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Atomic-scale Engeered Spins at a Surface

Atomic-scale Engeered Spins at a Surface

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Atomic-scale Engeered Spins at a Surface

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  1. Atomic-scale Engeered Spins at a Surface Chiung-Yuan Lin IBM Almaden Research Center

  2. Nanomagnetism and Information Technology • Magnetism is at the heart of data storage. • Many novel computations schemes are based on manipulation of magnetic properties. Courtesy of Hitachi J.R. Petta et al. Science309, 2180 (2005) A. Imre et al. Science311, 205 (2006)

  3. Nanomagnets • Fabricated nanomagnets can recreate model spin systems such as spin ice. • A small number of atomic spins can be coupled in metal clusters or molecular magnetic structures. R.F. Wang et al., Nature439, 303 (2006) Fe8, courtesy ESF. M.B. Knickelbein Phys. Rev. B70, 14424 (2004)

  4. Assembly and Measurement of Nanomagnets

  5. |ST,m> |ST,m> |1,+1> |5/2,+5/2> |5/2,+3/2> |1,0> |5/2,+1/2> Energy Energy |5/2,-1/2> |1,-1> |5/2,-3/2> |5/2,-5/2> |0,0> Magnetic Field Magnetic Field STM Studies of Atomic-Scale Spin-Coupling • Manipulation on thin insulators:build individual nanomagnets with an STM • Spin Excitation Spectroscopy: collective spin excitations of individual nanostructures 10Mn chain Mn atom Science 312, 1021 (2006)

  6. Keep it Simple: Free Mn Atom 4s • Half filled d-shell • Weak spin-orbit interactions 3d Mn: S = 5/2, L = 0, J = 5/2

  7. dI/dV V 0 Scanning Tunneling Spectroscopy: LDOS Ef eV tip sample Features in the local DOS are reflected in dI/dV.

  8. Thin insulating layer Magnetic Atoms on Surfaces Magnetic atom • Atom’s spin is screened by conduction electrons (Kondo effect) • A thin insulating layer may isolate the atomic spin Metal surface

  9. Ef Ef eV D D eV X tip sample tip sample D D |eV| < D Elastic Channel Open Inelastic Channel Closed |eV| > D Elastic Channel Open Inelastic Channel Open Inelastic Electron Tunneling Spectroscopy Non-magnetic tip Thin insulator Magnetic atom dI/dV kBT < D σe+σie σe Non-magnetic sample eV -D D 0

  10. Methods of Electronic-structure Calculation Plane wave Atomic spheres Atomic partial wave Atomic partial wave Interstitial region • Full-potential Linearized Augmented Plane Wave basis • Periodic-slab geometry (5-layer Cu + 8-layer vacuum) • Density Functional Theory Generalized Gradiant Approximation (GGA) PBE96: Perdew et al., PRL 77, 3865 (1996) • Structure Optimization

  11. Methods of Electronic-structure Calculation Cu Cu Cu Cu vacuum vacuum vacuum • FLAPW basis • Periodic-slab geometry (5-layer Cu + 8-layer vacuum) • Density Functional Theory Generalized Gradiant Approximation (GGA) PBE96: Perdew et al., PRL 77, 3865 (1996) • Structure Optimization

  12. Methods of Electronic-structure Calculation • FLAPW basis • Periodic-slab geometry (5-layer Cu + 8-layer vacuum) • Density Functional Theory Generalized Gradiant Approximation (GGA) PBE96: Perdew et al., PRL 77, 3865 (1996) • Structure Optimization

  13. N a0=Ö2d0 Cu d0 Thin Insulator: CuN Islands on Cu(100) d0=2.55Å a0=3.60Å 1nm • Atomic resolution on CuN • Mn atoms bind to Cu and N sites CuN Mn Mn Mn Mn Mn Mn Cu(100) CuN monolayer Cu(100)

  14. 0.25Å 1.80Å DFT Calculation of Electron Density in CuN N atoms are approximately coplanar with Cu atoms on CuN surface. N-1 N-1 Cu+0.5 Cu+0.5 Cu+0.5 Cu Cu

  15. Pick up Atom Manipulation of Mn on Cu(100) / CuN • Move tip in • Apply 2.0V • Pull tip back

  16. Pick up Atom Drop off Manipulation of Mn on Cu(100) / CuN • Move tip in • Apply -0.5V • Pull tip back

  17. N Cu Mn Mn Spectroscopy of Mn Dimers • Large step at ~6mV splits into three distinct steps at high fields

  18. |ST,m> E |1,+1> |1,0> |1,-1> |0,0> B Coupled Spins 5 4 … 1 0 • S=5/2 Ä S=5/2  ST = • For ST=0 (singlet) the first excited state is ST=1 (triplet) • Three excitations around constant energy shift

  19. IBM Almaden STM Lab has built chains of up to 10 Mn atoms on Cu binding sites Cu(100) 2 6 1nm 3 7 10Mn 1Mn 4 8 CuN 1nm 5 9 Mn Mn Mn Chains of Mn Atoms N Cu

  20. 2 6 1nm 3 7 4 8 5 9 Spectroscopy of Mn Chains 10 Spectra change dramatically with each additional Mn atom.

  21. Heisenberg Model of Spin Coupling J S • Phenomenological Exchange Coupling • J = Coupling strength • Si = spin of ith atom • Assumptions • All spins are the same • Nearest-neighbor coupling • All J are the same • J > 0 (antiferromagnetic coupling)

  22. Heisenberg Dimer Spectrum • SG=0 and SE=1 • Atomic spin affects numbers of levels but not spacing • First excited state at J J S

  23. Determination of Spin Coupling Strength • From the dimer spectrum J=6.2meV • Variations in J of ±5% for different dimers at various locations J=6.2meV

  24. Determination of Atomic Spin • Using J = 6.2meV, we find S=5/2 • STM determines both J and S! S=3 S=5/2 S=2 J=6.2meV

  25. Heisenberg Model for Longer Chains • Use J = 6.2meV and S=5/2 • Odd chains • ground state spin = 5/2 • excited state spin = 3/2 • Even chains • ground state spin = 0 • excited state spin = 1

  26. Unit Cells Used in Calculating Mn on CuN Single Mn, larger unit cell Single Mn, smallest unit cell Mn dimer, smallest unit cell N Cu Mn Mn 10.80Å 7.20Å 7.20Å

  27. Electron Density with an Adsorbed Mn Atom Mn+ N -1.5 N -1.5 Cu+0.5 Cu+0.5 Cu Cu Cu • N atoms move farther out of surface Cu layer towards Mn atom. • Cu atom being pushed into the surface. • This “isolates” the free spin of Mn atom.

  28. Mn Spin from DFT majority () minority () Free Mn atom 3d 5S=5/2

  29. A new kind of atomic-scale magnet • Surface N atoms isolate and bridge Mn atoms. • This is a “surface” assembled magnet. Mn Mn N N N Cu Cu Cu Cu Cu

  30. Control of Spin Coupling Strength J=6.2meV J=2.7meV STM can switch J by a factor of 2 by selecting the binding site

  31. GGA+U GGA+U (strong Coulomb repulsion on Mn 3d) Calculating U by constraint GGA • Calculating U • Lock d-orbital into the atomic sphere • Do GGA for Mn d3 d2.5 and d3 d1.5 • U=Δεd of the above two

  32. N Cu Calculating Exchange Coupling H=JS1·S2 |±|S=5/2, Sz=±5/2 DFT total energies = EE 2S2J=++|H|++  +-|H| +-

  33. Calculating Exchange Coupling (in meV)

  34. Summary of theoretical work • The nontrivial structure of the engineered spins requires DFT to determine. • Calculated structure shows a new kind of molecular magnets. • GGA+U produces correct S and very accurate J; very helpful for searching a system of desired S and J.

  35. What’s Next • Can we understand IETS processes? • matrix elements, selection rules, transition strengths • What is the origin of the exchange coupling? • superexchange, delocalized electrons • Are other interactions possible? • vary distances, shapes, types of atoms • Can we control anisotropy effects? • Find a way to store and transfer spin information:bits and circuits based on atomic spins

  36. Chris Lutz Andreas Heinrich Barbara Jones CyrusHirjibehedin Thanks to