Quantum dynamics of the Néel vector in the antiferromagnetic molecular wheelCsFe8

Abstract

The inelastic neutron scattering (INS) spectrum is studied for the antiferromagnetic molecular wheel $\mathrm{Cs}{\mathrm{Fe}}_{8}$, in the temperature range $2--60\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, and for transfer energies up to $3.6\phantom{\rule{0.3em}{0ex}}\mathrm{meV}$. A qualitative analysis shows that the observed peaks correspond to the transitions between the $L$-band states, from the ground state up to the $S=5$ multiplet. For a quantitative analysis, the wheel is described by a microscopic spin Hamiltonian (SH), which includes the nearest-neighbor Heisenberg exchange interactions and uniaxial easy-axis single-ion anisotropy, characterized by the constants $J$ and $D$, respectively. For a best-fit determination of $J$ and $D$, the $L$ band is modeled by an effective SH, and the effective SH concept extended such as to facilitate an accurate calculation of INS scattering intensities, overcoming difficulties with the dimension of the Hilbert space. The low-energy magnetism in $\mathrm{Cs}{\mathrm{Fe}}_{8}$ is excellently described by the generic SH used. The two lowest states are characterized by a tunneling of the N\'eel vector, as found previously, while the higher-lying states are well described as rotational modes of the N\'eel vector.