1 / 33

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.

luzr
Download Presentation

Single-Molecule Magnets: A Molecular Approach to Nanomagnetism

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  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

More Related