Zero-field Paramagnetic Resonance in Ferric Acetylacetonate

The zero-field energy levels have been determined for ferric acetylacetonate at 4, 80 and near 275 °K by direct zero-field spectrometry. The spin Hamiltonian is predominantly of the form DS z 2 , and below 90 °K the total splitting is about 25 Gc/s for aluminium and cobalt diluted salts and 47 Gc/s for the concentrated salt. The calculated Schottky specific heat tail at liquid helium temperatures is 0.58 degK 2 , expressed as CT 2 / R . It is shown that, with the exception of the Weiss constant which is sensitive to anisotropy, the exchange phenomena accord well with an isotropic exchange with |2 J/k | = 0.014 degK. At low temperatures in diluted crystals there are three distinct level systems, indicating a change from the crystal space group at room temperature, which demands only one system.