Discrete easy-axis tilting in Mn12-acetate, as determined by EPR: implications for the magnetic quantum tunneling mechanism

Details Discrete easy-axis tilting in Mn12-acetate, as determined by EPR: implications for the magnetic quantum tunneling mechanism Discrete easy-axis tilting in Mn12-acetate, as determined by EPR: implications for the magnetic quantum tunneling mechanism

2004-01-26

arxiv.org/abs/cond-mat/0401515v2

Physics

Condensed Matter

Mesoscale and Nanoscale Physics

39 pages, including 13 low-resolution figures, Phys. Rev. B (Sept. 1, 2004)

Scientific paper

The variation with microwave frequency and temperature of previously reported anomalous peaks in the EPR spectra of Mn12-acetate, under large transverse fields, reveals that the molecular easy magnetization axes are tilted with respect to the global symmetry direction. More importantly, on the basis of the angle-dependence of fine structures observed in the EPR spectra we infer that the tilt distribution must be discrete, as was previously suspected from studies which demonstrated the presence of a locally varying rhombic anisotropy [S. Hill et al., Phys. Rev. Lett. 90, 217204 (2003)]. The tilts are confined to two orthogonal planes, and the distribution extends up to ~1.7 degrees away from the the global easy (z-) axis. We ascribe the tilting to the hydrogen-bonding effect associated with the disordered acetic acid solvent molecules. The effect is considerably larger than deduced from x-ray diffraction analyses. These data constitute the sought-after evidence for the presence of transverse fields in Mn12-acetate, and provide a possible explanation for the lack of selection rules in the resonant quantum tunneling behavior seen in low-temperature hysteresis experiments for this S = 10 system.

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