Polymeric ferromagnets

Finally, synthetic metals made of polymeric organic molecules may also show the property of ferromagnetism. Organic materials of this kind were first demonstrated in 1987 by Ovchinnikov and his co-workers at the Institute of Chemical Physics in Moscow. The polymer they used was based on a polydiacetylene backbone, which contains alternating double-single and triple-single bonds between the carbon atoms of the molecule (10.2). 

The magnetic data of the polymeric complex at low temperature (T < 20 K) yield much higher values than those calculated using Eq. 1 and show strong ferromagnetic interactions. The magnetic behavior of the complex is different from those of discrete Cu-Ln complexes, which normally show antiferromagnetic interactions [81-85],... 

More insight into the reasons for magnetism of derivatives may be obtained by a closer look at other recently discovered ferromagnetic Cjo materials, which include complexes of cobaltocene with a CgQ-derivative [156, 157], but also metal complexes with europium [175-177] or cerium [178-180] or pure C q in a polymeric modification [181-183]. 

Oxidation of the polymers from Scheme 1-66 leads to the polymeric cation radicals with ferromagnetic coupling of spins. Surprisingly, however, the spin concentration in these polymer networks was extremely low. Only a few percent of the cation radical units actually carried an unpaired electron. 

As to the tailor-made polymers, a more serious, perhaps intrinsic, cause is discussed (Haare et al. 1998). These polymeric cation radicals form ir-macrodimers according to usual equilibria 2M+ <=> (M)l+. The ir-dimerization inhibits the formation of high-spin states. Maybe other n-conjugated ion radicals with a much lower tendency to form ir-dimers, such as polymeric cation radicals based on p-phenylenediamine, are a promising way to construct intrachain ferromagnetic coupling in polymers. 

Disordered low-dimensional C60 polymers also show interesting magnetic properties. In particular, Makarova et al. [16] reported that rhombohedral C60 polymerized at high temperatures near 6 GPa showed signs of weak ferromagnetism (nonlinear susceptibility and magnetic hysteresis loops, as shown in Fig. 16) at temperatures well exceeding room temperature. This has been verified... 

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