In Memory of H. C. Siegmann - the father of modern spin physics Joachim Stöhr SLAC

1. In Memory of H. C. Siegmann - the father of modern spin physics Joachim Stöhr SLAC

2. 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

3. Hans’ role in magnetism research a historical perspective

4. Magnetic structure of matter: atomic moments B. N. Brockhouse C. G. Shull 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 different the new era started in 1970s…

5. The pioneers of modern spin physics • 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 These discoveries have moved the quantum mechanical concept of the electron spin from its scientific discovery in the 1920s to a cornerstone of modern technology. They have triggered the spintronics revolution.

6. nm “for the discovery of giant magnetoresistance” A. Fert P. Grünberg • 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”

7. Beams of spin polarized atoms Walther Gerlach

8. c = a + b The Stern-Gerlach experiment 1921 - beginning of transient spins Postcard by Walther Gerlach to Niels Bohr, Feb. 8, 1922 m Note: neutral atoms (no charge) with spin +sz -sz QM wavefunction cos q = a2 – b2

9. 1969 – a key breakthrough 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 !

10. Discovery was prominently acknowledged Postcard from Walther Gerlach sent March 28, 1969 Letter from Sir Neville Mott sent March 268, 1971

11. Spin polarized photoemission has made important contributions Electronic structure of the oldest magnetic material: magnetite Fe3O4 130 K metallic state • At room temperature, magnetite is a half metal • Conduction by minority spins only explains Mott’s model of Verwey transition

12. Spin polarized photoemission has revealed theoretical limitations electronic correlations can only be approximately described Ni(110) spin polarized bands • 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

13. The beginning of spin-polarized electron beams - 1973 Garwin, Pierce, Siegmann, Helv. Phys. Acta, 47, 29 (1974) and The GaAs spin-polarized gun

14. Spin polarized electron beams in high energy physics

15. Invitation to SLAC - 1973

16. Beginnings of The Standard Model - The Electroweak Force SLAC test of a theoretical prediction Unification of the electromagnetic and weak forces into one “electroweak” force Abdus Salam Steven Weinberg Sheldon Glashow • 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

17. SLAC 1978: Test of Weinberg’s model 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 The SLAC experiment produced the first observation of parity violation in a neutral current interaction! 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)

19. Spin polarized electron beams in magnetism

20. Spin dependent transport in materials 1936: Mott’s “two-currrent model” explains Gerlach’s resisitivity of Ni The current flows independently in two spin channels -- no spin flips ! Mott’s “two-currrent model” explains the GMR effect

21. An ingenious test of the “two current model” probability of spin-conserving and “spin-flip” transitions Energy spin detector beam ballistic transport EV transport diffusive transport no suitable spin polarization detector

22. Test of the two current model with spin polarized beams Siegmann, Meier, Erbudak, Landolt, Adv. El. and El. Phys. 62, 1- 99 (1984) No spin flips are detected !

24. The end