Linewidth of single photon transitions in Mn12_{12}-acetate

We use time-domain terahertz spectroscopy to measure the position and linewidth of single photon transitions in Mn$_{12}$-acetate. This linewidth is compared to the linewidth measured in tunneling experiments. We conclude that local magnetic fields (due to dipole or hyperfine interactions) cannot be responsible for the observed linewidth, and suggest that the linewidth is due to variations in the anisotropy constants for different clusters. We also calculate a lower limit on the dipole field distribution that would be expected due to random orientations of clusters and find that collective effects must narrow this distribution in tunneling measurements.Comment: 5 pages, accepted to Physical Review

Radiofrequency and Mössbauer Spectra of a Paramaghetic Ion in the Presence of Hyperfine Interactions and of Tunnel Effect

The influence of a coherent tunnel effect on the magnetic hyperfine structure of a diluted paramagnetic ion, with an electronic ground state $|S_z=\pm S>(S > 1/2)$, is studied under the assumptions of zero longitudinal magnetic field and slow electronic relaxation. It is shown that the radiofrequency and Mössbauer spectra should be altered, with a change in the number and intensities of the lines. When the tunnelling parameter, $\Gamma$, is large compared to the hyperfine structure, the magnetic hyperfine structure splitting should be strongly reduced and it should vanish for $\Gamma \rightarrow \infty$. Tunnelling effects have already been observed in EPR experiments on ${\rm Tb}^{3+}$ diluted in diamagnetic matrices. The cases of NMR with $I=3/2$ and of Mössbauer spectroscopy for a transition $I_{\rm e}=3/2\rightarrow I_{\rm g}=1/2$ are considered

A simple Derivation of the Tunneling Splitting for Large Quantum Spins

With the help of a pertubation treatment to lowest order, general expressions for the tunneling splitting have been derived for a quantum spin $S$. This spin is assumed to have a main axial Hamiltonian (without applied longitudinal magnetic field), and to be submitted to perturbations with orthorhombic, tetragonal, trigonal or hexagonal symmetries, or to a transverse field. The formulas obtained are valid for general (not very small) $S$. They are compared with available results of the literature. Previous observations of tunneling splittings in terbium compounds by EPR are mentioned, with the relevant references

Effets du couplage spin-orbite sur les propriétés des composés magnétiques ioniques du groupe du fer à température de transition basse

Nous rassemblons ici un certain nombre de résultats relatifs à l'effet du couplage spin-orbite sur le niveau fondamental des ions du groupe du fer dans un cristal. A l'aide de quelques exemples simples, nous montrons l'importance de ces considérations dans les études de structures magnétiques