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single-molecule magnets

Single molecule exhibits domain behavior; e.g. magnetic hysteresis. Large ground state ... Hysteresis of Mn12-Ac, Basis of memory storage on single molecules ...

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single-molecule magnets

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    1. Single-Molecule Magnets & High-Field Electron Magnetic Resonance

    Naresh Dalal Florida State University, Tallahassee, FL SuperNet Teacher Workshop June 16-20 , 2008

    2. Outline

    What are Single-Molecule Magnets (SMM)? Advantages of High Field EMR Recent Chemical Applications Conclusions Acknowledgements

    3. What are Single-Molecule Magnets?

    Large molecular clusters containing paramagnetic transition metals Single molecule exhibits domain behavior; e.g. magnetic hysteresis Large ground state spin value Large magnetic anisotropy Quantum-mechanical behavior at the macroscopic level (steps in magnetic hysteresis loops) Exhibit quantum tunneling of the magnetization (QTM)

    6. Mn12-Ac

    Mn3+ (3d4): S=2 Mn4+ (3d3): S=3/2 8Mn3+ for S=16 ? 4Mn4+ for S=6 ? Total 16-6; S=10

    7. Hysteresis of Mn12-Ac, Basis of memory storage on single molecules

    Hysteresis results from individual molecules Steps indicate QTM Tunneling occurs at regular field intervals Friedmann et al. P.R.L. 76, 3830 (96)

    8. Fe8Br8

    Eight Fe3+ ions antiferromagnetically coupled J1-3 >> J1-5 > J3-5 Spin Frustration S = 6 x (5/2) 2 x (5/2) = 10

    Leuenberger, M.N.; Loss, D.; Nature, Vol. 410, pg. 789, 2001

    10. Quantum Computing in Molecular Magnets

    Potential for memory storage Currently 1 bit = ~109 molecules

    11. Barrier to Spin Orientation of Single-Molecule Magnets

    Large total Spin, S Negative axial anisotropy, D Magnetic Energy ~ DMs2 Thermally assisted tunneling Ground state tunneling Applied magnetic field tunneling (hysterisis loop)

    12. Energy level diagram of Mn12-acetate (S=10,9,8.)

    13. EPR transitions require high-field, high frequency spectrometer

    EPR transition between the ground state requires at least 30GHz

    14. excited states

    Validity of the single spin model Insight into the origin of ZFS parameters Differences in calculated and experimental* tunneling rates

    15. Why study excited states? ? Benchmark for magnetic structure

    cdc fit yields two sets of Js with excited states at different energies J1-2 = 20 cm-1 (102 cm-1) J1-3 = 120 cm-1 (120 cm-1) J1-5 = 15 cm-1 (15 cm-1) J3-5 = 35 cm-1 (35 cm-1) One set yields reasonable cdc fit with S=9 excited state > 25 cm-1 above ground state Other set gives much better cdc fit, with S=9 excited state ? 0.5 cm-1 above ground state * C. Delfs, et al., Inorg. Chem. 32, 3099 (1993).

    16. Fe8Br8 Experimental Spectrum

    131 GHz, 35 K a transitions correspond to S = 10 ground state* b peaks emerge with increasing temperature

    17. Temperature dependence of b transitions

    No evidence of b peaks at 5 K b peaks increase intensity with temperature b peaks arise from an excited state

    18. Spin Hamiltonian Parameters

    Simulated S = 10 peaks g = 2.00 D = -0.292 K S = 9 parameters g = 2.02 D = -0.27 K

    19. Energy Level Diagrams

    131 GHz 35 K

    20. Boltzmann Analysis

    I9/I10 = (P9/P10)exp(-DE10-9/kT) S=9 excited state lies 24 2 K above ground state

    21. Summary

    Fe8Br8 has a thermally accessible excited state with S = 9 at 24 K above the S = 10 ground state Yields benchmark for determination of J parameters S=9 state arises from perturbation on external Fe3+ ions Dipolar contribution to D parameter Possible breakdown of single spin model Impact on calculated quantum tunneling rates?

    22. Applications to Other Materials Beating Spin-Exchange Phenomenon

    EPR of Na3CrO8 powder Resolution enhancement by high field EPR spectroscopy Standard frequency NHMFL Highest commercial J. Mag. Reson., 124, 495 (1997) giso implies cubic symmetry Two peaks in g-value imply axial symmetry. In this case ground state is dx2-y2. 3d1 metal ion

    25. Recent Studies

    Fe8Br8 has been found to be a new class of MRI contrast agents, potentially improving medical imaging. SMMs have been found to be magnetic semiconductors and photoconductors. Potential utility for spintronics.

    26. Acknowledgements

    Drs. Andrew Harter, Dave Zipse, Micah North, Ashley Stowe, Chris Ramsey, Saritha Nellutla M.Pati Dr. Stephen Hill (UF) Drs. James Brooks , Hans Van Tol (NHMFL) Dr. Christou (UF) NSF/NIRT (Grant No. DMR 0506946)

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