E. M. Rumberger

Magnetization tunneling in single-molecule magnets

The quantum mechanical tunneling of the direction of magnetization is discussed for several examples of single-molecules magnets (SMM’s). SMM’s are molecules that function as nanomagnets. Magnetization tunneling is described for two crystallographically different forms of [Mn12O12(O2CC6H4-p-Me)16(H2O)4] solvate. The two Mn12 complexes are isomers that both differ in the positioning of the H2O and carboxylate ligands and also in the orientations of the Jahn–Teller elongation at the Mn III ions. The magnetization versus magnetic field hysteresis loop is quite different for the two isomeric Mn12 complexes. One Mn12 complex exhibits a magnetization hysteresis loop that is characteristic of considerably faster magnetization tunneling than in the other Mn12 isomer. The lower symmetry and greater rhombic zero-field splitting are the origin of the faster magnetization tunneling. Frequency-dependent ac magnetic susceptibility and dc magnetization decay data are presented to characterize the magnetization relaxation rate versus temperature responses of three mixed-valence Mn4 complexes. In all three cases, the Arrhenius plot of the logarithm of the magnetization relaxation rate versus the inverse absolute temperature shows a temperature-dependent region as well as a temperature-independent region. The temperature-independent magnetization rate is definitive evidence of magnetization tunneling in the lowest-energy zero-field component of the ground state.

Symmetry of Magnetic Quantum Tunneling in Single Molecule Magnet Mn12-Acetate

The symmetry of magnetic quantum tunneling has been studied in the prototype single molecule magnet Mn12-acetate using a micro-Hall effect magnetometer and superconducting high field vector magnet system. An average crystal fourfold symmetry is shown to be due to local molecular environments of twofold symmetry that are rotated by 90 degrees with respect to one another, confirming that disorder which lowers the molecule symmetry is as important to magnetic quantum tunneling. We have studied a subset of these lower (twofold) site symmetry molecules and present evidence for a Berry phase effect consistent with a local twofold symmetry.

Distribution of Tunnel Splittings in Mn12 Acetate

In magnetic fields applied parallel to the anisotropy axis, the relaxation of the magnetization of Mn(12)-acetate measured for different sweep rates collapses onto a single scaled curve. The form of the scaling implies that the dominant symmetry-breaking process responsible for tunneling is a locally varying second-order transverse anisotropy, forbidden by tetragonal symmetry in the perfect crystal, which gives rise to a broad distribution of tunnel splittings in a real crystal of Mn(12) acetate. Different forms applied to even- and odd-numbered steps provide a clear distinction between even resonances (associated with crystal anisotropy) and odd resonances (which require a transverse magnetic field).

Linewidth of single-photon transitions inMn12-acetate

We use time-domain terahertz spectroscopy to measure the position and linewidth of single photon transitions in Mn

Two new hexanuclear iron(III) complexes with S 5 ground states

_{12}

Characterisation of nanoscopic (Mn12O12(O2CR)16(H2O)4) single-molecule magnets: physicochemical properties and LDI- and MALDI-TOF mass spectrometry{

-acetate. This linewidth is compared to the linewidth measured in tunneling experiments. We conclude that local magnetic fields (due to dipole or hyperfine interactions) cannot be responsible for the observed linewidth, and suggest that the linewidth is due to variations in the anisotropy constants for different clusters. We also calculate a lower limit on the dipole field distribution that would be expected due to random orientations of clusters and find that collective effects must narrow this distribution in tunneling measurements.

Mn12-acetate: a prototypical single molecule magnet

The synthesis and magnetic properties of two new hexanuclear iron complexes [Fe6O2(OH)2(O2CR)10L2] (R = But (3), Me (4); LH = 2-(2-hydroxyethyl)pyridine (hepH) (3), 6-methyl-2-(hydroxymethyl)pyridine (Me-hmpH) (4)) are reported. Both compounds are prepared by treatment of [Fe3O(O2CR)6(H2O)3]+ with three equivalents of LH in MeCN. The X-ray crystal structure of 3·2CHCl3·2H2O is presented. It consists of a planar array of six Fe3+ ions comprising two [Fe3(μ3-O)] subunits that are related by an inversion centre and linked at two of their apices, each linkage consisting of one μ-hydroxo and two μ-carboxylato groups. DC magnetic susceptibility measurements at 1.0 and 0.10 Tesla in the 2.0–300 K range show an increase in the effective magnetic moment with decreasing temperature, corresponding to a high spin (S) ground state. The spin of the ground state was established by magnetization measurements in the 1.0–7.0 T field range and 1.7–4.0 K temperature range. Fitting of the reduced magnetization data by full matrix diagonalization, incorporating both axial and rhombic anisotropy, gave S = 5, g = 1.96, D = 0.46 cm−1 and |E| = 0.046 cm−1 for 3, and S = 5, g = 2.07, D = 0.27 cm−1 and |E| = 0 cm−1 for 4. Alternative fits with a negative ZFS were rejected based on their relative fitting error as well as on measurements of the magnetization relaxation behaviour of the complexes at very low temperature (≥0.04 K), where no hysteresis characteristic of a single-molecule magnet was observed.

Photon-induced magnetization reversal in the Fe8 single-molecule magnet

The syntheses of the two new single molecule magnets, [Mn12O12(O2CR)16(H2O)4]S (R = CHCHCH3, S = H2O (2) and R = C6H4C6H5, S = 2C6H5C6H4COOH (3)) and X-ray crystal structure of the first are described. Complex 2 crystallizes in the orthorhombic space group Ibca, which at 198 K has a = 21.2208(4), b = 21.2265(4), c = 42.2475(6) A, and Z = 8. Frequency-dependent out-of-phase ac signals are seen for both complexes, which indicates that these two complexes function as single-molecule magnets. Dc magnetization measurements for both complexes also exhibit hysteresis in the magnetization vs. external magnetic field plots with regular steps characteristic of quantum mechanical tunnelling of the direction of magnetization. The techniques of LDI- and MALDI-TOF mass spectrometry have been investigated to prospect their utility for the chemical characterization of Mn12 clusters. The technique is applied to known clusters as well as to two new compounds, and characteristic signals are found, especially predominant being that of the [Mn12O12(O2CR)14] cation or anion, showing the potential interest of this technique for those preparing this type of compounds.

First row wheels, {(tBu3SiS)MX}12(M = Co, X = Cl; M = Ni, X = Br), are common amongst both simpler and more complex aggregates

Abstract Single molecule magnets display fascinating quantum mechanical behavior, and may have important technological applications for information storage and quantum computation. A brief review is given of the physical properties of Mn12-acetate, one of the two prototypical molecular nanomagnets that have been most intensively investigated. Descriptions and discussions are given of the Mn12 magnetic cluster and the fundamental process of quantum tunneling of a nanoscopic spin magnetization; the distinction between thermally-assisted tunneling and pure quantum tunneling, and a study of the crossover between the two regimes; and a review of earlier investigations that suggest that the tunneling in this system is due to locally varying second-order crystal anisotropy which gives rise to a distribution of tunnel splittings. In the second part of the paper, we report results obtained by a new experimental method that confirm our earlier conclusion that the tunnel splittings in Mn12 are distributed rather than single-valued, as had been generally assumed.

Tunneling splittings in Mn12-acetate single crystals

We use millimeter-wave radiation to manipulate the populations of the energy levels of a single crystal of molecular magnet Fe8. When continuous-wave radiation is in resonance with the transition from the ground state to the first excited state, the equilibrium magnetization exhibits a peak or dip whose field position varies linearly with the radiation frequency. Our results provide a lower bound of 0.17 ns for transverse relaxation time and suggest the possibility that single-molecule magnets might be utilized for quantum computation.