Magnetization steps in the model magnetic dimer μ- oxalatotetrakis(acetylacetonato)Fe2(III)

The Fe³⁺ dimer in μ-oxalatotetrakis(acetylacetonato)Fe₂(III) is ideally suited for a magnetization-step study. Using pulsed magnetic fields up to 500 kOe (50 T), all five predicted magnetization steps (MST's) from this dimer were observed. The MST's appeared as peaks in the differential susceptibility dM/dH. Using the convention in which the exchange interaction between the two Fe(III) ions is -2JS₁.S₂, the magnetic fields at the MST's give an intradimer exchange constant J=-3.6+/-0.1 cm^-1. The powder sample was in a Delrin capsule that was immersed in a 1.5 K liquid-helium bath. The shapes and widths of the MST's were strongly affected by the thermal contact between the sample and the liquid-helium bath. Preventing the liquid helium from entering the capsule containing the sample apparently resulted in nearly adiabatic conditions during the milliseconds duration of the field pulse. In this case, as each of the MST's was approached, the strong cooling by the magnetocaloric effect caused the dM/dH peaks to be very narrow. Much broader dM/dH peaks were observed when liquid helium entered the capsule containing the sample. In addition to magnetization-step experiments in pulsed fields, the magnetic susceptibility χ was measured between 2 and 170 K using a steady magnetic field of 3 kOe. The susceptibility led to an intradimer exchange constant which was in good agreement with the value obtained from the MST's.