Single crystal EPR determination of the spin Hamiltonian parameters for Fe8 molecular clusters

Abstract

We use a multi-high-frequency resonant cavity technique to obtain EPR spectra 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 the magnetic field parallel to the easy axis, we can directly measure the g z -value. Measurements performed with the magnetic field parallel to the intermediate and hard axes may be used to constrain further the Hamiltonian parameters. Our results are in broad agreement with recent inelastic neutron scattering data. In addition, analysis of individual resonances (which we can assign to known transitions) reveals a pronounced M S dependence of the resonance line widths. Furthermore, the line positions exhibit complex (again M S dependent) temperature dependences that cannot be reconciled with the standard spin Hamiltonian.

Detailed single-crystal EPR line shape measurements for the single-molecule magnetsFe8BrandMnHill1, Maccagnano2, Park3 et al. 2002Phys. Rev. B1188771View full textAdd to dashboardCite

“…Indeed, even for aligned polycrystals, significant discrepancies can be noted between the observed and calculated EPR transitions, with respect to both the peak positions and intensities, as well as the line shapes and widths [33]. As an example, it has been reported in [33] that the linewidths in Fe 8 Br approximately scale as M 2 S , whereas our earlier studies [29,37] have shown a dominant linear dependence of the linewidths on M S , both for Fe 8 Br and Mn 12 −ac. This fact, along with the asymmetric lineshapes observed in high field EPR studies, may account for differences in the spin Hamiltonian parameters deduced by single-pass (i.e.…”

“…Orientation of Mn 12 −ac crystal was straightforward, due to the fact that its easy axis is defined by the needle axis of the sample. Alignment of the Fe 8 Br crystal necessitated measurements at many angles [29]. All of the data presented in this paper were obtained with the DC magnetic field aligned approximately parallel to the samples' easy axes (hard axis data are presented elsewhere [26,29]).…”

“…An important breakthrough in the spectroscopy of SMMs has been the development of a sensitive cavity perturbation technique for carrying out multi-high-frequency (40 to > 200 GHz) EPR measurements on single crystals [26,27,28,29,30]. In particular, this technique leads to at least a three orders of magnitude improvement in detection sensitivity [28] relative to conventional single-pass multi-high-frequency EPR techniques [8,31,32,33,34,35,36], which opens up many new avenues for probing SMMs.…”

“…In particular, this technique leads to at least a three orders of magnitude improvement in detection sensitivity [28] relative to conventional single-pass multi-high-frequency EPR techniques [8,31,32,33,34,35,36], which opens up many new avenues for probing SMMs. To date, our single crystal investigations have focused primarily on the energy dependence of the positions in magnetic field of EPR absorptions corresponding to transitions between different spin quantum levels [26,27,28,29].…”

“…This fact, along with the asymmetric lineshapes observed in high field EPR studies, may account for differences in the spin Hamiltonian parameters deduced by single-pass (i.e. cavity-less) high-field EPR, as compared to other techniques [16,26,28,29]. The zero field coherent time-domain THz technique employed by Parks et al [36], is limited in energy resolution (resolution is a fifth of the broadest EPR line width), as well as sensitivity.…”