Remnant magnetization of Fe8 high-spin molecules: X-ray magnetic circular dichroism at 300 mK.

Fe8 high-spin molecules exhibit quantum spin tunneling at very low temperatures. Eight Fe³⁺ ions are sixfold coordinated and magnetically coupled through oxygen bridges. The net magnetization (M_S=20 μ_B per molecule) results from competing antiferromagnetic interactions between the various 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<2 K and temperature independent below 400 mK. In order to reach temperatures low enough to observe a remnant magnetization, we developed a ³He-⁴He dilution refrigerator. This device, that takes advantage of the thermodynamic particularities of the ³He-⁴He mix, let us obtain x-ray magnetic circular dichroism (XMCD) spectra at 300 mK for this molecule. It has been possible to register an XMCD remnant signal, without magnetic field applied, at the iron L_2,3 edges. XMCD coupled with ligand field multiplet calculations has allowed to determine the spin and orbit contributions to the magnetization of the Fe³⁺ ions. [ABSTRACT FROM AUTHOR]