Neutron transitions within theS=10ground multiplet of aFe8magnetic cluster

Inelastic neutron scattering has been used to determine the anisotropic spin Hamiltonian that splits the $S=10\mathrm{}$ ground multiplet of the cluster $\left(\mathrm{tacn}{)}_6{\mathrm{Fe}}_8{\mathrm{O}}_2\right(\mathrm{OH}{)}_{12}{\mathrm{Br}}_{4.3}\left({\mathrm{ClO}}_4{)}_{3.7}6{\mathrm{H}}_2\mathrm{O}\hspace{0.5em}\right({\mathrm{Fe}}_8\mathrm{PCl}),$ where (tacn) is the organic ligand 1,4,7-triazacyclononane. This single-molecule magnet,which is obtained from the well known ${\mathrm{Fe}}_8\mathrm{Br}$ cluster, by partial replacement of the bromide ions by perchlorate anions, presents a larger transverse anisotropy than the parent compound, the $E/D$ ratio being 0.22 instead of 0.16. This is in agreement with the relaxation measurements in the pure quantum tunneling regime. The details of the neutron spectra detected at three different temperatures (2, 10, and $15\hspace{0.5em}K)$ allowed us to determine the spin Hamiltonian parameters up to fourth order and to identify the main source of the transition linewidth broadening with the occupational disorder of the bromide and perchlorate ions, rather than with the random internal dipolar field distribution.