Probing spin frustration in high-symmetry magnetic nanomolecules by inelastic neutron scattering

Low temperature inelastic neutron scattering studies have been performed to characterize the low energy magnetic excitation spectrum of the magnetic nanomolecule $\left\{{\mathrm{Mo}}_{72}{\mathrm{Fe}}_{30}\right\}$. This unique highly symmetric cluster features spin frustration and is one of the largest discrete magnetic molecules studied to date by inelastic neutron scattering. The $30\mathrm{s}=5∕2$ ${\mathrm{Fe}}^{\mathrm{III}}$ ions, embedded in a spherical polyoxomolybdate molecule, occupy the vertices of an icosidodecahedron and are coupled via nearest-neighbor antiferromagnetic interactions. The overall energy scale of the excitation and the gross features of the temperature dependence of the observed neutron scattering are explained by a quantum model of the frustrated spin cluster. However, no satisfactory theoretical explanation is yet available for the observed magnetic field dependence.

Figure 1

(Color online) (a) The 30 ${\mathrm{Fe}}^{\mathrm{III}}$ ions in $\left\{{\mathrm{Mo}}_{72}{\mathrm{Fe}}_{30}\right\}$ occupy the vertices of an icosidodecahedron. In a classical description at $0\mathrm{K}$ the 30 spins can be divided into three sublattices each of 10 parallel spins. There is a relative angle of $120°$ for any pair of nearest-neighbor spin vectors. (b) Lowest energy levels calculated from a solvable quantum rotational band model (Ref. 7). Allowed transitions for the $S=0$ ground state to states with $S=1$ are indicated. The energy levels of the first excited rotational band are shown shaded (schematic only) to reflect the fact that the high degeneracy of these levels is partially lifted for the realistic nearest-neighbor Heisenberg model of $\left\{{\mathrm{Mo}}_{72}{\mathrm{Fe}}_{30}\right\}$.

Figure 2

(Color online) (a) Inelastic neutron scattering spectrum at a nominal temperature of $65\mathrm{mK}$. The intensity axis is on a logarithmic scale. The solid line shows a best estimate of the nonmagnetic background (see text). Inset: the background subtracted scattering. (b) Raw data for several different temperatures plotted on a linear scale.

Figure 3

(Color online) Temperature dependence of the inelastic neutron scattering, plotted with the nonmagnetic background subtracted. Inset, theoretical scattering for several temperatures, calculated using the quantum rotational band model with simplifying assumptions (see text and Ref. 20).

Figure 4

(Color online) Intensities versus neutron energy-loss showing the effect of a $7\mathrm{Tesla}$ magnetic field on the main excitation. In an applied magnetic field, the intensity of the main excitation peak decreases and two side-bands at lower and higher energies are observed. The solid line describes the convolution of the three shaded bands for the $7\mathrm{Tesla}$ data. Inset, linear field dependence of the high energy side-band.