On the origin of anomalous EPR peaks observed in Mn12–Ac

Abstract

We present a detailed investigation of the temperature and frequency dependence of the anomalous electron paramagnetic resonance (EPR) transitions first observed in Mn12–Ac by Hill et al. [Phys. Rev. Lett. 80 (1998) 2453]. 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. Recent studies by Amigó et al. [Phys. Rev. B 65 (2002) 172403] have attributed these additional peaks to a strain-induced transverse quadratic anisotropy which gives rise to distinct Mn12–Ac site symmetries, 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–Ac. 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. These findings compare favorably with available neutron scattering data.