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2002 London NIRT: Fe 8 EPR linewidth data

2002 London NIRT: Fe 8 EPR linewidth data. M S dependence of Gaussian widths is due to D -strain Energies  M S 2 , therefore energy differences  M S s D = 0.6% D -strain  disorder; multiple environments. 89 GHz. 117 GHz.

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2002 London NIRT: Fe 8 EPR linewidth data

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  1. 2002 London NIRT: Fe8 EPR linewidth data • MS dependence of Gaussian widths is due to D-strain • Energies  MS2, therefore energy differences  MS • sD= 0.6% • D-strain  disorder; multiple environments 89 GHz 117 GHz • Temperature dependence of Gaussian widths is due to intermolecular spin-spin interactions (dipolar and exchange) • S. Hill, S. Maccagnano, K. Park, R. M. Achey, J. M. North and N. S. Dalal, Phys. Rev. B 65, 224410 (2002).

  2. High-frequency EPR data • Magnetic dipole transitions (Dms = ±1) - note frequency scale! S = 10 field//z z, S4-axis Bz

  3. 2002 London NIRT: Fe8 EPR linewidth data • MS dependence of Gaussian widths is due to D-strain • Energies  MS2, therefore energy differences  MS • sD= 0.6% • D-strain  disorder; multiple environments 89 GHz 117 GHz • Temperature dependence of Gaussian widths is due to intermolecular spin-spin interactions (dipolar and exchange) • S. Hill, S. Maccagnano, K. Park, R. M. Achey, J. M. North and N. S. Dalal, Phys. Rev. B 65, 224410 (2002).

  4. Attempts to model this behavior (spin-spin interactions)

  5. Fe8Br - easy axis line position data • Temperature dependent shifts are due to competing short range ferromagnetic exchange (J = -7 gauss) interactions and longer range antiferromagnetic dipolar coupling (20 gauss) • Quantitative agreement with simulations taking both interactions into account • First evidence for exchange in this widely studied SMM • Kyungwha Park, M.A. Novotny, N.S. Dalal, S. Hill, P.A. Rikvold, Phys. Rev. B 65, 14426 (2002). • Kyungwha Park, M.A. Novotny, N.S. Dalal, S. Hill, P.A. Rikvold, Phys. Rev. B (In press, October 2002); cond-mat/0204481.

  6. High-frequency EPR data • Obtain the axial terms in the z.f.s. Hamiltonian: • Magnetic dipole transitions (Dms = ±1) - note frequency scale! field//z z, S4-axis Bz

  7. Fe8Br (S = 10) - easy axis linewidth data 89 GHz 117 GHz -10 Hill et al., Phys. Rev. B 65, 224410 (2002)

  8. All Js are antiferromagnetic • Intra-dimer J = 4.45 meV (36 cm-1) • J' = 0.51 meV (4 cm-1) • Jf < J' is frustrating interaction c b a • To lowest order, treat as independent spin-½ dimers • [Cu2+]2 Hamiltonian has perfect cylindrical [U(1)] symmetry Body-centered tetragonal magnetic lattice  J' [Cu2+]2 dimer J J' Each Cu2+ provides a spin-½ Jf • Intra-dimer separation: 2.74 Å • NN inter-dimer distance: 7 Å • NNN inter-dimer distance: ~10 Å

  9. Properties of the isolated dimer Heisenberg: Zeeman: J T+ Energy Triplet (T ) T0 Singlet (S) S T- Magnetic field

  10. Temperature dependence – Low T S. Sebastian et al., cond-mat/0606244.

  11. Angle dependence – origin of anisotropy Dipolar interaction

  12. Insight from the two leg ladder J' J i = 1 2 3 4 5..... K.E. P.E. C.P. F. Mila, Euro Phys. J. B. 6, 201 (1998). T. Giamarchi & A. M. Tsvelik, PRB 59, 11398 (1999). Mobile quasiparticles  dispersion (bandstructure)

  13. Temperature dependence – Low T S. Sebastian et al., cond-mat/0606244.

  14. Spin-1 chain with easy-plane anisotropy 8 sin exact diagonalization

  15. Antiferromagnetic exchange in a dimer of Mn4 SMMs Monomer Zeeman diagram [Mn4O3Cl4(O2CEt)3(py)3] m1 m2 D = -0.75(1) K B04 = 5 × 10-5 K J 0.12(1) K Wolfgang Wernsdorfer, George Christou, et al., Nature, 2002, 406-409

  16. Antiferromagnetic exchange in a dimer of Mn4 SMMs EPR     To zeroth order, the exchange generates a bias field BJ = Jm'/gmB which each spin experiences due to the other spin within the dimer • Bias should shift the single spin (monomer) EPR transitions. Wolfgang Wernsdorfer, George Christou, et al., Nature, 2002, 406-409 [Mn4O3Cl4(O2CEt)3(py)3] Dimer Zeeman diagram m1 m2 D = -0.75(1) K B04 = 5 × 10-5 K J 0.12(1) K

  17. S1 = S2 = 9/2; multiplicity of levels = (2S1 + 1) (2S2 + 1) = 100 Look for additional splitting (multiplicity) and symmetry effects (selection rules) in EPR.

  18. S1 = S2 = 9/2; multiplicity of levels = (2S1 + 1) (2S2 + 1) = 100 Look for additional splitting (multiplicity) and symmetry effects (selection rules) in EPR.

  19. Clear evidence for coherent transitions involving both molecules f = 145 GHz Experiment Simulation D = -0.75(1) K B04 = -5 × 10-5 K J 0.12(1) K Jz = Jxy = 0.12(1) K S. Hill et al., Science302, 1015 (2003)

  20. Ni4 SMMs Although most aspects of earlier EPR line width studies on Mn12Ac and Fe8 have been understood in terms of competing exchange and dipolar interactions,20–22 an explanation for the behavior of the ground-state resonance (mS ) -4 to -3 in the present study) has remained elusive for kBT < Δ0. We speculate that this behavior is related to the development of short-range intermolecular magnetic correlations/coherences (either ferro- or antiferromagnetic) which are exchange averaged at higher temperatures. Inorg. Chem. 47, 1965-1974 (2008).

  21. Exchange biased S = 4 Ni4 SMM A. Ferguson et al., Dalton Trans., 2008, 6409 - 6414, DOI: 10.1039/b807447j

  22. Exchange biased S = 4 Ni4 SMM A. Ferguson et al., Dalton Trans., 2008, 6409 - 6414, DOI: 10.1039/b807447j

  23. S = 4 Mn6 SMMs

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