Transcript Si(557)
Magnetic Nanostructures F. J. Himpsel, Dept. of Physics, UW-Madison • Limits of Data Storage • Magnetoelectronics • One-Dimensional Structures on Silicon SSSC Meeting, Irvine, Oct. 4, 2001 All of the information ... accumulated in all the books in the world can be written … in a cube of material 1/200 inch wide. Use 125 atoms to store one bit. Richard Feynman Caltech, December 29th, 1959 In pursuit of the ultimate storage medium 1 Atom per Bit Writing a Zero Before After Filling all Sites Natural Occupancy: 50% After Si Evaporation: 100% Smaller Bits Less Energy Stored Slower Readout Use Highly-Parallel Readout Array of Scanning Probes Array of Shift Registers ( Millipede, IBM Zrich ) ( nm m ) Magnetic Storage Media Magnetic Force Microscope Image (IBM) 600 nm 17 Gbits/inch2 commercial Hundreds of particles per bit Single particle per bit ! 50 nm 10 nm particle Perfect Magnetic Particles FePt Sun, Murray , Weller, Folks, Moser, Science 287, 1989 (2000) Magnetoelectronics Spin Currents instead of Charge Currents Giant Magnetoresistance: Spin-Polarized Tunneling: GMR and Spin - Dependent Scattering Parallel Spin Filters Resistance Low Opposing Spin Filters Resistance High Filtering mechanisms • Interface: Spin-dependent Reflectivity Quantum Well States • Bulk: Spin-dependent Mean Free Path Magnetic Doping Spin-polarized Quantum Well States Minority spins discrete, Majority spins continuous High Resolution Photoemission Ni Energy Relative to EF [eV] 4 2 0 -2 0.7 0.9 k|| along [011] 1.1 [Å-1 ] -4 -6 -8 States near the Fermi level determine magneto-transport -10 K X ( 3.5 kT = 90 meV ) Magnetic Doping Fe doped Magnetic Impurity Selects Spin Carrier One-Dimensional Structures on Silicon Why Silicon ? Couple Nano- to Microelectronics Utilize Silicon Technology Storage Media: 1 Particle (Atom) per Bit Atomically Precise Tracks Step Arrays as Templates: 2 - 80 nm 1 Kink in 20 000 Atoms Emulate Lithography: CaF2 Masks Selective Deposition Atomic Wires: Exotic Electrons in 1D Si(111) 77 Control the step spacing in units of 2.3 nm = 7 atom rows x - Derivative of the STM Topography “Illumination from the Left Casting Shadows” Step Step Stepped Silicon Template 1 Kink in 20 000 Atoms 15 nm Si(557) 5.73 nm Regular Step Spacing Si(557) 77 Unit + Triple Step = 17 Atomic Rows Stepped Silicon Templates triple 6 nm single 15 nm Tobacco Mosaic Virus bunched 80 nm CaF2 Mask Selective Adsorption DPP Molecule Selective Deposition via Photolysis of Ferrocene Troughs converted to Fe wires Clean Si(557) 6 nm 2 nm Decoration of Steps Atomic Wires + Gold Si(557) - Au Spin - Charge Separation in a One-Dimensional Metal Photoelectron EF Hole Holon + Spinon EF = Spinon Holon Crossing at EF Zacher, Arrigoni, Hanke, and Schrieffer, PRB 57, 6379 (1998) Si(557)-Au • Splitting persists at EF • Electron count is even Not spin charge separation EFermi Two degenerate orbitals ? Antibonding E1 E2 Bonding Tailoring the Electronic Structure stepped flat Electron count even, Electron count odd, two bands, metallic one band, “gap” Si(111) - Au http://uw.physics.wisc.edu/~himpsel