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Single Molecular Magnets

Single Molecular Magnets. Ge,Weihao. Introduction. What is Single Molecular Magnet? Configuration: metal atom linked by oxygen, packed in ligands Described as a total single spin; size and anisotropy has great effect Why are they interesting?

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Single Molecular Magnets

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  1. Single Molecular Magnets Ge,Weihao

  2. Introduction • What is Single Molecular Magnet? • Configuration: metal atom linked by oxygen, packed in ligands • Described as a total single spin; size and anisotropy has great effect • Why are they interesting? • Theoretical: quantum behavior at mesoscopic level • Application: • Quantum computer: • quantum interference and coherence • High-density storage device • Internal memory effect • high integration • Other applications • Two kinds of these clusters • Big integer spin • ½ spin big molecule

  3. Superparamagnetism • Superparamagnetism • paramagnetism below Curie’s temperature • large susceptibility • superparamagnetism limit • Origin of superparamagnetism • magnetism: result of spin alignment • thermal excitation, ferromagnetism <-> paramagnetism • small scale, below Tc: • thermal excitation destroys the ordering between the clusters • thermal excitation cannot upset alignment within the cluster • ferro~ inside & para~ outside => treated as a large spin as a whole • Experiment results • stepped hysteresis can be found below certain temperature. • frequency dependent AC susceptibility

  4. Quantum Tunneling Magnetization • Experiment • steps found in hysteresis of Mn12 cluster • Model • two – well model • resonant tunneling • thermally assisted QTM & pure QTM • A commonly used form of Hamiltonian • axial anisotropic term • Zeeman splitting term • transverse anisotropic term

  5. Integer Spin SMM: Mn12 • Significance • archeological reason: first synthesized SMM & QTM first observed • most widely studied • Structure • Mn3+, external octagon; Mn4+, internal tetrahedron. • Ground state: S=10 • Hamiltonian • symmetry: • lowest even power of transverse terms is 4 • transition: • probably exist low-ordered odd-powered terms

  6. Integer Spin SMM: Fe4 • Structure • Fe3+: centered triangle, C2 symmetry • Ground state: S=5 • Hamiltonian • general form • Advantages over Mn12 in application • More efficient tunneling • longer relaxation time • less affected when attached to a surface • stability

  7. ½ spin big molecule: V15 • Structure • a center triangle between two hexagons • S=1/2, no large energy barrier, large zero field splitting • Experimental result • hysteresis observed • Rabi oscillation • coherence time: ~100 μs • Theoretical approaches • dissipative two-level system: • Landau - Zener transition • exchange interaction: • “spin rotation in a phonon bath”

  8. Summary • General introduction to single molecular magnets • quantum behavior beyond the microscopic scale in these clusters • Origin of the magnetism of SMM • Quantum Tunneling Magnetization • A result of size and anisotropy • Integer spin clusters and ½ spin clusters • integer: • easy to interpreted by large-spin approximation • ½ spin: • lack of barrier, tunneling caused by spin-phonon interaction • long-lived coherence

  9. References QTM: Gatteschi,D.; Sessoli,R. “Quantum tunneling magnetization and related phenomena in Molecular Materials.” Angew. Chem.Ed. 42(3), 2003,pp.268 Mn12: Friedman,J., Sarachik,M. “Mesoscopic Measurement of Resonant Magnetization Tunneling in High-Spin Molecules.” PRL, 76(20),1996, pp.3830 Barra, A., et.al. “High-frequency EPR spectra of a molecular nanomagnet: Understanding quantum tunneling of the magnetization.” PRB. 56(13), 1997, pp.8192 Fe4: Accorsi,S., et.al. “Tuning Anisotropy Barriers in a Family of Tetraion(III) Single-Molecule Magnets with an S = 5 Ground State” JACS. 128(14), 2006, pp.4742-4755 Wernsdorfer,W., et.al. “X-ray Magnetic Circular Dichroism Picks out Single-Molecule Magnets Suitable for Nanodevices.” Adv.Mater. 21, 2009, pp.167-171 Sessoli,R., et.al. “Magnetic memory of a single-molecule quantum magnet wired to a gold surface.” Nature Mater. 8, 2009, pp.194 V15: Müller,A., et.al. “Quantum Oscillation in a molecular magnet.” Nature lett. 453, 2008 pp.203 Choirescu, et.al. “Environmental effects on big molecule with spin ½.” J.Appl.Phys.87(9) 2000 pp.5496

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