Abstract
The nature of the magnetic ground state of the spin-frustrated molecular magnet {Mo72Fe30} is studied using polarized neutron scattering and specific heat methods. The magnetic scattering cross section up to 2.30 Å−1 has been determined by means of diffuse neutron scattering with xyz polarization analysis at temperatures from 1.5 to 100 K. The spin correlations observed at 1.5 K agree well with the simulation based on the three-sublattice spin configuration model for {Mo72Fe30}. The specific heat of {Mo72Fe30} has been measured from 60 mK to 15 K under zero external magnetic field. The energy gaps, 0.09(1) and 0.64(1) meV, identified by the Schottky-type anomalies in the specific heat data are in reasonable agreement with the low-lying magnetic excitations predicted by the quantum rotational band model for the three-sublattice spin configuration model. Therefore, our experimental study provides strong support that the three-sublattice spin configuration model is a good approach to the magnetic ground state of {Mo72Fe30}.
Export citation and abstract BibTeX RIS
GENERAL SCIENTIFIC SUMMARY Introduction and background. Molecular magnets have attracted intense interest because of their potential applications in future quantum computing. The polyoxomolybdate {Mo72Fe30} is one of the largest molecular magnets that have been synthesized. It has a highly symmetric magnetic structure, in which the 30 Fe3+ ions occupy the vertices of an icosidodecahedron. Antiferromagnetic exchange together with a Kagome-lattice type interconnectivity leads to a high degree of geometrical frustration. A classical three-sublattice spin configuration has been suggested to describe the magnetic ground state of {Mo72Fe30}. However, the experimental support of this three sublattice spin model remains very indirect, as long as there exist no direct observations of the magnetic ground state of the {Mo72Fe30} molecule.
Main results. In our paper, the spin correlations of {Mo72Fe30} were measured by means of diffuse neutron scattering with polarization analysis at various temperatures. The spin correlations observed at 1.5 K agree well with the simulation based on the three-sublattice spin configuration model for {Mo72Fe30}. The low-lying magnetic excitations are studied by low-temperature specific heat. Schottky anomalies with their unique shape for two-state systems are observed and serve as a clear signature to identify the energy gaps, which are consistent with the prediction from the three-sublattice spin model.
Wider implications. The present paper provides direct and strong support for the validity of the three-sublattice spin model in describing the magnetic ground state of {Mo72Fe30}. It may have significance in studying other complex geometrical spin-frustrated systems by means of the polarized neutron scattering technique.
Figure. Magnetic diffuse scattering (red circles) at 1.5 K in absolute unit for {Mo72Fe30}, together with the simulation of the spin pair-correlations for one {Mo72Fe30} molecule under the three-sublattice spin configuration model (blue line). Black line: magnetic form factor of free Fe3+ ion (S=5/2). Green dashed line: 91% scaled simulation curve. Insert: {Mo72Fe30} molecule with 30 Fe3+ spins in the three-sublattice model.