Neutron diffraction studies of the molecular compound [Co _2(bta)]_n (H_4bta =1,2,4,5-benzenetetracarboxylic acid): In the quest of canted ferromagnetism

摘要

The exchange mechanism and magnetic structure of the organic-inorganic layered molecule-based magnet [Co_2(bta)]_n (1) (H _4bta =1,2,4,5-benzenetetracarboxylic acid) have been investigated through variable-temperature magnetic susceptibility measurements and supported with a series of neutron diffraction experiments. Cryomagnetic studies have shown an antiferromagnetic ordering at a transition temperature of 16 K that is followed by the appearance of a weak ferromagnetism below 11 K. The weak antiferromagnetic interlayer interaction plays an important role in this system in spite of the long interlayer separation. A ferromagnetic ordering is induced by applied magnetic fields greater than 1800 G (metamagnetic behavior), and a slow magnetic relaxation from this ferromagnetic phase to the antiferromagnetic one is observed. The magnetic structure of 1 has been elucidated at low temperatures in zero field by neutron powder diffraction measurements and was found to be of antiferromagnetic nature with the local cobalt(II) spins (magnetic moments) being aligned ferromagnetically in the ac plane and antiferromagnetically coupled along the crystallographic b axis. No evidence for a long-range spontaneous ferromagnetic component below 11 K was observed in the neutron experiment.