On the origin of anomalous EPR peaks observed in Mn 12 –Ac

Abstract

We present a detailed investigation of the temperature and frequency dependence of the anomalous electron paramagnetic resonance (EPR) transitions first observed in Mn 12 Á/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 Mn 12 Á/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 Mn 12 Á/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 S0/9 multiplet having very similar zero-field crystal parameters to the S0/10 state. These findings compare favorably with available neutron scattering data. #

Discrete easy-axis tilting inMn12-acetate, as determined by EPR: Implications for the magnetic quantum tunneling mechanism

“…1: D = −0.455 cm −1 , B 0 4 = −2.0 × 10 −5 cm −1 and B 4 4 = ±3.2 × 10 −5 cm −1 . We note that this parameter set yields the optimum agreement with a large body of single crystal EPR data, 14,30,31,32 including the present study, which additionally considers the effects of E-strain and easy axis tilting. At this stage, we do not explicitly include lower symmetry interactions in Eq.…”

“…1(b), which compares various calculations of the ␣ and ␤ transition frequencies with actual 8 K EPR peak positions. 30,31 For these earlier experiments, data were obtained with the field applied parallel to the hard plane of a single-crystal sample to within an accuracy of ϳ1.5°͑ = 90°± 1.5°͒. Within the hard plane, a crude attempt was made to align the magnetic field along the medium fourfold axis.…”

“…The same trend has been noted for field alignment along different directions within the hard plane. [30][31][32] For comparison, Fig. 2 displays representative experimental EPR spectra obtained with the field approximately along the hard fourfold axis ͑ϳ =0°͒, also within ±1.5°of the hard/ medium plane ͑ = 90°± 1.5°͒; 32 note that the ␤ resonances are again seen to the lowest frequencies studied ͑44 GHz͒.…”

“…The ␥ resonance has been previously attributed to a small fraction ͑Ͻ5%͒ of Mn 12 molecules having lower symmetry (i.e., different crystal field parameters) than the acetate. 30 1(b). The opposite is true for the ␤ transitions.…”

Inelastic-neutron-scattering study of excited spin multiplets and low-energy phonons in theFe8nanomagnet: Implications for relaxation

“…Similar disagreement between EPR spectra and INS results occurs for the other prototype nanomagnet Mn 12 . [19][20][21][22] …”