The principal and mean magnetic moments of Fe acac3 (acac = acetylacetonate) and K3Fe(oxalate),,3H,0 are reported over the temperature range 80--300%. This appears to be the first time that anisotropy of the 6Alp ground-state of the iron( Ill) ion has been detected by crystal magnetic susceptibility measurements. The anisotropy is interpreted in terms of the zero-field splitting of the ground state arising from a low-symmetry ligand-field and spin-orbit coupling ; values of the zero-field splitting are reported and discussed with reference to the molecular structures of the two complexes.THE origin of the zero-field splitting in 3d5 systems to explain ground-state splittings of the order of 0.001 and the effect of such splitting on the magnetic properties cm.-l in nearly cubic systems were based on highof FeIII and MnII have long been of interest in studies order perturbations due to the cubic field and spinof specific heats, adiabatic demagnetisation, and, orbit coupling.lS2 Such an explanation was queried by more recently, paramagnetic resonance. Early attempts Meijer who found that the discrepancy between the 4
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