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 measure d to investigate the magnetic properties of the first oxalate-bpy mixed-lig and 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) syst em 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), respec tively. The nature of the ordering in Ni(ox)(bpy) is antiferromagnetic, how ever 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 b ut with canting, or so-called weak ferromagnetism. The residual moment resu lting from the canting is larger in Fe(ox)(bpy) than in Co(ox)(bpy). The va lues of mu(eff) obtained by fitting chi(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. (C) 2000 American I nstitute of Physics. [S0021-8979(00)91708-0].