David Zipse

Advantages of superconducting quantum interference device-detected magnetic resonance over conventional high-frequency electron paramagnetic resonance for characterization of nanomagnetic materials

A dc-detected high-frequency electron paramagnetic resonance (HF-EPR) technique, based on a standard superconducting quantum interference device (SQUID) magnetometer, has significant advantages over traditional HF-EPR based on microwave absorption measurements. The SQUID-based technique provides quantitative determination of the dc magnetic moment as a function of microwave power, magnetic field and temperature. The EPR spectra obtained do not contain variability in the line shape and splittings that are commonly observed in the standard single-pass transmission mode HF-EPR. We demonstrate the improved performance by comparing EPR spectra for Fe8 molecular nanomagnets using both SQUID-based and conventional microwave-absorption EPR systems.

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.

Resonant microwave power absorption and relaxation of the energy levels of the molecular nanomagnet Fe8 using superconducting quantum interference device-based magnetometry

Energy levels and saturation of molecular nanomagnet Fe8 crystals were investigated using a 95 and 141 GHz electron paramagnetic resonance (EPR) technique based on a standard superconducting quantum interference device (SQUID) magnetometer. The technique provides quantitative determination of the dc magnetic moment as a function of microwave power, magnetic field, and temperature.

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.

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.

An Fe4 cluster with a slowly relaxing paramagnetic excited state: [Fe4O(OH)5(tacn)4]I7⋅2.5 H2O

We report the synthesis, x-ray structure, and magnetic susceptibility of a magnetic compound [Fe4O(OH)5(tacn)4]I7⋅2.5 H2O (Fe4), where tacn=1, 4, 7 triazacyclononane. Fe4 crystallizes in the monoclinic space group, P2(1)/c, with cell parameters a=18.7604(9) A, b=12.5840(6) A, c=24.064(1) A, α=90°, β=108.783(1)°, and γ=90°. Dc susceptibility, χdc, experiments show the magnetic moment of Fe4 saturates at approximately 0.2 μB, giving a spin ground state of S=0. However, on application of dc fields, a peak develops in χdc, which is ascribed to the presence of a low lying excited magnetic state. Interestingly, ac susceptibility experiments show frequency dependence of χ″ and thermally activated magnetization switching dynamics, with a potential energy barrier of 26 K. The origin of these observations still remains unexplained.