In Memory of H. C. SiegmannIn Memory of H. C. Siegmann- -

the father of modern spin physics the father of modern spin physics

Joachim Stöhr SLAC

• Ph.D. in 1961, LMU Munich - last student of Walther Gerlach • Full professor at ETH Zurich in 1974• 33 years at ETH: supervised 120 Diploma and 62 PhD dissertations • Robert Wichard Pohl Prize 1992 from the German Physical Society• After retirement from ETH in 2000, guest Professor at SLAC• Co-authored textbook on magnetism and helped supervise 15 PhD dissertations

Hans’ role in magnetism research

a historical perspective

Magnetic structure of matter: atomic moments

Bulk atomic and magnetic structure e.g. NiO

Prize came at the end of an era in magnetism…intensive work in 1950 - 60 by 1970s much was known about bulk magnetic structure

Modern magnetism is differentthe new era started in 1970s…

B. N. Brockhouse C. G. Shull

1969 - 76: H. C. Siegmann

• Spin polarized photoemission - spin dependent electronic structure of matter

• Invention of the GaAs spin polarized source spin polarized electron beams

1986 - 88: P. Grünberg and A. Fert

Discovery of the GMR effect Atomic engineering of novel materials

The pioneers of modern spin physics

These discoveries have moved the quantum mechanical concept of the electron spinfrom its scientific discovery in the 1920s to a cornerstone of modern technology.

They have triggered the spintronics revolution.

“for the discovery of giant magnetoresistance”

A. Fert P. Grünberg

nm

Discovery combined two concepts:

• atomic engineering of “sandwiches” with different magnetization directions

• current becomes spin polarized and flows in two separate (“up” and “down”) spin channels with different resistivity – Mott’s “two current model”

Beams of spin polarized atoms

Walther Gerlach

The Stern-Gerlach experiment 1921 - beginning of transient spins

m

Postcard by Walther Gerlachto Niels Bohr, Feb. 8, 1922

= a + b

cos = a2 – b2

+sz

-sz

Note: neutral atoms (no charge) with spin

QM wavefunction

1969 – a key breakthroughbreakthrough the discovery of spin-polarized photoemission or from spin polarized atoms to spin polarized electrons

Importance:

• electron spin polarization present in material is preserved when electrons are liberated with photons

• spin polarized electrons can reveal spin structure of materials !

Postcard from Walther Gerlach sent March 28, 1969

Discovery was prominently acknowledgedDiscovery was prominently acknowledged

Letter from Sir Neville Mott sent March 268, 1971

• At room temperature, magnetite is a half metal

• Conduction by minority spins only explains Mott’s model of Verwey transition

Spin polarized photoemission has made important contributionsSpin polarized photoemission has made important contributions Electronic structure of the oldest magnetic material: magnetite Fe3O4

130 K metallic state

• Observed exchange splitting smaller than calculated

• Observed splitting depends on position in Brillouin zone

• Observed bandwidth is narrower than calculated

• Observed temperature dependence cannot be calculated

Spin polarized photoemission has revealed theoretical limitationsSpin polarized photoemission has revealed theoretical limitations electronic correlations can only be approximately described

Ni(110) spin polarized bands

The beginning of spin-polarized electron beams - 1973The beginning of spin-polarized electron beams - 1973

The GaAs spin-polarized gun

Garwin, Pierce, Siegmann, Helv. Phys. Acta, 47, 29 (1974) and

Spin polarized electron beams Spin polarized electron beams in high energy physicsin high energy physics

Invitation to SLAC - 1973Invitation to SLAC - 1973

Beginnings of The Standard Model - The Electroweak ForceBeginnings of The Standard Model - The Electroweak Force

Steven Weinberg Sheldon Glashow Abdus Salam

• Proposed in the 1960s by Weinberg, Glashow, and Salam

• In 1967 Weinberg publishes “A Model of Leptons” which met all theoretical goals

The model produces a major controversy:

The proposed electroweak model violates parity

SLAC test of a theoretical predictionSLAC test of a theoretical prediction Unification of the electromagnetic and weak forces into one “electroweak” forceUnification of the electromagnetic and weak forces into one “electroweak” force

If parity is violated, oppositely polarized electrons will scatter with different probabilities

GaAs Source Characteristics:• High intensity: up to 5x1011 electrons per pulse at 120 Hz• Good polarization: ~ 40%• Fast reversal of polarization

C. Y. Prescott et. al., Physics Letters 77B, 347 (1978); C. Y. Prescott et. al., Physics Letters 84B, 524 (1979);Physics Today 9, 17 (1978)

The SLAC experiment produced the first observation of parity violation in a neutral current interaction!

SLAC 1978: Test of Weinberg’s modelSLAC 1978: Test of Weinberg’s model

Spin polarized electron beams Spin polarized electron beams in magnetismin magnetism

The current flows independently in two spin channels -- no spin flips ! Mott’s “two-currrent model” explains the GMR effect

1936: Mott’s “two-currrent model”explains Gerlach’s resisitivity of Ni

Spin dependent transport in materialsSpin dependent transport in materials

An ingenious test of the “two current model” probability of spin-conserving and “spin-flip” transitions

transport

no suitable spin polarization detector

spindetector

beam

EV

diffusivetransport

ballistictransport

Energy

Test of the two current model with spin polarized beamsTest of the two current model with spin polarized beams

No spin flips are detected !

Siegmann, Meier, Erbudak, Landolt, Adv. El. and El. Phys. 62, 1- 99 (1984)

The end