XMCD signals of the high spin molecules Fe8

Abstract

Fe8 clusters are molecules which show quantum spin tunneling at very low temperature. They have eight iron ions that are six-fold coordinated and magnetically coupled through oxygen bridges. The net magnetization (Ms = 20 μB per molecule) results from competing antiferromagnetic interactions between the Fe3+ ions (S = 5/2). Because of the structural anisotropy of these molecules, the magnetization curve presents a hysteresis loop with staircases below 2 K. The staircases of the hysteresis loop are due to the quantum spin tunneling, which is temperature dependent for 400 mK < T < 3 K and temperature independent below 400 mK. Using a 3He–4He dilution refrigerator, we have recorded XMCD spectra at iron L2,3 edges at 300 mK for this molecule. It has been possible to record an XMCD remnant signal, without magnetic field applied, on these high spin molecules. XMCD coupled with Ligand Field Multiplet calculations has allowed to determine the spin and orbit magnetizations of Fe3+.