EPR studies on (NBu4)2Co2[Cu(opba)]3·S, where opba=ortho-phenylenebis(oxamato) and S=Solvent: unusual case of long-range magnetic order in weakly interacting systems

Electron paramagnetic resonance (EPR) investigations were carried out on two bimetallic molecular magnets (NBu4)2Co2[Cu(opba)]3·S, where S=3DMSO·3H2O and 2H2O, known to exhibit long-range magnetic order below 32 and 34 K, respectively, in the temperature range 300–15 K. Temperature dependence of the intensity of the EPR signal arising from Cu2+ in these compounds showed a rapid decrease in intensity upon cooling below 65 K, which was accompanied by a disappearance of the super-hyperfine structures, observed on the Cu2+ signal at higher temperatures, pointing to a Co2+Cu2+ spin–spin interaction following the slowing down of Co2+ spin–lattice relaxation. The appearance of a new signal at zero field (ZFS) below 40 K is attributed to the onset of long-range magnetic ordering in both the compounds.