Transcript LBNL Lecture, October 2005 - Stanford Synchrotron Radiation

The Magic of Magnetism
From Physical Attraction to Spin Doctors
Joachim Stöhr
Stanford Synchrotron Radiation Laboratory
Stanford Linear Accelerator Center
Magnetism and Humans
Magnetic Personality
- Attraction -
“Animal Magnetism” –
Mesmerizing –
Magnetic Healing
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
The term “magnetism”
Legend:
Shepherd by the name of Magnés found that his iron tipped cane was attracted to rocks
More probable:
Name comes from city of Magnesia (modern Turkey) surrounded by magnetic rock deposits
Thales of Miletus
(Greek, ~ 634 - 546 BC)
first described “magnetism” as the attraction of iron by ``lodestone“
- lodestone is the naturally occurring mineral magnetite Fe3O4
Si Nan - the south governor
China, ca. 200 BC
magnetic lodestone
spoon
south
bronze plate
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.
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
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 field mostly from electric currents in the liquid iron outer core
Field changes direction about every 500,000 years
Around 1800: mythical magnetism
Franz Anton Mesmer theorized that "animal magnetism" accounted for
his ability to heal.
Mesmerizing a patient
Franz Anton Mesmer
(Germany/Austria 1734-1815)
“The mystery of magnetism, explain that to me !
Around 1800: No greater mystery but love and hate.”
magnetism is fundamental, it is just there
– it does not need to be explained
Natural Philosophy:
two opposites make a “whole”
love – hate
black – white
north – south
plus – minus
Johann Wolfgang von Goethe
(Germany, 1749-1832)
…contains important modern element:
there are no magnetic monopoles!
1819 – a breakthrough discovery
Hans Christian Oersted
(Denmark, 1777-1851)
The right hand rule:
1831: The foundation of the industrial society
Michael Faraday
(England, 1791 – 1867)
Electric generator:
motion
electricity
Electric motor:
electricity
motion
Electricity is big $$$$$$$
US electricity retail sales:
$ 250 billion/year ($ 0.25 trillion)
Total Gross Domestic Product (trillions of $)
1. United States
2. Japan
3. Germany
4. United Kingdom
5. France
6. Italy
7. China
10.9 (electricity is 2.5 %)
Hydro-electric - water drives turbine
4.3
2.4 Nuclear - heats water - steam - turbine
1.8
1.7
1.5 Fuel - burns - hot gases - turbine
1.4
1845: Magnetism and light – a connection !
again…
Michael Faraday
1791 - 1867
Light must have a magnetic component
1864: Maxwell’s equations: coupling of electric and magnetic phenomena
Light is an electromagnetic wave
James Clerk Maxwell
Ludwig Boltzmann (Germany,1844-1906)
(Scotland, 1831 – 1879)
“Was it a god who wrote these symbols
…unveiling the forces of nature?”
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
Aligning or “switching” a magnetic material
- Oersted’s method still used after 186 years ! Magnetic material, e.g iron
current
magnetic regions
“domains”
Magnetism and Magnetic Materials today……
the high-tech society…
The spin on electronics:
magnetic devices in computers
“chip - RAM”
“disk”
“0” and “1” bits = small magnetic areas with opposite magnetization direction
Magnetic recording technology
Courtesy of Jan Thiele, Hitachi
The Data Storage Industry
$53 billion/year worldwide revenues
IBM RAMAC 1956:
1 song
stack of 50 giant discs, 24" diameter
total 4.4 MB - $35,000/year rent
Today:
200 GB = 200, 000 MB - $ 90
10,000 songs
Brightness of x-ray sources
“smaller and faster”
X-rays have come a long way……
x-ray images … then and now
1
0 “bits”
10,000 times
smaller
1895
W. Röntgen, Würzburg
1993
Magnetic x-ray image
ALS
The Power of X-Rays
A Glimpse of the Future……
exploring spin-electronics
“spintronics”
Beyond Oersted’s method – “spin doctoring”
The concept of a “spin polarized current”
copper
electrons
spins
copper
magnetic atoms
in iron
spin polarized current
Spin currents: a new way of magnetic switching:
electric current creates magnetic field
“Oersted field”
spin current acts like an
“Exchange field”
sensor
layer
reference
layer
Weak, long range
sensor
layer
Strong, short range
Switching of magnetic memory cells (MRAM)
cell to
be switched
switching
better: switching
by Oersted
byfield
current
around
in wire
wire
current
current
X-ray microscopy image of spin injection structure
100 x 300 nm
y
Detector
x
leads for ns
current pulses
x-ray image
Movie 1
X-Ray microscopy images of magnetization in buried sensor layer
After current in one direction
After current in other direction
magnetization has complicated “horseshoe” state
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...
Acknowledgements
Yves Acremann, SSRL
Ann Mueller, SSRL
Michael 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