Evidence for a very low-lying S = 9 excited state of the S = 10 single molecule magnet Mn12-acetate

TL;DR

This study provides evidence for a low-lying S=9 excited state in Mn12-acetate, revealed through temperature-dependent EPR measurements, suggesting a new understanding of the molecule's magnetic states and their role in quantum tunneling.

Contribution

The paper identifies and characterizes a very low-lying S=9 excited state in Mn12-acetate, expanding knowledge of its magnetic energy levels beyond the well-known S=10 ground state.

Findings

Anomalous EPR peaks vanish at low temperature.

The excited state is approximately 10-15 K_B above the ground state.

Results align with neutron scattering data.

Abstract

We present a detailed investigation of the temperature and frequency dependence of the anomalous EPR transitions first observed in Mn12-acetate by Hill et al. [Phys. Rev. Lett. 80, 2453 (1998)]. The most dominant of these transitions manifest themselves as an extra series of EPR absorption peaks for spectra obtained with the DC field applied within the hard magnetic plane of a single crystal sample. Recent studies by Amigo et al. [Phys. Rev. B 65, 172403 (2002)] have attributed these extra peaks to a strain induced transverse quadratic anisotropy which gives rise to distinct Mn12-acetate species, each having a distinct EPR spectrum; on the basis of these measurements, it has been suggested that this transverse anisotropy is responsible for the tunneling in Mn12-acetate. Our temperature and frequency dependent measurements demonstrate unambiguously that these anomalous EPR absorptions vanish as the temperature tends to zero, thereby indicating that they correspond to transitions from an excited state of the molecule. We argue that this low lying excited state corresponds to an S = 9 multiplet having very similar zero-field crystal parameters to the S = 10 state, and lying only about 10-15 k_B above the S = 10 (M_S = +/-9) ground state. These findings also compare favorably with available neutron scattering data.