Static and dynamic magnetic properties of an $\left[{\mathrm{Fe}}_{13}\right]$ cluster

The static and dynamic magnetic properties of an $\left[{\mathrm{Fe}}_{13}\right]$ cluster were investigated using several experimental techniques. The cluster crystallizes in a cubic space group, but careful investigation of the crystallographic data revealed that the symmetry is distorted locally. dc magnetic susceptibility measurements showed the presence of competing antiferromagnetic isotropic exchange interactions leading to a high-spin ground state and many low-lying excited spin states, which was confirmed by inelastic neutron scattering measurements. From high-field electron paramagnetic resonance measurements a small zero-field splitting of the spin ground state was inferred, which supports the local symmetry distortion found in the crystallographic studies. The spin dynamics slows down at sub-kelvin temperatures, where ac susceptibility measurements indicated that part of the sample shows superparamagnetic-like behavior and the other part relaxes through magnetization tunneling. The Mössbauer data confirmed the slowing down of the spin dynamics, indicating that this occurs mainly in the peripheral spins.

Figure 1

View along the [111] axis, showing one pyridinium cation and the three surrounding clusters, emphasizing the crystal packing. The big spheres in the clusters are ${\mathrm{Fe}}^{\mathrm{III}}$ ions, the small, light spheres are oxygen atoms, and the small, dark spheres are fluoride ions. The methanolate methyl groups have been omitted for clarity.

Figure 2

Observed (●) and calculated (—) $\chi T$ values. The molar magnetic susceptibility data were recorded at an applied field of $H=0.1\mathrm{T}$. The fit was obtained with $J_1=43{\mathrm{cm}}^{-1}$ and $J_2=30{\mathrm{cm}}^{-1}$.

Figure 3

(a) Spin topology of $\left[{\mathrm{Fe}}_{13}\right]$. (b) Spin topology after neglecting one of the intratriangle interactions.

Figure 4

Plot of the energies of the various spin levels as a function of the ratio of the exchange interaction constants, $J_2∕J_1$. The inset shows the low-lying spin states in the $J_2∕J_1=0.70$ region which is the $J_2∕J_1$ ratio that results from the fit (see text). The bar indicates the energy scale for the fitted value of $J_1=43{\mathrm{cm}}^{-1}$.

Figure 5

INS spectra recorded on IN5 using incident neutrons with $\lambda =11\mathrm{\AA }$ (energy resolution $12\mu \mathrm{eV}$). Data have been collected at $T=2\mathrm{K}$ (closed circles) and $T=10\mathrm{K}$ (open circles). Solid lines represent the superposition of the elastic peak and four Gaussian line shapes centered at $\pm 0.05$ and $\pm 0.10\mathrm{meV}$. Individual components are shown by broken lines.

Figure 6

INS spectra recorded on IN5 using incident neutrons with $\lambda =6\mathrm{\AA }$ (energy resolution $60\mu \mathrm{eV}$). Data have been collected at $T=2\mathrm{K}$ (closed circles) and $T=15\mathrm{K}$ (open circles). The intensity results from the sum over the whole detector bank, covering an angular range from 14.5°–132.5°.

Figure 7

High-frequency EPR spectra recorded on a pressed powder sample of $\left[{\mathrm{Fe}}_{13}\right]$ at $\nu =285\mathrm{GHz}$ at various temperatures as indicated in the figure.

Figure 8

The real (${\chi }^{\prime }$, top) and imaginary (${\chi }^{″}$, bottom) parts of the ac susceptibility recorded in zero applied field at various driving frequencies ranging between 275 and $11451\mathrm{Hz}$ as indicated in the figure.

Figure 9

Measurement time scale as the inverse of the driving frequency as a function of the inverse temperature corresponding to the maximum in ${\chi }^{″}$ at that frequency (symbols) and fit (drawn line) according to the Arrhenius law, with ${\tau }_0=(7.5\pm 1.4)\times {10}^{-13}\mathrm{s}$, and $\Delta E=13.10\pm 0.12\mathrm{K}$.

Figure 10

Mössbauer spectra collected at (starting from the top) $T=2.55$, 3, 4, 5, 7.9, and $13\mathrm{K}$. Solid lines represent fits obtained as described in the text.

Figure 11

Spectrum components relative to the four sites, at (a) $T=2.55\mathrm{K}$; (b) $T=5\mathrm{K}$; (c) $T=13\mathrm{K}$.