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The temperature and magnetic field dependence of the magnetization and susceptibility of singlecrystal decamethylferrocenium tetracyanoethenide demonstrate that this material is the first molecular compound with a ferromagnetic ground state. A spontaneous magnetization is observed for r<4.8 K.The results are consistent with a crossover from a dominance of one-dimensional ferromagnetic exchange interaction to a 3D mean-field-like interaction at =16 K. The critical exponents are in accord with mean-field behavior. A generalized Hubbard model is proposed to account for the unusual ferromagnetic exchange interactions in this system.

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Much effort has been expended to improve metal sidewall coverage of via contacts in multilevel metallization schemes because of the possibility of an enhanced rate of electromigration failure at the sidewall. Multilayered conductors, which incorporate a refractory material beneath an Al alloy layer, will be more resistant to these voiding failures. These metallizations are not ideal, however, because of the discontinuities in the flux of electromigrating atoms at the Al/refractory interfaces of the via, which render the interfaces vulnerable to voiding. This paper examines the relative importance of electromigration at the sidewall and metal interface of vias containing AlSiCu/TiN multilayer conductors. Our results demonstrate that voiding failure is not associated with poor metal step coverage, and that the interface between the metal levels is the region most vulnerable to voiding. Formation of voids between the metal levels occurs by drift of the Al layer away from the Al/TiN interface, and there is a strong dependence of failure on the direction of electron flow through the via. This directional dependence arises because the Al drift velocity is determined by the current density in the stripes attached to the via. We present an expression for the via failure time that incorporates the stripe current density.