J. M. North

Definitive spectroscopic determination of the transverse interactions responsible for the magnetic quantum tunneling in Mn(12)-acetate.

We present detailed angle-dependent single crystal electron paramagnetic resonance (EPR) data for field rotations in the hard plane of the S=10 single molecule magnet Mn(12)-acetate. A clear fourfold variation in the resonance positions may be attributed to an intrinsic fourth-order transverse anisotropy (O(4)/(4)). Meanwhile, a fourfold variation of the EPR line shapes confirms a recently proposed model wherein disorder associated with the acetic acid of crystallization induces a locally varying quadratic (rhombic) transverse anisotropy [O (2)/(2) identical with E(S (2)/(x)-S(2)/(y))]. These findings explain most aspects of the magnetic quantum tunneling observed in Mn(12)-acetate.

A Raman study of the single molecule magnet Mn12-acetate and analogs

Abstract Recent theoretical studies suggest that low-lying Mn–O vibrations play a significant role in the mechanism of quantum tunneling of magnetization (QTM) exhibited by single molecule magnets like Mn 12 -acetate (Mn 12 ). We report a Raman study of Mn 12 , deuterated Mn 12 , both with S =10, and of Mn 8 Fe 4 isostructural, with Mn 12 but with S =2 ground state. Comparison of the detected mode frequencies with theoretical predictions and earlier infrared data supports their role in the spin–vibron interactions and the mechanism of anisotropy and QTM in Mn 12 .

Semiconductive and photoconductive properties of the single-molecule magnetsMn12-acetate andFe8Br8

Resistivity measurements are reported for single crystals of Mn 1 2 -acetate and Fe 8 Br 8 . Both materials exhibit a semiconductor-like, thermally activated behavior over the 200-300 K range. The activation energy E a obtained for Mn 1 2 -acetate was 0.37′0.05 eV, which is to be contrasted with the value of 0.55 eV deduced from the earlier reported absorption edge measurements and the range of 0.3-1 eV from intramolecular density of states calculations. assuming 2E a =E g , the optical band gap. For Fe 8 Br 8 , E a was measured as 0.73 ′0.1 eV, and is discussed in light of the available approximate band structure calculations. Some plausible pathways are indicated based on the crystal structures of both lattices. For Mn 1 2 -acetate, we also measured photoconductivity in the visible range; the conductivity increased by a factor of about 8 on increasing the photon energy from 632.8 nm (red) to 488 nm (blue). X-ray irradiation increased the resistivity, but E a was insensitive to exposure.

A spectroscopic comparison between several high-symmetry S=10 Mn12 single-molecule magnets

We report angle-dependent high-field electron-paramagnetic-resonance data collected for single-crystal samples of Mn12–Ac. The spectra reveal fine structures associated with various Mn12 species corresponding to different disordered local environments. Each of the fine structures exhibits a distinct dependence on the field orientation, thereby highlighting the discrete nature of the disorder. We compare these data with the spectra obtained for two recently discovered analogs of Mn12–Ac, differing only in their ligand and solvent molecules. None of the fine structures seen for Mn12–Ac are found for the recently discovered Mn12 complexes, thus confirming that the solvent significantly influences the magnetization dynamics in Mn12–Ac.

On the origin of anomalous EPR peaks observed in Mn12–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 –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 Amigo 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 –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 –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 S =9 multiplet having very similar zero-field crystal parameters to the S =10 state. These findings compare favorably with available neutron scattering data.

Semiconductivity, spin delocalization, and excited states of the single molecule magnets Fe8Br8 and Mn12-acetate (invited)

Some newly found properties of the single-molecule magnets Mn12–acetate and Fe8Br8 are summarized: they are semiconductors and their 20 unpaired (S=10) electrons are delocalized not only on the metal ions, but also quite significantly (several percent) on the organic linkers (ligands). Fe8Br8 has an S=9 excited state located at about 24 K (17 cm−1) above the S=10 ground state. The zero-field (D and E) parameters of the S=9 state differ from those of the S=10 state by 7%–8%. The band gap for Mn12–acetate was found to be 0.37 eV, while that for Fe8Br8 was 0.73 eV. Details of the experimental techniques used (EPR, NMR, isotope-labeling, four-point conductivity) are provided, together with the discussions of the results in the context of other experimental techniques, and theoretical calculations.Some newly found properties of the single-molecule magnets Mn12–acetate and Fe8Br8 are summarized: they are semiconductors and their 20 unpaired (S=10) electrons are delocalized not only on the metal ions, but also quite significantly (several percent) on the organic linkers (ligands). Fe8Br8 has an S=9 excited state located at about 24 K (17 cm−1) above the S=10 ground state. The zero-field (D and E) parameters of the S=9 state differ from those of the S=10 state by 7%–8%. The band gap for Mn12–acetate was found to be 0.37 eV, while that for Fe8Br8 was 0.73 eV. Details of the experimental techniques used (EPR, NMR, isotope-labeling, four-point conductivity) are provided, together with the discussions of the results in the context of other experimental techniques, and theoretical calculations.

Environmental factors influencing EPR in Mn12-Ac and Fe8Br

Abstract A multi-high-frequency (40–200 GHz) resonant cavity perturbation technique yields distortion-free high-field EPR spectra for oriented single crystal samples of the uniaxial and biaxial spin S =10 single molecule magnets (SMMs) Mn 12 -Ac and Fe 8 Br. We examine quantitatively the temperature dependence of the EPR linewidths and line shifts for fixed frequency measurements with an applied magnetic field along the easy axis. Simulations of the obtained experimental data take into account various environmental couplings, including intermolecular spin–spin interactions (dipolar and exchange), as well as distributions in the zero-field crystal field parameters. The temperature dependence of the linewidths and the line shifts are mainly caused by spin–spin interactions. For Fe 8 Br and Mn 12 -Ac, the calculated line shifts and linewidths agree well with the observed experimental trends. The linewidths for Fe 8 Br reveal a stronger temperature dependence than those for Mn 12 -Ac because, for the latter, a much wider distribution in D overshadows the temperature dependence of the spin–spin interactions. For Fe 8 Br, the line-shift analysis suggests two competing interactions: a weak effective ferromagnetic exchange coupling between neighboring molecules, and a longer-range antiferromagnetic dipolar interaction. For Mn 12 -Ac, a pronounced modulation of the EPR lineshapes for transverse applied fields suggests the possibility of a solvent-disorder-induced transverse anisotropy, as has recently been proposed by other groups. These findings could have implications for the mechanism of quantum tunneling of magnetization in both of these SMMs.

Raman and infrared modes of the single molecule magnet Fe8Br8 and analogs

We present Raman and infrared data for the S=10 single-molecule magnet (SMM) Fe8Br8, 17O-labeled Fe8Br8 and their analogs, Fe8Br6.4(ClO4))1.6 and Fe8Br4(ClO4))4, over a range of 100–1600 cm−1. The Raman modes were assigned through group theoretical analysis of smaller model compounds. These results could help understand the structural basis of the SMM behavior of these compounds. Additionally, Raman scattering appears to have high potential as an analytical technique for the identification of SMM analogs.

EPR and NMR characterization of theS=9 excited state and spin density distribution in the single-molecule magnet Fe8Br8: Implications to theS=10 model and magnetization tunneling pathways

High-sensitivity, variable-frequency, high-field electron paramagnetic resonance (EPR) measurements and nuclear magnetic resonance (NMR) measurements are reported for theS=10 single-molecule magnet Fe8Br8. We find that theS=10 state is nested with its first excited state, withS=9, located at only 24±2 K above. Also reported are some preliminary81Br NMR measurements of the unpaired electron spin density on the Br− sites. The results provide new insight and benchmarks for improved electronic and magnetic structural calculations and macroscopic tunneling pathways of this class of single-molecule magnets.

Single Crystal High Frequency Cavity-based EPR Spectroscopy of Single Molecule Magnets

We report high frequency electron paramagnetic resonance (EPR) investigations of a series of high spin (total spin up to S = 10) manganese and nickel complexes which have been shown to exhibit single molecule magnetism, including low temperature (below - IK) hysteresis loops and resonant magnetic quantum tunneling. A cavity perturbation technique enables high sensitivity oriented single crystal EPR measurements spanning a very wide frequency range (16 to 200+ GHz). Fitting of the frequency and field orientation dependence of EPR spectra allows direct determination of the effective spin Hamiltonian parameters. Studies on a range of materials with varying (approximately axial) site symmetries facilitates an assessment of the role of transverse anisotropy (terms in the Hamiltonian that do not commute with S,) in the magnetic quantum tunneling phenomenon.