The Magic of Magnetism From Physical Attraction to

The Magic of Magnetism From Physical Attraction to Spin Doctors Joachim Stöhr Stanford Synchrotron Radiation Laboratory Stanford Linear Accelerator Center

Magnetism and Humans “Animal Magnetism” – Mesmerizing – Magnetic Healing Magnetic Personality - Attraction - h t y m o r F m to c s n ie e c … . .

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 Fe 3 O 4

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 11 th or early 12 th century compass became known in Europe sometime later in the 12 th 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)

Around 1800: “The mystery of magnetism, explain that to me ! 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

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” copper electrons spins copper magnetic atoms in iron spin polarized current

Spin currents: a new way of magnetic switching: electric current creates magnetic field spin current acts like an “Oersted field” curre nt “Exchange field” sensor layer curre nt 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 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/stohrgroup New book: J. Stöhr and H. C. Siegmann Magnetism: From Fundamentals to Nanoscale Dynamics Springer Verlag, February/March 2006

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Magnetic recording: Small and fast ! 1 mile/hour 1000 miles/hour e e ed r c In ib ll a m ly s g n i d a – s t i b in l b i d cre r d n a g n i writ s s d e pe