Andreas Sieber, Colette Boskovic, Roland Bircher, Oliver Waldmann, Stefan T. Ochsenbein, Grégory Chaboussant, Hans U. Güdel, Nadeschda Kirchner, Joris van Slageren, Wolfgang Wernsdorfer, Antonia Neels, Helen Stoeckli-Evans, Stefan Janssen, Fanni Juranyi and Hannu Mutka
Synthesis and Spectroscopic Characterization of a New Family of Ni₄ Spin Clusters
Inorg. Chem. 44, 4315-4325 (2005)                             

Abstract: A new family of tetranuclear Ni complexes [Ni₄(ROH)₄L₄] (H₂L = salicylidene-2-ethanolamine; R = Me (1) or Et (2)) has been synthesized and studied. Complexes 1 and 2 possess a [Ni₄O₄] core comprising a distorted cubane arrangement. Magnetic susceptibility and inelastic neutron scattering studies indicate a combination of ferromagnetic and antiferromagnetic pairwise exchange interactions between the four Ni^II centers, resulting in an S = 4 spin ground state. Magnetization measurements reveal an easy-axis-type magnetic anisotropy with D -0.93 cm^–1 for both complexes. Despite the large magnetic anisotropy, no slow relaxation of the magnetization is observed down to 40 mK. To determine the origin of the low-temperature magnetic behavior, the magnetic anisotropy of complex 1 was probed in detail using inelastic neutron scattering and frequency domain magnetic resonance spectroscopy. The spectroscopic studies confirm the easy-axis-type anisotropy and indicate strong transverse interactions. These lead to rapid quantum tunneling of the magnetization, explaining the unexpected absence of slow magnetization relaxation for complex 1.