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 )

50 nm 10 nm particle

Magnetic Storage Media

600nm

17 Gbits/inch2 commercial

Hundreds of particles per bit

Single particle per bit !

Magnetic ForceMicroscopeImage (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 )

-10

-8

-6

-4

-2

0

2

4

XK

Ni

En

erg

y R

ela

tiv

e t

o E

F [

eV

]

0.7 0.9 1.1

k|| along [011] [Å-1 ]

Magnetic Doping

Magnetic Impurity Selects Spin Carrier

Fe doped

Why Silicon ? Couple Nano- to MicroelectronicsUtilize Silicon Technology

Storage Media: 1 Particle (Atom) per BitAtomically 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

One-Dimensional Structures on Silicon

Si(111) 77

Control the step spacing

in units of

2.3 nm = 7 atom rows

Step Step

x - Derivative of the STM Topography

“Illumination from the Left Casting Shadows”

Stepped Silicon

Template

1 Kink in

20 000 Atoms

15 nm

Si(557) Regular Step Spacing

5.73 nm

77 Unit + Triple Step

Si(557)

= 17 Atomic Rows

Stepped Silicon Templates

80 nm15 nm6 nm

triple single bunched

Tobacco Mosaic Virus

CaF2 Mask Selective Adsorption

DPP Molecule

Selective Deposition

via Photolysis of Ferrocene

Troughs converted to Fe wires

Clean Si(557)

+ Gold

Decoration of Steps Atomic Wires2 nm6 nm

Si(557) - Au

Hole Holon + Spinon

EF

Photoelectron

Spin - Charge Separation

in a One-Dimensional Metal

Zacher, Arrigoni, Hanke, and Schrieffer, PRB 57, 6379 (1998)

Spinon

Holon

EF =

Crossing at EF

Si(557)-Au

• Splitting persists at EF

• Electron count is even

Not spin charge separation

EFermi

Two degenerate orbitals ?

Bonding

Antibonding

E2E1

Tailoring the Electronic Structure

Electron count even,

two bands, metallic

Electron count odd,

one band, “gap”

stepped flat

Si(111) - Au

http://uw.physics.wisc.edu/~himpsel