Magnetic properties of a molecule-based Fe[TCNE]2 magnet*

The temperature dependence of magnetization $M(T)$ of Fe[TCNE]$_{2}$ {\textbullet}xCH$_{2}$Cl$_{2}$, (x$\sim $0.3-1; TCNE = tetracyanoethylene) molecule-based magnet$^{1}$ demonstrates two phase transitions (at $T_{1}$ $\sim $ 95-100 K and at $T_{2} \quad \sim $10-25 K) whose temperatures depend upon the degree of disorder. The position of the ac susceptibility peak related to the high-temperature transition is almost frequency independent, while the low-temperature peak demonstrates a shift per frequency decade (\textit{$\phi $} = d$T$/$T_{1})$/d(log $\omega )$ of 0.03 characteristic for spin-glasses. A coercive field $H_{cr}$ (400 Oe at 2 K) decreases exponentially on warming and vanishes when approaching $T_{1}$. This behavior may be qualitatively described in terms of the Heisenberg mean-field model developed for the systems with competing ferrimagnetic (FM) and spin-glass ordering. At $T_{1}$ the system passes from PM to FM state (the $M$ component parallel to $H$ orders while the perpendicular $M_{tr}$ freely rotates). On further cooling, the system begins to acquire a static $M_{tr}$ component (non-vanishing $H_{cr})$. At $T_{2}$, a crossover between weak and strong irreversibility occurs and a spin- or a cluster-glass state is achieved. [1] K. Pokhodnya, N. Petersen, and J. S. Miller. \textit{Inorg. Chem.} \textbf{41}, 1996 (2002)