-PYBNN)(center dot center dot)] and phenol-2,4-diylbis(nitronyl nitroxide) [(2,4-POBNN)(center dot center dot)], were investigated by using SQUID magnetometers. The diradical cation salt (2,6-PYBNN)Cl--+(-) shows intramolecular ferromagnetic (FM) interaction with the exchange coupling constant J/k = +9.0 K and intermolecular antiferromagnetic (AFM) interaction with the Weiss constant theta = -0.75 K in the low temperature phase below 4.5 K. Although the intramolecular FM interaction is close to that observed in the neutral diradical 2,6-PYBNN.., the intermolecular AFM interaction is significantly reduced from J'/k = -57 K due probably to the increase of intermolecular distances by the incorporation of Cl- ions into the salt. We also present and discuss the magnetic behavior in the diradical cation salt (2,6-PYBNN)center dot center dot+BF4- and the diradical anion salt. (C4H9)(4)N+(2,4-POBNN)(center dot center dot-). (c) 2005 Elsevier Ltd. All rights reserved.