Single crystal EPR study at 95 GHz of a large Fe based molecular nanomagnet: Toward the structuring of magnetic nanoparticle properties

摘要

A W-band single-crystal EPR study has been performed on a molecular cluster comprising 19 iron(iii) ions bridged by oxo- hydroxide ions, Fe_(19), in order to investigate magnetic nanosystems with a behavior in between the one of Magnetic NanoParticles (MNP) and that of Single Molecule Magnets (SMM). The Fe_(19) has a disk-like shape: a planar Fe_7 core with a brucite (Mg(OH)_2) structure enclosed in a "shell" of 12 Fe(iii) ions. EPR and magnetic measurements revealed an S = 35/2 ground state with an S = 33/2 excited state lying ～ 8 K above. The presence of other low-lying excited states was also envisaged. Rhombic Zero Field Splitting (ZFS) tensors were determined, the easy axes lying in the Fe_(19) plane for both the multiplets. At particular temperatures and orientations, a partially resolved fine structure could be observed which could not be distinguished in powder spectra, due to orientation disorder. The similarities of the EPR behavior of Fe_(19) and MNP, together with the accuracy of single crystal analysis, helped to shed light on spectral features observed in MNP spectra, that is a sharp line at g = 2 and a low intensity transition at g = 4. Moreover, a theoretical analysis has been used to estimate the contribution to the total magnetic anisotropy of core and surface; this latter is crucial in determining the easy axis-type anisotropy, alike that of MNP surface.