Three-dimensional ordering, spin excitations, and dilution effects in the quasi-1D ferromagnetic (DMeFc)(TCNE)

The linear chain S =1/2 molecular ferromagnet decamethylferrocenium tetracyanoethanide [(DMeFc)(TCNE)] has been studied by frequency- dependent ac susceptibility experiments. Χ'(T) and Χ"(T) exhibit maxima in the temperature range of 4.8- 4.9 K, which corresponds to the transition temperature to the three- dimensional (3D) ferromagnetic ground state. The 3D ordering is due to the presence of a finite interchain ferromagnetic interaction, as determined earlier by dc magnetic susceptibility and specific heat experiments. The results of ac susceptibility measurements below T<SUB>c</SUB> are analyzed in terms of a Debye-like model to investigate the presence of gaps in the spin excitation spectrum of the system. Considerable changes are observed in the critical temperature on introducing spinless sites into the system. The transition temperature of the dilution series (DMeFc)<SUB>x</SUB>(DMeCo)<SUB>1-x</SUB>(TCNE), where (DMeCo)<SUP>+</SUP>=(decamethylcobaltocenium<SUP>+</SUP>) has spin S=0, decreases with increasing (1- x)