Magnetic properties and critical behavior of $\mathrm{Fe}\left(\mathrm{tetracyanoethylene}{)}_2\cdot x\right({\mathrm{CH}}_2{\mathrm{Cl}}_2):$ A high-$T_c$ molecule-based magnet

We report magnetic studies of $\mathrm{Fe}\left(\mathrm{TCNE}{)}_2\cdot x\right({\mathrm{CH}}_2{\mathrm{Cl}}_2),$ a member of the family of high-$T_c$ molecule-based magnets, $M\left(\mathrm{TCNE}{)}_x\cdot y\mathrm{}\right(\mathrm{solvent})\mathrm{}$ $(M=\mathrm{V},$ Mn, TCNE=tetracyanoethelyne). Based on extensive static and dynamic magnetic measurements we show that this system has a complex magnetic behavior, with a mixture of ferrimagnetic and random anisotropy characteristics. The constricted hysteresis curve with a spin-flop shape, the ac susceptibility in the presence of a dc field, consistent with the spin-flop picture, and the remanent magnetization suggest ferrimagnetic behavior. The ac susceptibility data in zero dc field have modest frequency dependence suggesting glassiness, while the field-cooling/zero-field-cooling magnetization data show irreversibilities, starting at $\sim 97$ K, and increasing below $\sim 20$ K, all consistent with the behavior of reentrant random anisotropy magnets (RAM). Ferromagneticlike scaling analyses provide a critical temperature $T_c=97\mathrm{}$ K and the critical exponents $\beta =0.45\mathrm{}$ and $\delta =2.5,$ relatively consistent with random anisotropy magnet predictions. Also, the curvature of the $T<\mathrm{}{T}_c$ data in the modified Arrott plot is characteristic for RAM. Correlating the static and dynamic magnetic studies and analyzing the similarities with other members of this family of hybrid organic/inorganic compounds, we discuss the origins of anisotropy and randomness and the possible interconnections between ferrimagnetism and RAM (sperimagnetism) in $\mathrm{Fe}\left(\mathrm{TCNE}{)}_2\cdot x\right({\mathrm{CH}}_2{\mathrm{Cl}}_2).\mathrm{}$