Neutron diffraction and magetometrer measurements: a powerful tandem to understand complex magnetic behaviors

Abstract

One of the areas of solid-state chemistry that has shown remarkable growth over the last two decades is that involving the synthesis and characterization of organic and metal-organic magnetic materials. The exchange mechanism and magnetic structure of the organic-inorganic layered molecule-based magnet [Co2(bta)]n (H4bta = 1,2,4,5-benzenetetracarboxylic acid) have been investigated combining variable-temperature magnetic susceptibility and neutron diffraction measurements at zero-field and under external magnetic fields. Cryomagnetic studies have shown a complex magnetic behavior with four different regions. Nevertheless most of the irreversibility and irreproducibility of the magnetic properties observed by different authors could not be explained. However, they can be easily understood because of the occurrence of a slow magnetic relaxation from the field-induced ferromagnetic phase towards the antiferromagnetic state. In view of the diversity of magnetic phenomena generated in this three-dimensional framework, we have decided to carry out neutron diffraction measurements in order to shed light into this controversial magnetic behavior. The magnetic structure of [Co2(bta)]n has been elucidated at low temperatures in zero field by neutron powder diffraction measurements and 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. Nevertheless, no evidence for a long-range spontaneous ferromagnetic component below 11 K was observed in the neutron experiment. It should be noted that symmetry analysis and the fact of the propagation vector is on the surface of the Brillouin zone evidence that the observed ferromagnetic component cannot be explained in terms of an intrinsic effect, as the previous proposed spin-canting phase. Therefore, the most plausible solution is based on the presence of stacking faults, which produce a net macroscopic ferromagnetic component, as demonstrated by neutron diffraction data taken under applied magnetic field.