Linear Chains of Magnetic Ions Stacked with Variable Distance: Ferromagnetic Ordering with a Curie Temperature above 20K

We have studied the magnetic properties of the SURMOF-2 series of metal-organic frameworks (MOFs). Contrary to bulk MOF-2 crystals, where Cu2+ ions form paddlewheels and are antiferromagnetically coupled, in this case the Cu2+ ions are connected via carboxylate groups in a zipper-like fashion. This unusual coupling of the spin 1 /2 ions within the resulting one-dimensional chains is found to stabilize a low-temperature, ferromagnetic (FM) phase. In contrast to other ordered 1D systems, no strong magnetic fields are needed to induce the ferromagnetism. The magnetic coupling constants describing the interaction between the individual metal ions have been determined in SQUID experiments. They are fully consistent with the results of abinitio DFT electronic structure calculations. The theoretical results allow the unusual magnetic behavior of this exotic, yet easy-to-fabricate, material to be described in a detailed fashion.