Mössbauer study of the hyperfine interactions and spin dynamics inα-iron(II) phthalocyanine

Abstract

The $^{57}\mathrm{Fe}$ M\"ossbauer spectroscopy on $\ensuremath{\alpha}$-iron(II) phthalocyanine (FePc) as a function of temperature $(1.3<\mathrm{T}<295\phantom{\rule{0.3em}{0ex}}\mathrm{K})$ and applied field $(0<\mathrm{B}<10\phantom{\rule{0.3em}{0ex}}\mathrm{T})$ has been used to study the peculiar magnetic properties of this ferromagnetic quasilinear chain type compound. One sextet with an internal hyperfine field ${B}_{\mathrm{int}}=66.2\phantom{\rule{0.3em}{0ex}}\mathrm{T}$ was observed at $1.3\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, a very large value for a bivalent iron with $S=1$ pointing to the existence of large positive orbital and dipolar contributions in the investigated FePc. Under an applied field, the experimental spectra exhibited two nonequivalent Fe positions, due to spin canting, with the values for the hyperfine fields of the split sextets increasing with increasing field, an indication that unlike most cases, ${B}_{\mathrm{int}}$ in $\ensuremath{\alpha}\text{\ensuremath{-}}\mathrm{Fe}\mathrm{Pc}$ is positive, i.e., parallel to the magnetic moment of iron. Therefore, the origin of the large hyperfine field is the orbital moment rather than the Fermi's contact interaction. This fact is ascribed to the orbital degeneracy of the ground state of Fe(II) in the present configuration, where an unpaired hole occupies the orbital doublet (${d}_{xz}$, ${d}_{yz}$). This feature supports and explains the magnetization and susceptibility data as well as the anomalously high hyperfine field observed at $^{57}\mathrm{Fe}$ nucleus. The relaxational behavior in the ac susceptibility and M\"ossbauer spectra found in the region $5--20\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ was ascribed to solitonlike motion of domain walls within the magnetic chains, with a single-kink activation energy of $72\phantom{\rule{0.3em}{0ex}}\mathrm{K}$.