Magnetization Density in an Iron(III) Magnetic Cluster. A Polarized Neutron Investigation

Abstract

Magnetic molecular clusters, formed by a large number of strongly interacting metal ions, have been extensively investigated during recent years,1 as models of nanometer-sized single-domain magnetic particles. Particular attention2 has been devoted to clusters with high spins in the ground state and Ising-type anisotropy showing slow relaxation of the magnetization at low temperature, which eventually relaxes with a tunneling mechanism. Slow relaxation and the even more interesting phenomenon of pure quantum tunneling of the magnetization have been recently reported3a for an octanuclear iron(III) cluster, {[Fe8O2(OH)12(tacn)6]Br7‚H2O}[Br‚8H2O], Fe8, where tacn ) 1,4,7-triazacyclononane. It is characterized by a ground S ) 10 state originated by the presence of competing antiferromagnetic interactions between the S ) /2 spins3b-c of the iron atoms and by very weak dipolar intercluster interactions. A model for the coupling scheme, presented in Figure 1, has been proposed,3c but given the complexity of the system and the large number of independent exchange pathways, the exact nature of the ground state cannot be unambiguously described by fitting thermodynamic properties such as magnetic susceptibility. On the other hand, the spin structure and the dipolar magnetic fields inside the sample seem to play an important role in the tunneling mechanism,3d requiring a detailed knowledge of the global spin density. Therefore, to obtain a better description of the ground state it is necessary to use experimental techniques, which give access to the unpaired magnetization density of the cluster, like EPR, NMR, and neutron diffraction. In the latter case, two types of techniques were used: polarized and unpolarized neutron diffraction. However, the latter, which has been applied on a Mn12 cluster,4 seems to provide results which are not very accurate. Polarized neutron diffraction, PND, applies to single crystals of paramagnetic species in which the magnetization density is aligned by an external magnetic field. The incident neutron beam is polarized either parallel (v) or antiparallel (V) to the applied magnetic field, and the so-called flipping ratios R between the intensities I and I respectively at the Bragg positions (hkl) are measured