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EPR Studies of Heavy Atom Molecule-Based Magnets

This talk discusses the idea behind studying heavy atom molecule-based magnets using electron paramagnetic resonance (EPR). It explores recent examples such as radical ferromagnets and mononuclear lanthanide-based magnets. The talk highlights the importance of EPR spectroscopy in understanding the electronic and magnetic properties of these systems.

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EPR Studies of Heavy Atom Molecule-Based Magnets

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  1. EPR Studies of Heavy Atom Molecule-Based Magnets Stephen HillNHMFL and Florida State University, Physics • Outline of talk: • Idea behind the title of this talk • Nice recent example: Radical Ferromagnet • Mononuclear nanomagnets based on Lanthanide ions • CW and pulsed EPR studies of Ho system • Coherent quantum tunneling dynamics

  2. EPR Studies of Heavy Atom Molecule-Based Magnets Stephen HillNHMFL and Florida State University, Physics In collaboration with: Radical Ferromagnets: Steven Winter and Richard Oakley, U. Waterloo Saiti Datta and AlexeyKovalev (NHMFL Postdocs) Holmium polyoxometallate: Saiti Datta and SanhitaGhosh (FSU/NHMFLpostdoc/student) Eugenio Coronado and Salvador Cardona-Serra, U. Valencia, Spain Enrique del Barco, U. Central Florida

  3. Heavy Atom Radical Ferromagnets Record: Tc = 17K Hc = 0.15 T Oakley et al., JACS 130, 14791 (2008); JACS 131, 7112 (2009)

  4. Radicals well known to EPRspectroscopists Tryptophan (Trp) radical in azurin, an electron transfer protein S. Stoll, D. Britt UC Davis • g tensor characteristic of microenvironment . • Compare to electronic structure calculations. • Crucial for systems with small g anisotropy (tryptophans, tetra-pyrroles, e.g., chloro-phylls, and organic photovoltaic materials) Stoll et al., JACS132, 11812 (2010); JACS131, 1986 (2009).

  5. Heavy Atom Radical Ferromagnets Record: Tc = 17K Hc = 0.15 T Most importantly: huge (record) coercive field (1.4 kOe at 2 K) 9.0 8.7 Resonance field (tesla) 8.4 1: HA = 0.8 T 2: HA = 0.45 T 8.1 7.8

  6. Heavy Atom Radical Ferromagnets Record: Tc = 17K Hc = 0.15 T Hubbard Hamiltonian with spin-orbit (s) and hopping (h) perturbations

  7. Mononuclear Lanthanide Single Molecule Magnets Hund’s rule coupling for Ho3+: L = 6, S = 2, J = 8; 5I8 Axial ligand-field: mJ = ±5 I = 7/2 nuclear spin (100%) Ishikawa et al.,

  8. Mononuclear Lanthanide Molecular Nanomagnets Based on Polyoxometalates [Ln(W5O18)2]9- (LnIII = Tb, Dy, Ho, Er, Tm, and Yb) ~D4d AlDamen et al.,

  9. Mononuclear Lanthanide Molecular Nanomagnets Based on Polyoxometalates Er3+ compound Er3+ and Ho3+ Exhibit some SMM characteristics AlDamen et al.,

  10. Mononuclear Lanthanide Molecular Nanomagnets Based on Polyoxometalates Fits to cmT & NMR D4d(f≠45o) AlDamen et al.,

  11. Mononuclear Lanthanide Molecular Nanomagnets Based on Polyoxometalates Hund’s rule coupling for Ho3+: L = 6, S = 2, J = 8; 5I8 gJ = 5/4 Ground state: mJ = ±4 Ho3+: [Xe]4f10 AlDamen et al., D = 0.600 cm-1, B04 = 6.94 ×10-3 cm-1, B06 = -4.88 ×10-5 cm-1

  12. Mononuclear Lanthanide Molecular Nanomagnets Based on Polyoxometalates Hund’s rule coupling for Ho3+: L = 6, S = 2, J = 8; 5I8 gJ = 5/4 • Other relevant details: • 100% I = 7/2 nuclear spin • Strong hyperfine coupling • Dilution: [HoxY1-x(W5O18)2]9- • Na+ charge compensation • H2O solvent Ho3+: [Xe]4f10 AlDamen et al., D = 0.600 cm-1, B04 = 6.94 ×10-3 cm-1, B06 = -4.88 ×10-5 cm-1

  13. High(ish) frequency EPR of [HoxY1-x(W5O18)2]9-(x = 0.25) Broad 8 line spectrum due to strong hyperfine coupling to Ho nucleus, I = 7/2 B//c

  14. High(ish) frequency EPR of [HoxY1-x(W5O18)2]9-(x = 0.25) • Nominally (strongly) forbidden transitions: mJ = -4  +4, DmI = 0 • This suggests mixing (tunneling) of mJ states (no EPR for f > 100 GHz) Next excited level at least 20-30 cm-1 above 1 K = 21 GHz 1 cm-1 = 30 GHz B//c

  15. Angle-dependence:[HoxY1-x(W5O18)2]9-single crystal (x=0.25) • Indicative of strong anisotropy associated with J = 8 ground state • Note: hyperfine splitting also exhibits significant anisotropy

  16. Full Matrix Analysis of the Angle-dependence gz = 1.06 A = 835 MHz (0.0278 cm-1) • Simulations assume isotropic g • data do not constrain gxy so well • Free ion g = 1.25 D = 0.600 cm-1, B04 = 6.94 ×10-3 cm-1, B06 = -4.88 ×10-5 cm-1 Ligand field parameters from: AlDamen et al., Inorg. Chem. 48, 3467 (2009)

  17. Standard CW X-band EPR of [HoxY1-x(W5O18)2]9-(x = 0.25) Multi-frequency studies: does D4d parameterization hold water? f ~ 9.5 GHz

  18. Standard CW X-band EPR of [HoxY1-x(W5O18)2]9-(x = 0.25) Multi-frequency studies: does D4d parameterization hold water? f ~ 9.5 GHz

  19. Standard CW X-band EPR of [HoxY1-x(W5O18)2]9-(x = 0.25) D4d symmetry approximate → natural to add: ~9 GHz tunneling gap - D f ~ 9.5 GHz f≠45o

  20. Standard CW X-band EPR of [HoxY1-x(W5O18)2]9-(x = 0.25) Standard B1 B0 configuration Parallel mode (B1//B0)

  21. Pulsed X-band EPR of [HoxY1-x(W5O18)2]9-(x = 0.25) Rabi oscillations: remarkably long T2 T1 ~ 1 ms T2 ~ 140 ns T = 4.8 K Hahn echo sequence

  22. Pulsed X-band EPR of [HoxY1-x(W5O18)2]9-(x = 0.25) Rabi oscillations: remarkably long T2 Cr7Ni (S = 1): 0.2mg/mL, T2 ~300 ns @ 5K Ardavan et al., PRL98, 057201 (2007) Fe4: 0.5g/mL, 95 GHz and B = 0 Schlegel et al., PRL101, 147203 (2008) Fe8: 240 GHz and 4.6 T (kBT ~ 11.5 K) Takahashi et al., PRL102, 087603 (2009) Fe4 S = 5 T = 4.8 K T2 ~ 140 ns Fe8 S = 10

  23. Pulsed X-band EPR of [HoxY1-x(W5O18)2]9-(x = 0.25) Echo-detected spectrum is T2 weighted Spectrum also sensitive to pulse sequence

  24. Pulsed X-band EPR of [HoxY1-x(W5O18)2]9-(x = 0.25) Competing anisotropies (TUNNELING): → no longer obvious what is parallel/perpendicular DmI = 0 DmI = ±1

  25. Pulsed X-band EPR of [HoxY1-x(W5O18)2]9-(x = 0.25) Cancelation resonances → significant reduction in decoherence Bi (I = 9/2) in Si1 Note: excitation bandwidth Comparable to linewidth 1Mohammady et al., Phys. Rev. Lett. 105, 067602 (2010)

  26. Pulsed X-band EPR of [HoxY1-x(W5O18)2]9-(x = 0.25) • COHERENT QUANTUM TUNNELING Note: excitation bandwidth Comparable to linewidth

  27. Pulsed X-band EPR of [HoxY1-x(W5O18)2]9-(x = 0.1) • Sample not perfectly aligned; shift to consistent with simulations • Cancelation resonances now stronger than the standard ones!! • T2 factor of two larger for cancelation resonances Impurity in cavity T2 ~ 200 ns

  28. Pulsed X-band EPR: concentration dependence Electron-Spin-Echo- Envelope-Modulation (ESEEM) 1.2 ms ESEEM frequency Consistent with Coupling to protons

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