A Crystal Field Approach to Orbitally Degenerate SMMs: Beyond the Spin-Only Hamiltonian

Single-Molecule Magnets (SMMs) with large magnetization reversal barriers are promising candidates for high-density information storage. Recently, a large uniaxial magnetic anisotropy was observed for a mononuclear trigonal bipyramidal (TBP) [Ni$^{\mathrm{II}}$Cl$_{\mathrm{3}}$(Me-abco)$_{\mathrm{2}}$] SMM [1]. High-field EPR studies analyzed on the basis of a spin-only Hamiltonian give \textbrokenbar D\textbrokenbar \textgreater 400 cm$^{\mathrm{-1}}$, which is close to the spin-orbit coupling parameter $\lambda =$ 668 cm$^{\mathrm{-1\thinspace \thinspace }}$for Ni$^{\mathrm{II}}$, suggesting an orbitally degenerate ground state. The spin-only description is ineffective in this limit, necessitating the development of a model that includes the orbital moment. Here we describe a phenomenological approach that takes into account a full description of crystal field, electron-electron repulsion and spin-orbit coupling effects on the ground state of a Ni$^{\mathrm{II}}$ ion in a TBP coordination geometry. The model is in good agreement with the high-field EPR experiments, validating its use for spectroscopic studies of orbitally degenerate molecular nanomagnets. [1] K. E. Marriott et al., Chem Sci (published Online)