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

Abstract

The variation with microwave frequency and temperature of previously reported anomalous peaks in the EPR spectra of Mn 12 -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 ϳ1.7°degrees away from the global easy ͑z͒ 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 Mn 12 -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.

Homochiral Mn3+ Spin-Crossover Complexes: A Structural and Spectroscopic Study

“…We therefore believe that the powder sample also contains multiple S = 2 species with a small spread in ZFS parameters; 118 to reflect this, we employ broader/lighter simulated curves in Figure 5 . The existence of different species may be due to distinct sites within the lattice of an individual crystal, 119 or they could be due to crystal-to-crystal variations within the powder. A conservative analysis indicates the following axial ZFS parameter, D = +6.9(3) cm –1 , and a much more significant spread in E , with values from 0.17 to 0.63 cm –1 needed to reproduce all observed resonances (see Figures 5 and S3–S5 ).…”

The Properties of the [Mn12O12(O2CR)16(H2O)4] Single-Molecule Magnets in Truly Axial Symmetry: [Mn12O12(O2CCH2Br)16(H2O)4]·4CH2Cl2

“…It has, however, now become apparent that rather than 1 being a paragon of a Mn 12 SMM, it is instead a mixture of six isomers differing in the number n ( n = 0−4) of OH···O hydrogen-bonding interactions with the lattice MeCO 2 H molecules of crystallization, − with only two of these isomers ( n = 0 and 4) possessing S 4 symmetry. Many previous studies on 1 have thus involved studies of a mixture of species, − and the obtained data will have been affected by this to various extents, depending on the technique.…”

“…Many previous studies on 1 have thus involved studies of a mixture of species, − and the obtained data will have been affected by this to various extents, depending on the technique. In some cases, such as the use of a sensitive technique such as EPR, this manifests itself as inhomogeneously broadened 16-21 and even multiple signals corresponding to the various isomers. ,,− Of course, “one does not know what one is missing until one finds it”, and it was only the recent studies with sensitive techniques (EPR, NMR) on new Mn 12 derivatives with tetragonal symmetry, and no mixture of isomers, that finally provided an inkling of the high quality data that can be obtained for Mn 12 in truly axial symmetry and with essentially no distributions of molecular environments in the crystal. ,− …”

Terminological confusions and problems at the interface between the crystal field Hamiltonians and the zero-field splitting Hamiltonians—Survey of the CF=ZFS confusion in recent literature

“…is physically misleading in view of the origin ofH T SH . Takahashi et al [135] for Mn 12 -acetate invoked an awkward terminology: 'quadratic and quartic transverse crystal-field interactions in the spin Hamiltonian ½Ô named as 'higher order crystal field interactions'. Barra [136] invoked similar fourth and sixth-order terms named as 'higher order terms of the crystal field interaction'.…”