Magnetic Quantum Tunneling in the Single-Molecule Magnet Mn12-Acetate

Abstract

The symmetry of magnetic quantum tunneling (MQT) in the single molecule magnet Mn12-acetate has been determined by sensitive low-temperature magnetic measurements in the pure quantum tunneling regime and high frequency EPR spectroscopy in the presence of large transverse magnetic fields. The combined data set definitely establishes the transverse anisotropy terms responsible for the low temperature quantum dynamics. MQT is due to a disorder induced locally varying quadratic transverse anisotropy associated with rhombic distortions in the molecular environment (2 nd order in the spin-operators). This is superimposed on a 4 th order transverse magnetic anisotropy consistent with the global (average) S4 molecule site symmetry. These forms of the transverse anisotropy are incommensurate, leading to a complex interplay between local and global symmetries, the consequences of which are analyzed in detail. The resulting model explains: (1) the observation of a twofold symmetry of MQT as a function of the angle of the transverse magnetic field when a subset of molecules in a single crystal are studied; (2) the non-monotonic dependence of the tunneling probability on the magnitude of the transverse magnetic field, which is ascribed to an interference (Berry phase) effect; and (3) the angular dependence of EPR absorption peaks, including the fine structure in the peaks, among many other phenomena. This work also establishes the magnitude of the 2 nd and 4 th order transverse anisotropy terms for Mn12-acetate single crystals and the angle between the hard magnetic anisotropy axes of these terms. EPR as a function of the angle of the field with respect to the easy axis (close to the hard-medium plane) confirms that there are discrete tilts of the molecular magnetic easy axis from the global (average) easy axis of a crystal, also associated with solvent disorder. The latter observation provides a very plausible explanation for the lack of MQT selection rules, which has been a puzzle for many years.

Transverse anisotropy in the mixed-valent Mn2IIMn4IIIMn3IV single-molecule magnet

“…It is well documented that lowfield data ͑especially extrapolations to B =0͒ obtained for fields close to the easy axis are insensitive to transverse anisotropy terms. 8 As can be seen from Table I, the obtained axial parameters agree very well with previous magnetic and spectroscopic measurements. 4,5 Rotation about a single axis guarantees field-alignment in the hard plane, although the orientation of the field within the hard plane is not known.…”

“…It is well documented that HFEPR measurements with B Ќ c provide information concerning transverse terms. 8 In the following analysis, we constrain the axial terms ͑D and B 4 0 ͒ on the basis of the simulations in Fig. 2.…”

Effects of quantum mechanics on the deflagration threshold in the molecular magnetMn12acetate

“…The magnetic properties of the Mn 12 -Ac single crystals were found to be similar to those previously published. 4,6,[14][15][16] We used a relatively large crystal of Mn 12 -Ac that was about 2 mm long and 0.5 mm wide. Smaller crystals did not show magnetic avalanches on sweeping the magnetic field due to rapid escape of heat through the surface ͑i.e., the magnetization relaxes slowly with increasing magnetic field before the avalanche condition is achieved͒.…”

Evolution of thermodynamic properties and inelastic neutron scattering intensities for spin-12antiferromagnetic quantum rings

“…Nanomagnetism continues to be a highly-active research area in condensed matter physics due to the potential for spintronic devices through the coupling of electronic and magnetic degrees of freedom [1][2][3][4][5][6][7][8][9][10][11][12][13] . The interactions in nanomagnetic systems range from long-range interactions that produce spin-waves and skyrmionic orders to short-range magnetic cluster that exhibit discrete excitations.…”