Generalized Heisenberg-Type Magnetic Phenomena in Coordination Polymers with Nickel-Lanthanide Dinuclear Units

A new family of 3d–4f coordination polymers with the molecular formula [NiIILnIII(L)(dca)2(NO3)]n [Ln = Eu (1), Gd (2), Tb (3), Dy (4), and Ho (5); H2L = N,N′-bis(2-hydroxy-3-methoxy-5-methylbenzyl)homopiperazine; dca = dicyanamide] has been synthesized in search for new single-molecule magnet (SMM) materials containing highly anisotropic lanthanides. The magnetic properties of these materials have been established by DC and AC magnetometry and explained quantitatively by comprehensive phenomenological modeling based on a generalized Heisenberg-type model, hypothesized previously based on first-principles calculations. Single-crystal X-ray diffraction has shown that the compounds are isostructural, with lanthanide atoms occupying a nine-coordination site with muffin-like geometry and individual NiII–LnIII units linked by dca anions. Other than the paramagnetic compound 1, 2–5 exhibit intra-unit ferromagnetic 3d–4f interactions, favorable for a large spin ground state. A slow field-induced relaxation of magnetization has been observed in compound 4 (only), displaying a substantial energy barrier of Ueff/kB = 26.2(5) K, below 6 K. This is attributed to an easy-plane anisotropy and is consistent with the relaxation in systems with a Kramers ground state doublet and hyperfine interactions.