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 29 th, 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 Z  rich ) ( nm   m )

50 nm 10 nm particle Magnetic Storage Media 17 Gbits/inch 2 commercial Hundreds of particles per bit Single particle per bit ! Magnetic Force Microscope Image (IBM)

Perfect Magnetic Particles Sun, Murray, Weller, Folks, Moser, Science 287, 1989 (2000) FePt

Giant Magnetoresistance: Spin-Polarized Tunneling: Magnetoelectronics Spin Currents instead of Charge Currents

Filtering mechanisms Interface: Spin-dependent Reflectivity  Quantum Well States Bulk: Spin-dependent Mean Free Path  Magnetic Doping Parallel Spin Filters  Resistance Low Opposing Spin Filters  Resistance High GMR and Spin - Dependent Scattering

Minority spins discrete, Majority spins continuous Spin-polarized Quantum Well States

High Resolution Photoemission States near the Fermi level determine magneto-transport (  3.5 kT = 90 meV ) Ni Energy Relative to E F [eV] k || along [011] [Å -1 ]

Magnetic Doping Magnetic Impurity Selects Spin Carrier Fe doped

Why Silicon ? Couple Nano- to Microelectronics Utilize Silicon Technology Storage Media: 1 Particle (Atom) per Bit Atomically Precise Tracks Step Arrays as Templates: nm 1 Kink in Atoms Emulate Lithography: CaF 2 Masks Selective Deposition Atomic Wires: Exotic Electrons in 1D One-Dimensional Structures on Silicon

Si(111) 7  7 Control the step spacing in units of 2.3 nm = 7 atom rows Step x - Derivative of the STM Topography “Illumination from the Left Casting Shadows”

Stepped Silicon Template 1 Kink in Atoms 15 nm

Si(557) Regular Step Spacing 5.73 nm

7  7 Unit + Triple Step Si(557) = 17 Atomic Rows

Stepped Silicon Templates 80 nm15 nm6 nm triplesinglebunched Tobacco Mosaic Virus

CaF 2 MaskSelective Adsorption DPP Molecule

Selective Deposition via Photolysis of Ferrocene Troughs converted to Fe wires

Clean Si(557) + Gold Decoration of Steps  Atomic Wires 2 nm 6 nm

Si(557) - Au

Hole  Holon + Spinon EFEF Photoelectron Spin - Charge Separation in a One-Dimensional Metal Zacher, Arrigoni, Hanke, and Schrieffer, PRB 57, 6379 (1998) Spinon Holon E F = Crossing at E F

Si(557)-Au Splitting persists at E F Electron count is even  Not spin charge separation E Fermi Two degenerate orbitals ? Bonding Antibonding E2E2 E1E1

Tailoring the Electronic Structure Electron count even, two bands, metallic Electron count odd, one band, “gap” steppedflat

Si(111) - Au