High Frequency Electron Paramagnetic Resonance Studies of High Spin Co(II) Complexes

Variable-High-Frequency Electron Paramagnetic Resonance (HFEPR) data have been collected for single crystals of [Zn(hmp)(dmb)Cl]$_{4}$ (\textbf{1}) doped with a small quantity of high-spin Co(II), and an isostructural tetranuclear cobalt complex [Co$^{II}$(hmp)(dmb)Cl]$_{4}$ (\textbf{2}). Crystals of complex \textbf{2 }exhibit low temperature hysteresis, suggesting that it may be a single molecule magnet (SMM).$^{1}$ However, HFEPR data for complex \textbf{2} cannot be fit to a standard Giant Spin model, as is routinely the case for other SMMs. HFEPR data obtained for complex \textbf{1} indicate that the ground state of the Co$^{II}$ ions is an effective spin \textit{S$\prime $}~=~$^{1}$/$_{2}$ Kramers' doublet with a highly anisotropic $g$-tensor. The anisotropy is of the easy-axis type, with the individual easy axis directions tilted away from the crystallographic $c$ direction by 58$^{o}$. We will attempt to rationalize the EPR spectrum obtained for complex \textbf{2} (as well as its possible SMM behavior) in terms of a simple model involving anisotropic exchange coupling between four effective spin \textit{S$\prime $}~=~$^{1}$/$_{2}$ Co$^{II }$ions, with the local anisotropy entering only through the anisotropic g-tensor at each Co$^{II}$ site. $^{1}$ E.-C. Yang, J. Appl. Phys. \textbf{91}, 7382 (2002).