Phase Transitions in Low-Dimensional Molecular Magnetic Systems

The magnetic phenomena of molecular electron transfer salts are investigated. The linear chain, S = 1/2, molecular ferromagnet, decamethylferrocenium tetracyano-ethanide, (FeCp_sp{2}{*}) (TCNE), has been studied by frequency dependent ac susceptibility experiments. chi^'(T) and chi^{'' }(T) exhibit a maxima at approximately 4.8 K which corresponds to the transition temperature to the 3D ferromagnetic ground state^1. 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_ {c} are analyzed in terms of a Debye like model to investigate the presence of gaps in the spin spectrum of the system. Considerable changes are observed in the critical temperature on introducing spinless sites into the system. We have systematically introduced spinless defects in the quasi-one-dimensional ferromagnetic system decamethylferrocenium tetracyanoethanide, (FeCp_sp{2} {*}) (TCNE). The 3-D ordering temperature T_{c} decreases rapidly with the concentration of spinless sites. The rate of decrease of T_{c} with concentration is in the order of the exchange spatial anisotropy (i.e., J_{interchain}/J _{intrachain}) of the parent system in accordance with long standing theoretical predictions. The effects of magnetic defects are also discussed briefly. The molecular electron transfer slat (MnCp _sp{2}{*}) (TCNE) undergoes a ferromagnetic transition at the Curie temperature, T_{c} ~ 8.2 K. Anomalous Metamagnetic Behavior in Molecular Decamethylmanganocenium 2,3-Dichloro-5,6-dicyanobenzoquinoneide, (MnCp _sp{2}{*}) (DDQ) is observed and studied in detail. The molecular electron transfer salt (MnCp_sp{2}{ *}) (DDQ) is shown to have a metamagnetic behavior below the Neel temperature 8.5 K. Anomalous behavior occurs below 4 K in the form of a large hysteresis and remnant magnetization. We derive an equilibrium phase diagram and an unique set of two non-equilibrium phase diagrams from our experimental results. The molecular complex V(TCNE_{x }y(CH_2Cl_2 ) at room temperature exhibits spontaneous magnetization M and a hystertic behaviour with a coercive field ~40 G. The near zero field is fit well to a spin wave result for a ferrimagnet at low temperature (T < 50 K).