Single crystal EPR determination of the spin Hamiltonian parameters for Fe-8 molecular clusters

We use a multi-high-frequency resonant cavity technique to obtain EPR spect ra for single crystal samples of the biaxial molecular magnet Fe-8(tacn)(6) O-2(OH)(12)]Br-8. 9H(2)O (Fe-8). By performing measurements at many closely spaced frequencies, we are able to extrapolate data back to zero magnetic field and, thereby, obtain accurate estimates of the zero-field splittings. Furthermore, from the (low-) field dependence of these splittings, with th e magnetic field parallel to the easy axis, we can directly measure the g(z )-value. Measurements performed with the magnetic field parallel to the int ermediate and hard axes may be used to constrain further the Hamiltonian pa rameters. Our results are in broad agreement with recent inelastic neutron scattering data. In addition, analysis of individual resonances (which we c an assign to known transitions) reveals a pronounced M-s dependence of the resonance line widths. Furthermore, the line positions exhibit complex (aga in M-s dependent) temperature dependences that cannot be reconciled with th e standard spin Hamiltonian. (C) 2001 Elsevier Science Ltd. All rights rese rved.