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

TL;DR

This study uses EPR spectroscopy to reveal that the easy-axis tilts in Mn12-acetate are discrete and confined to two planes, providing insights into the magnetic quantum tunneling mechanism and the role of transverse fields.

Contribution

It demonstrates that the easy-axis tilts are discrete and confined to two planes, linked to hydrogen-bonding effects, explaining quantum tunneling behavior in Mn12-acetate.

Findings

Tilt distribution extends up to ~1.7 degrees from the easy axis.

Tilts are confined to two orthogonal planes.

Evidence for transverse fields affecting quantum tunneling.

Abstract

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.