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Molecular control of the quantum dynamics of single-molecule magnets

Molecular control of the quantum dynamics of single-molecule magnets. Stephen Hill Department of Physics, University of Florida, Gainesville, FL32611 George Christou (UF), David Hendrickson (UCSD), Naresh Dalal (FSU), Enrique del Barco (UCF), Andy Kent (NYU).

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Molecular control of the quantum dynamics of single-molecule magnets

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  1. Molecular control of the quantum dynamics of single-molecule magnets Stephen Hill Department of Physics, University of Florida, Gainesville, FL32611 George Christou (UF), David Hendrickson (UCSD), Naresh Dalal (FSU), Enrique del Barco (UCF), Andy Kent (NYU) • Introduction to single-molecule magnet (SMM) • Emphasis on quantum dynamics • Effect of molecule symmetry (Mn4) • Quantum entanglement of two SMMs ([Mn4]2) • Influence of ligand environment (Mn12) • Low-symmetry Mn12 • Summary Supported by: NSF, Research Corporation, NHMFL, & University of Florida

  2. "down" "up" { } ± Single-molecule magnets • Molecule that exhibits magnetic bistability at T < TB • Magnetization hysteresis property of individual molecule • Slow magnetization dynamics at T < TB • Eventually quantum dynamics as T  0. • Can serve as prototype magnetic nanostructures • Molecular control (supramolecular chemistry) • Crystalline arrays, surfaces, etc..

  3. Spin projection - ms  S Energy E-4 E4 E-5 E5 E-6 E6 • Large uniaxial (negative) magnetoanisotropy E-7 "up" E7 E-8 E8 E-9 E9 "down" E-10 E10 Requirements for a single-molecule magnet • Well defined giant spin ground state • Clusters of transition metal oxides: Mn, Fe, Ni, Co, etc.. • Ferro-, Ferri-, or frustrated magnetic interactions • Giant spin approximation

  4. Symmetry • Fast quantum dynamics Requirements for a single-molecule magnet • Well defined giant spin ground state • Clusters of transition metal oxides: Mn, Fe, Ni, Co, etc.. • Ferro-, Ferri-, or frustrated magnetic interactions • Giant spin approximation • Large uniaxial (negative) magnetoanisotropy Other considerations.... • The environment • Intermolecular interactions, nuclear hyperfine interactions, etc. • Disorder (ligand disorder)

  5. A. J. Tasiopoulos et al., Angew. Chem. Int. Ed. 43, 2117 (2004). CHRISTOU GROUP

  6. MnIII: 3 × S = 2 - MnIV: S = 3/2  S = 9/2 Mn4 single molecule magnets (cubane family) [Mn4O3Cl4(O2CEt)3(py)3] Distorted cubane • MnIII (S = 2) and MnIV (S = 3/2) ions couple ferrimagnetically to give an extremely well defined ground state spin of S = 9/2. • Anisotropy due to Jahn-Teller distortion on the MnIII ions.

  7. Zeeman diagram for S = 9/2, D < 0 system B // z-axis of molecule

  8. HFEPR for high symmetry (C3v) Mn4 cubane Field // z-axis of the molecule (±0.2o)

  9. Fit to easy axis data - yields diagonal crystal field terms

  10. Routes to incredible # of SMMs • Core ligands (X): Cl-, Br-, F- NO3-, N3-, NCO- OH-, MeO-, Me3SiO- Jahn-Teller points towards core ligand 0D Wonderland • Peripheral ligands: (i) carboxylate ligands: -O2CMe, -O2CEt (ii) Cl-, py, HIm, dbm-, Me2dbm-, Et2dbm- J. Am. Chem. Soc. 126, 12503 (2004).

  11. High-symmetry S = 9/2 [Mn4O3Cl4(O2CEt)3(py)3] complex W. Wernsdorfer • Slow magnetization relaxation (hysteresis) • Resonant magnetic quantum tunneling N. Aliaga-Alcalde et al., JACS 126, 12503 (2004)

  12. Distorted S = 9/2 [Mn4O3(O2CPh-R)3(dbm)3] complex W. Wernsdorfer • Fast magnetization relaxation at B = 0 N. Aliaga-Alcalde et al., JACS 126, 12503 (2004)

  13. Low symmetry accounts for the fast relaxation Partially aligned powder EPR D = -0.646 cm-1, E = 0.14 cm-1 N. Aliaga-Alcalde et al., JACS 126, 12503 (2004)

  14. Antiferromagnetic exchange in a dimer of Mn4 SMMs Monomer Zeeman diagram [Mn4O3Cl4(O2CEt)3(py)3] m1 m2 D = -0.52(1) cm-1 B04 = -3.5 × 10-5 cm-1 J 0.08(1) cm-1

  15. Antiferromagnetic exchange in a dimer of Mn4 SMMs     [Mn4O3Cl4(O2CEt)3(py)3] Monomer Zeeman diagram B//z m1 m2 D = -0.52(1) cm-1 B04 = -3.5 × 10-5 cm-1 J 0.08(1) cm-1 Multiplicity increases from (2S +1) to (2S +1)2

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

  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. Clear evidence for coherent transitions involving both molecules 9 GHz f = 145 GHz Experiment Simulation Jz = Jxy = 0.08(1) cm-1 S. Hill et al., Science302, 1015 (2003)

  19. Easy-axis anisotropy due to Jahn-Teller distortion on Mn(III) • Crystallizes into a tetragonal structure with S4 site symmetry • Organic ligands ("chicken fat") isolate the molecules The first single molecule magnet: Mn12-acetate Lis, 1980 Mn(III) S = 2 S = 3/2 Mn(IV) Oxygen Carbon [Mn12O12(CH3COO)16(H2O)4]·2CH3COOH·4H20 R. Sessoli et al. JACS 115, 1804 (1993) • Ferrimagnetically coupled magnetic ions (Jintra 100 K) Well defined giant spin (S = 10) at low temperatures (T < 35 K)

  20. Single-crystal, high-field/frequency EPR • Obtain the axial terms in the z.f.s. Hamiltonian: • Magnetic dipole transitions (Dms = ±1) - note frequency scale! field//z z, S4-axis Hz

  21. Disorder lowers the symmetry of the molecules E. del Barco et al., PRL 91, 047203 (2003) S. Hill et al., PRL 90, 217204 (2003)

  22. Determination of transverse crystal-field interactions in Mn12-Ac f f = 51 GHz, T = 14 K Hard-plane (xy-plane) rotations

  23. Determination of transverse crystal-field interactions in Mn12-Ac f f = 51 GHz, T = 14 K • Two-fold line shifts associated with the high- and low-field shoulders due to a quadratic transverse interaction in HT • S. Hill et al., PRL 90, 217204 (2003) • del Barco et al., JLTP 140, 119 (2005). Incompatible with the crystallographic symmetry!

  24. J. Appl. Phys. 97 10M510 (2005) Phys. Rev. B 70, 054426 (2004) [Mn12O12(O2CMe)16(H2O)4] + 16 RCO2H [Mn12O12(O2CR)16(H2O)4] + 16 MeCO2H CH2Cl2 [Mn12O12(O2CMe)16(H2O)4]·2MeCO2H·4H2O vs. [Mn12O12(O2CCH2But)16(MeOH)4]·MeOH Synthesis: S4 Mn12-Ac Mn12-tBuAc • Less solvent of crystallization • Bulky R group: well separated molecules • Well aligned

  25. Angle-dependent EPR of fast relaxing Mn12 (Jahn-Teller isomerism) W. Wernsdorfer Mn12-tBuAc + CH2Cl2 MeNO2 c Smaller coercive field M. Soler et al., Chem. Commun. 2003, 2672

  26. Angle-dependent EPR of fast relaxing Mn12 (Jahn-Teller isomerism) f Alignment <0.2o is essential!!! D/E ~ 5 (very similar to Fe8)

  27. Summary • Single-molecule magnets provide a rich playground for studying quantum dynamics of magnetic nanostructures • Brief examples: • Influence of molecule symmetry • Quantum entanglement of two SMMs • Sensitivity to solvent environment • Many other examples: Fe, Ni and Co systems Useful references: Mn4 Monomers: Aliaga-Alcalde et al., JACS 126, 12503 (2004) Mn4 Dimer: Hill et al., Science 302, 1015 (2003) Mn12-Ac review: del Barco et al., J. Low Temp. Phys. 140, 119-174 (2005) More on Mn12: Petukhov et al., Phys. Rev. B 70, 054426 (2004) Takahashi et al., Phys. Rev. B 70, 094429 (2004) Hill et al., Phys. Rev. Lett. 90, 217204 (2003) Ni4 systems: Yang et al., Inorg. Chem., 44, 3827-3836 (2005).

  28. Many collaborators ...illustrates the interdisciplinary nature of this work UF Physics Rachel Edwards Alexey Kovalev John Lee Susumu Takahashi Jon Lawrence Norman Anderson Tony Wilson Cem Kirman Shaela Jones Sara Maccagnano FSU Chemistry Naresh Dalal Micah North David Zipse Randy Achey Chris Ramsey UF Chemistry George Christou Nuria Aliaga-Alcalde Monica Soler Nicole Chakov Sumit Bhaduri Muralee Murugesu Alina Vinslava Dolos Foguet-Albiol NYU Physics Andy Kent Enrique del Barco UCSD Chemistry David Hendrickson En-Che Yang Evan Rumberger Also: Kyungwha Park (NRL) Marco Evangelisti (Leiden) Hans Gudel (Bern) Wolfgang Wernsdorfer (Grenoble) Mark Novotny (MS State U) Per Arne Rikvold (CSIT - FSU)

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