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Single-Molecule Magnets: A Molecular Approach to Nanomagnetism

Single-Molecule Magnets: A Molecular Approach to Nanomagnetism. George Christou Department of Chemistry, University of Florida Gainesville, FL 32611-7200, USA christou@chem.ufl.edu. m s = 0. 0. -3. 3. m s = -1. 4. m s = 1. -2. 1. 2. -4. -1. 5. -5. m s = -2. m s = 2. 6.

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Single-Molecule Magnets: A Molecular Approach to Nanomagnetism

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  1. Single-Molecule Magnets: A Molecular Approach to Nanomagnetism George Christou Department of Chemistry, University of Florida Gainesville, FL 32611-7200, USA christou@chem.ufl.edu

  2. ms = 0 0 -3 3 ms = -1 4 ms = 1 -2 1 2 -4 -1 5 -5 ms = -2 ms = 2 6 -6 Energy ms = -3 ms = 3 -7 7 ms = 4 ms = -4 -8 8 ms = -5 ms = 5 -9 9 ms = 6 ms = -6 ms=10 -10 = ms ms = -7 ms = 7 E Magnetization Direction (z) ms = -8 ms = 8 U ms = -9 ms = 9 ms = -10 ms = 10 Single-Molecule Magnets (SMMs) The barrier to magnetization relaxation in SMMs is not due to inter-spin Interactions, as in traditional magnets, but to Ising (easy-axis) molecular anisotropy. Each molecule is a separate, nanoscale magnetic particle. • Requirements for SMMs: • Large ground state spin (S) • Negative ZFS parameter (D) Anisotropy barrier (U) = S2|D| Integer spin or (S2-1/4)|D| Half- integer spin

  3. Molecular Advantages of SMMs over Traditional Nanoscale Magnetic Particles Properties • truly monodisperse particles of nanoscale dimensions • crystalline, therefore contain highly ordered assemblies • well-defined ground state spin, S • truly quantum spin systems • synthesized by room temperature, solution methods • enveloped in a protective shell of organic groups (ligands) • truly soluble (rather than colloidal suspensions) in organic solvents • the organic shell (ligands) around the magnetic core can be easily modified, providing control of e.g. coupling with the environment Synthesis

  4. 40 mK -7 7 -8 8 -9 9 -10 10 Hysteresis Loops for an S = 10 SMM S = 10 21 energy states MS = -S, -S+1, …, S

  5. The Mn12 Family of Single-Molecule Magnets (SMMs) [Mn12O12(O2CR)16(H2O)4] (or Mn12) Mn12-Ac (i.e. R = CH3) has axial symmetry (tetragonal space group I4(bar)), and has therefore been considered the best to study in detail. • Properties of the Mn12 family • S = 10 • D = -0.40 to -0.50 cm-1 (-0.58 to -0.72 K) • Magnets below 3K Volume of the Mn12O12 magnetic core ~ 0.1 nm3 Carboxylate Substitution [Mn12O12(O2CMe)16(H2O)4] + 16 RCO2H [Mn12O12(O2CR)16(H2O)4] + 16 MeCO2H

  6. A Mn12 Complex with Tetragonal (Axial) Symmetry: [Mn12O12(O2CCH2Br)16(H2O)4](Mn12-BrAc) [Mn12O12(O2CMe)16(H2O)4] + 16 BrCH2CO2H [Mn12O12(O2CCH2Br)16(H2O)4] + 16 MeCO2H crystallizes as [Mn12-BrAc]·4CH2Cl2 tetragonal space group I42d --- almost no symmetry-lowering contacts between [Mn12BrAc] and the four CH2Cl2. --- in contrast to Mn12-Ac, where each molecule has strong H-bonding to 0,1, 2, 3 or 4 acetic acid molecules (Cornia et al., P.R.L. 2002, 89, 257201)

  7. A New Mn12 Complex with Tetragonal (Axial) Symmetry: [Mn12O12(O2CCH2But)16(MeOH)4]·MeOH(Mn12-ButAc) --- no symmetry-lowering contacts with the solvent molecules in the crystal --- bulky R group : well separated molecules Mn12-ButAc Mn12-Ac Muralee Murugesu

  8. Excited state tunneling Ground state tunneling Excited state tunneling Ground state tunneling The Sharpness of the Hysteresis Loops in Mn12-ButAc allows Steps due to Excited State Tunneling to be seen Wernsdorfer, Murugesu and Christou, Phys. Rev. Lett., in press Summary: Researchers have thought for over 10 years that axial Mn12-Ac is the best one to study, but it is not. More interesting physics is now being discovered with cleaner, truly axial Mn12 SMMs

  9. 0.03 - 0.24 0.56 0.86 0.34 Current 50 mA 0.64 0.95 0.61 0.91 0.29 10 mA 1.6 1.2 0.8 0.4 0.0 - 0.4 Potential (V) Spin Injection into Mn12 Electrochemistry Redox Potentials CyclicVoltammetry Differential Pulse Voltammetry CH2Cl2 solution vs. ferrocene One - electron additions to Mn12 in bulk Mn12 + I - [Mn12] - + ½ I2 Mn12 + 2 I - [Mn12] 2- + I2

  10. Effect of Electron Addition to Mn12 • added electrons localized on two Mn • S does not change significantly • |D| decreases with added electrons • barrier decreases with added electrons [Mn12]2- Also: Quantum Phase Interference and Spin-Parity in Mn12 Single-Molecule Magnets Phys. Rev. Lett. 2005, 95, 037203

  11. 19F NMR in solution R = C6F5 Mn12 [Mn12]- [Mn12]2-

  12. X MnIII MnIII MnIII O O O MnIV Mn4 SMMs with S = 9/2 • 3Mn3+, Mn4+ • C3v virtual core symmetry • Mn3+ Jahn-Teller axial elongations • Core ligands (X): Cl-, Br-, F-, NO3-, N3, NCO-, OH-, MeO-, Me3SiO-, etc • Advantages • Variation in core X group, for given organic groups • Variation in organic groups, for a given Mn4O3X core. • Soluble and crystalline • Mn4O3X core volume ~ 0.01 nm3

  13. energy magnetization orientation S = 9/2 Properties of Mn4 SMMs • S = 9/2 • D = – 0.65 to – 0.75 K • U = ΔE = (S2-1/4)|D| = 20 D E

  14. Mn4 SMMs with S = 9/2 Hi= – D Ŝiz2 + Hitrans + g μB μ0 Ŝi H (2Si + 1) energy states S = 9/2 : 10 energy states MS = -S, -S+1, …, S D = XgµB/kB(X is the step separation)

  15. Supramolecular Dimers of Mn4 SMMs: Exchange-biased Quantum Tunnelling of Magnetization [Mn4Pr]2 Hexagonal R3(bar) (S6 symmetry) Distances C…Cl 3.71 Å C-H…Cl 2.67 Å Cl…Cl 3.86 Å Angle C-H…Cl 161.71° Pr group

  16. Quantum Tunnelling in an [Mn4]2 dimer of SMMs using [Mn4Pr]2·MeCN (NA3) D = - 0.50 cm-1 = - 0.72 K Jintra = - 0.07 cm-1 = - 0.1 K Wernsdorfer, Christou, et al. Nature 2002, 416, 406

  17. Derivatives of [Mn4O3Cl4(O2CR)3(py-p-R´)3]2 Dimers Nuria Aliaga-Alcalde Nature Science

  18. 1 NA3 0.5 0.04 K s M/M 0 0.140 T/s 0.070 T/s -0.5 0.035 T/s 0.017 T/s 0.008 T/s 0.004 T/s -1 -1.2 -0.8 -0.4 0 0.4 0.8 1.2 µ H (T) 0 1 1 NA2 NA11 0.5 0.5 0.04 K 0.04 K s s M/M 0 0 M/M 0.140 T/s 0.070 T/s 0.035 T/s -0.5 -0.5 0.035 T/s 0.017 T/s 0.017 T/s 0.008 T/s 0.008 T/s 0.004 T/s -1 -1 -1 -0.5 0 0.5 1 -1 -0.5 0 0.5 1 µ H (T) µ H (T) 0 0 Variation of Exchange-bias and Fine-structure in [Mn4]2 SMM Dimers [Mn4Ac]2· MeCN [Mn4Pr]2· MeCN (Nature) [Mn4Pr]2· hexane (Science) • Two effects: • Variation in exchange-bias (Jintra) • Variation in fine structure (Jinter) • The properties of [Mn4]2 are very sensitive to the ligands and the solvent in the crystal

  19. Quantum Superpositions in Exchange-coupled [Mn4]2 Dimers -- with [Mn4Pr]2 · hexane (NA11) Jz Jx, Jy Hill,Edwards, Aliaga-Alcalde, Christou, Science 2003,302, 1015

  20. Hysteresis evidence for inter-dimer exchange interactions ↓ ↓ ↓ ↓ (↓ ↓ ↓) (↑↓↓) View down S6 axes (↑↑ ↓) R. Tiron, W. Wernsdorfer, N. Aliaga-Alcalde, and G. Christou, Phys. Rev. B 2003, 68, 140407(R) (↑ ↑ ↑)

  21. A New Generation of Mn Clusters: A Mn84 Torus [Mn12O12(O2CR)16(H2O)4] + MnO4- in MeOH/MeCO2H The structure consists of six Mn14 units i.e. [Mn14]6 Tasiopoulos et al. Angew. Chem. Int. Ed. 2004, 43, 2117

  22. Side-view Front-view

  23. cM′Tvs T cM′′T vs T 10 9 10 7 10 5 10 3 10 1 DC 10 -1 AC 10 -3 10 -5 20 0 5 10 15 S = 6 Ueff = 18 K τ0 = 5.7 x 10-9 s (s) t 1/T (1/K)

  24. Comparison of Sizes and Néel Vectors 3 nm Co nanoparticle Mn30 Mn12 Mn84 Mn4 N 1 10 100 1000 Quantum world Classical world Molecular (bottom-up) approach Classical (top-down) approach A meeting of the two worlds of nanomagnetism Tasiopoulos et al. Angew. Chem. Int. Ed. 2004, 43, 2117

  25. A Mn70 Torus EtOH yields a smaller torus of five units i.e. [Mn14]5 1.4 nm Alina Vinslava Anastasios Tasiopoulos 3.7 nm

  26. ~ 3.7 nm ~ 1.4 nm 10 9 10 7 10 5 (s) 10 3 t 10 1 10 -1 DC 10 -3 10 -5 ~ 1.2 nm 0 5 10 15 20 1/T (1/K) Ueff = 23 K τ0 = 1.7 x 10-10 s Compare Mn84: Ueff = 18 K, τ0 = 5.7 x 10-9 s

  27. pdmH2 A C B A Mn25 SMM with a Record S = 51/2 Spin for a Molecular Species MnCl2 + pdmH2 + N3- + base → [Mn25O18(OH)2(N3)12(pdm)6(pdmH)6]2+ MnIV, 18 MnIII,6 MnII green, MnIV; purple, MnIII; yellow, MnII; red, O; blue, N S = 51/2, D = - 0.022 cm-1 A B C Murugesu et al., JACS, 2004, 126, 4766 S = 15/2 + 0 + 21/2 + 0 + 15/2 = 51/2 ABC BA

  28. Summary and Conclusions • Mn12 and many other SMMs can be easily modified in various ways; this is one of the major advantages of molecular nanomagnetism • Two more Mn12 SMMs, Mn12BrAc and Mn12ButAc, with tetragonal (axial) symmetry have been studied, and they exhibit data of far superior quality than Mn12Ac • “All tetragonal Mn12 complexes have axial symmetry, but some are • more axial than others” (with apologies to George Orwell, Animal Farm) • The techniques of molecular and supramolecular chemistry can be used to modify the quantum properties of SMMs • Giant SMMs represent a meeting of the two worlds of nanoscale magnetism, the traditional (‘top-down’) and molecular (‘bottom-up’) approaches

  29. Acknowledgements Dr. Tasos Tasiopoulos (Mn84, Mn12) Dr. Muralee Murugesu (Mn25) Dr. Philippa King (Mn12, Mn18) Nuria Aliaga (Mn4, [Mn4]2) Nicole Chakov (Mn12) Alina Vinslava (Mn84, Mn70) Khalil Abboud (U. of Florida, X-ray) Naresh Dalal (Florida State Univ) Stephen Hill (U. of Florida, Physics) Ted O’Brien (IUPUI) Wolfgang Wernsdorfer (Grenoble) $$ NSF and NSF/NIRT$$

  30. Magnetic Properties of [Mn12O12(O2CCH2But)16(MeOH)4] (Mn12ButAc) DC Reduced Magnetization fit AC Susceptibility S = 10 D = - 0.51 cm-1 = - 0.73 K Arrhenius Plot Ueff = 67.8 K τ0 = 5.58 x 10-8s Muralee Murugesu

  31. Landau-Zener Tunnelling (1932) Tunnelling probability at an avoided level crossing

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