The Magic of Magnetism From Physical Attraction to Spin Doctors

Joachim Stöhr

Stanford Synchrotron Radiation LaboratoryStanford Linear Accelerator Center

Magnetism and Humans

Magnetic Personality

- Attraction -

“Animal Magnetism” –Mesmerizing –

Magnetic Healing

From myth

to sc

ience …

..

Magnetic Objects

Magnetic Devices

Overview of Talk

• A historical perspective … the spin on magnetism …

• Magnetism today… the hi-tech society

• A Glimpse of the Future…… spin electronics, “spintronics”

A historical perspective

first described “magnetism” as the attraction of iron by ``lodestone“- lodestone is the naturally occurring mineral magnetite Fe3O4

Thales of Miletus (Greek, ~ 634 - 546 BC)

Legend: Shepherd by the name of Magnés found that his iron tipped cane was attracted to rocks

The term “magnetism”

More probable:Name comes from city of Magnesia (modern Turkey) surrounded by magnetic rock deposits

Si Nan - the south governor China, ca. 200 BC

bronze plate

south

direction pointers likely used for feng shui - the method of achieving harmony with the forces of nature by properly aligning buildings and placing of objects.

magnetic lodestone spoon

In 1600 William Gilbert published his book“De Magnete“ - “About the Magnet“

he explained that compass needle points north-south because earth is magnetic

William Gilbert (England, 1544-1603)

Earth is magnetic - compass follows its field

Naval navigation:China in late 11th or early 12th century compass became known in Europe sometime later in the 12th century

Earth field mostly from electric currents in the liquid iron outer core

Field changes direction about every 500,000 years

Franz Anton Mesmer theorized that "animal magnetism" accounted forhis ability to heal.

(Germany/Austria 1734-1815)

Around 1800: mythical magnetism

Franz Anton Mesmer

Mesmerizing a patient

“The mystery of magnetism, explain that to me ! No greater mystery but love and hate.”

Natural Philosophy:

two opposites make a “whole”love – hateblack – white north – southplus – minus

magnetism is fundamental, it is just there – it does not need to be explained

Johann Wolfgang von Goethe (Germany, 1749-1832)

Around 1800:

…contains important modern element: there are no magnetic monopoles!

(Denmark, 1777-1851) Hans Christian Oersted

1819 – a breakthrough discovery

The right hand rule:

1831: The foundation of the industrial society

Electric generator: motion electricity

(England, 1791 – 1867)

Michael Faraday

Electric motor: electricity motion

Fuel - burns - hot gases - turbine

US electricity retail sales:

$ 250 billion/year ($ 0.25 trillion)

Electricity is big $$$$$$$

Nuclear - heats water - steam - turbine

Hydro-electric - water drives turbine

Total Gross Domestic Product (trillions of $)

1. United States 10.9 (electricity is 2.5 %) 2. Japan 4.33. Germany 2.44. United Kingdom 1.85. France 1.7 6. Italy 1.57. China 1.4

1845: Magnetism and light – a connection !

1791 - 1867

again…

Michael Faraday

Light must have a magnetic component

(Scotland, 1831 – 1879)

1864: Maxwell’s equations: coupling of electric and magnetic phenomena

Light is an electromagnetic wave

Ludwig Boltzmann (Germany,1844-1906)

“Was it a god who wrote these symbols…unveiling the forces of nature?”

James Clerk Maxwell

only wavelength is different

The electro-magnetic spectrum: “wireless waves at the speed of light”

Visible light

It is hard to imagine our society without wireless technology !

From

classical magnetism & electro-magnetism

to

spin doctors

1921-1928: Getting to the origin of magnetism 7 years that changed physics and magnetism

• The concept of the spin (Pauli, Uhlenbeck & Goudsmit, Dirac)

• Quantum theory (Heisenberg, Schrödinger, Dirac)

• Magnetic exchange interaction in materials (Heisenberg)

The mystery of spin: when measured, it is always along a “quantization axis” – “up” or “down”

The inside of magnets: origin of magnetism

magnet = arrangement of atoms with aligned spins

current

Aligning or “switching” a magnetic material - Oersted’s method still used after 186 years ! -

magnetic regions “domains”

Magnetic material, e.g iron

Magnetism and Magnetic Materials today……

the high-tech society…

The spin on electronics: magnetic devices in computers

“0” and “1” bits = small magnetic areas with opposite magnetization direction

“chip - RAM”

“disk”

Magnetic recording technology

Courtesy of Jan Thiele, Hitachi

stack of 50 giant discs, 24" diameter

total 4.4 MB - $35,000/year rent

Today:

200 GB = 200, 000 MB - $ 90

IBM RAMAC 1956:

The Data Storage Industry $53 billion/year worldwide revenues

10,000 songs

1 song

“smaller and faster”

Bri

gh

tnes

s o

f x-

ray

sou

rces

x-ray images … then and now

1895W. Röntgen, Würzburg

1993 Magnetic x-ray image

X-rays have come a long way……

10,000 times smaller

01 “bits”

The Power of X-Rays ALS

A Glimpse of the Future……

exploring spin-electronics

“spintronics”

Beyond Oersted’s method – “spin doctoring”

The concept of a “spin polarized current”

electrons magnetic atoms in iron

copper copper

spinsspin polarized current

Weak, long range

spin current acts like an

“Exchange field”

electric current creates magnetic field

“Oersted field”

Strong, short range

Spin currents: a new way of magnetic switching:

referencelayer sensor

layer

current

sensorlayer

current

Switching of magnetic memory cells (MRAM)

current current

cell to be switched

better: switching by current in wireswitching by Oersted field around wire

X-ray microscopy image of spin injection structure

100 x 300 nm

Detector

xy

leads for nscurrent pulses

x-ray image

Movie 1

magnetization has complicated “horseshoe” state

X-Ray microscopy images of magnetization in buried sensor layer

After current in one direction After current in other direction

Movie 2

Next time…

not cartoons but …

real x-ray movies !

In Summary…..

Magnetism ….

• is an old phenomenon and topic

• has many important practical applications navigation … electricity … wireless communication… medical imaging

• underlies modern data storage technology • “spin doctoring” may be used for switching of computer memory and “quantum computing”

• remains an exciting research topic

... the mystery of magnetism continues...

AcknowledgementsYves Acremann, SSRLAnn Mueller, SSRLMichael Hyde, SLAC

Science website:http://www.howstuffworks.com

My research group:http://www-ssrl.slac.stanford.edu/stohr

http://www-ssrl.slac.stanford.edu/stohrgroup

New book:

J. Stöhr and H. C. Siegmann Magnetism: From Fundamentals to Nanoscale Dynamics Springer Verlag, February/March 2006

The end

Magnetic recording: Small and fast !

1 mile/hour

1000 miles/hour

Incredibly small bits – incredible writing and reading speeds