Anisotropy and two-dimensional effects in the ESR properties of OsF6-graphite intercalation compounds.

Abstract

Systematic ESR and susceptibility studies were carried out on highly oriented pyrolytic graphite intercalated with ${\mathrm{OsF}}_{6}$. For all stages and ${\mathrm{OsF}}_{6}$ concentrations the ESR results show anisotropic g values, thermal broadening, and residual widths. The anisotropic g value is explained assuming a spin Hamiltonian with an effective spin, S=(3/2), appropriate to the quartet ground state of the 5${d}^{3}$ configuration of ${\mathrm{Os}}^{5+}$ in a slightly distorted octahedral symmetry. The susceptibility measurements support this identification and yield zero-field splitting parameters D=12\ifmmode\pm\else\textpm\fi{}5 K for stage I and D=30\ifmmode\pm\else\textpm\fi{}10 K for higher stages. The differences may be partially correlated with variation of the ``sandwich'' thickness of these compounds. The charge transfer per intercalant species is one electron\char22{}independent of stage. The anisotropic thermal broadening is attributed to a Korringa-like mechanism due to anisotropic exchange interaction between the localized 5d and the \ensuremath{\pi}-like mobile electrons. Using the theory of Redfield, we have developed equations for the ESR thermal broadening under conditions of anisotropic exchange and g values. Comparison with experimental data suggests almost isotropic 5d-\ensuremath{\pi} exchange interaction for stage I, but some anisotropy for higher stages. The data suggest that the density of carriers at the Fermi level is almost stage independent. By using second-moment calculations the angular variation of the residual width can be explained by two-dimensional dipolar interaction as well as small anisotropy in the ion-ion spin exchange interaction.