Ferromagnetic coupling and magnetic anisotropy in molecular Ni(II) squares

Abstract

We investigated the magnetic properties of two isostructural Ni(II) metal complexes $[{\mathrm{Ni}}_{4}{L}_{8}^{b}]$ and $[{\mathrm{Ni}}_{4}{L}_{8}^{c}].$ In each molecule the four Ni(II) centers form almost perfect regular squares. Magnetic coupling and anisotropy of single crystals were examined by magnetization measurements and in particular by high-field torque magnetometry at low temperatures. The data were analyzed in terms of an effective spin Hamiltonian appropriate for Ni(II) centers. For both compounds, we found a weak intramolecular ferromagnetic coupling of the four Ni(II) spins and sizable single-ion anisotropies of the easy-axis type. The coupling strengths are roughly identical for both compounds, whereas the zero-field-splitting parameters are significantly different. Possible reasons for this observation are discussed.