Magnetic ordering in M(ox)(bpy) system (M=Fe, Co, Ni; ox=C2O42−; bpy=4,4′-bipyridine)

The temperature and field dependence of the magnetization have been measured to investigate the magnetic properties of the first oxalate–bpy mixed-ligand framework with a general formula M(ox)(bpy) (M = Fe(II), Co(II), Ni(II); ox = C2O42−; bpy = 4,4′-bipyridine). All compounds in the M(ox)(bpy) system crystallize in an orthorhombic structure with space group Immm (No. 71), in which the magnetic M ions form one-dimensional chains along the a axis. Spontaneous magnetic orderings with transition temperatures of 12, 13, and 26 K have been found for Fe(ox)(bpy), Co(ox)(bpy), and Ni(ox)(bpy), respectively. The nature of the ordering in Ni(ox)(bpy) is antiferromagnetic, however it may exhibit a metamagnetic transition at high magnetic fields. The type of ordering in Fe(ox)(bpy) and Co(ox)(bpy) is also antiferromagnetic but with canting, or so-called weak ferromagnetism. The residual moment resulting from the canting is larger in Fe(ox)(bpy) than in Co(ox)(bpy). The values of μeff obtained by fitting χ(T) indicate a high spin state for M ions in all three compounds. The origin of the magnetic ordering in these compounds is attributed to antiferromagnetic exchange interactions between the M ions through the bridging oxalate molecules along the magnetic chain and interchain M–M couplings along the [111] direction.