Single-Molecule Magnets

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Single-Molecule Magnets High-Field Electron
Magnetic Resonance

• Naresh Dalal
• Florida State University, Tallahassee, FL

SuperNet Teacher Workshop June 16-20 , 2008
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Outline

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

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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)

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Mn12-Ac

• Mn3 (3d4) S2
• Mn4 (3d3) S3/2
• 8Mn3 for S16 ?
• 4Mn4 for S6 ?
• Total 16-6 S10

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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)
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Fe8Br8

• Eight Fe3 ions antiferromagnetically coupled
• J1-3 gtgt J1-5 gt J3-5
• Spin Frustration
• S 6 x (5/2) 2 x (5/2) 10

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Quantum Computing in Molecular Magnets
Leuenberger, M.N. Loss, D. Nature, Vol. 410,
pg. 789, 2001

• Potential for memory storage
• Currently 1 bit 109 molecules

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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)

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Energy level diagram of Mn12-acetate (S10,9,8.)
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EPR transitions require high-field, high
frequency spectrometer

EPR transition between the ground state requires
at least 30GHz
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excited states

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

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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 S9
  excited state gt 25 cm-1 above ground state
• Other set gives much better cdc fit, with S9
  excited state ? 0.5 cm-1 above ground state

C. Delfs, et al., Inorg. Chem. 32, 3099 (1993).
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Fe8Br8 Experimental Spectrum

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

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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

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Spin Hamiltonian Parameters

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

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Energy Level Diagrams

• 131 GHz
• 35 K

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Boltzmann Analysis

• I9/I10 (P9/P10)exp(-DE10-9/kT)
• S9 excited state lies 24 2 K above ground
  state

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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
• S9 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?

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• Applications to Other Materials
• Beating Spin-Exchange
• Phenomenon

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

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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)