1 / 9

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?

tea
Télécharger la présentation

Single Molecular Magnets

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 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

More Related