1. Soft X-Ray Studies of Surfaces, Interfaces and Thin Films: From Spectroscopy to Ultrafast Nanoscale Movies Joachim Stöhr SLAC, Stanford University http://www-ssrl.slac.stanford.edu/stohr http://www-ssrl.slac.stanford.edu/stohr Work supported by the DOE Office of Basic Energy Science

2. Overview of my talk The Power of Soft X-rays Polarized X-Ray Absorption Spectroscopy  Liquid crystal alignment on surfaces X-Ray Spectro-Microscopy  Ferromagnetic alignment on an antiferromagnetic surface Time Dependent X-Ray Spectro-Microscopy  Switching of magnetic nano-structures with spin currents A Glimpse of the Future

3. What are soft x-rays anyway? VUVHard X-Rays 30 eV3000 eV Soft X-Rays 100 eV ~ 10 nm 1000 eV ~1 nm

4. Opening the soft x-ray region – late 1970s Stanford Synchrotron Radiation Lab 500 eV800 eV Oxygen SEXAFS oxidized Al surface 12/ 5/1977 Photon flux Photon energy (eV) O K-edge Grasshopper monochromator Spectroscopy in the important region 280 – 1000 eV became possible

5. Tunable x-rays offer atom specific valence shell information through guided transitions Element specificity, Chemical specificity, Valence properties magnetic multilayerpolymer

6. Polarized x-ray absorption determines charge and spin orientation Antiferromagnetic order Orientational order of bonds Ferromagnetic order

7. Liquid crystal alignment on rubbed polymer surfaces …discovered in 1907… Use of soft x-rays to solve a 100 year old puzzle Note LC “pretilt” out of plane

8. A $30 billion world-wide business Alignment is basis of liquid crystal displays

9.  X-ray diffraction on polyimide suggests epitaxy-like nucleation  Oldest model assumes micro grooves in polymer surface Conventional models of alignment mechanism Models cannot explain LC “pretilt” angle up from plane

10. A key observation in 1998: Directional ion beam irradiated polymers also align liquid crystals Pretilt direction is opposite !

11. LCs align on a-carbon surface layer, not on polymer substrate Is LC alignment due to bond orientation on substrate surface? X-ray spectroscopy of ion beam modified polymer surface reference sample

12. Do not need polymers at all ! start with a-Carbon – align with ion beam Rubbing and ion beam create molecular level orientational order Highest resolution displays today use ion beam aligned carbon films Nature 411, 56 (2001); Science 292, 2299 (2001)

13. Polarization Dependent Imaging with X-Rays Oriented molecular regions Antiferromagnetic regions Ferromagnetic regions

14. Tackling a 50 year old mystery with x-rays: “Exchange bias” How can a “neutral“ antiferromanet bias a ferromagnet? Effect remained a puzzle ever since its discovery in 1956 Conventional techniques could not study the all-important interface Key modern magnetic building blocks are based on fixed (“pinned”) ferromagnetic reference layers does not turn in external fields pinned by antiferromagnet turns in external fields: “0” or “1” bits Reference layer

15.  X-Rays reveal interfacial coupling of FM and AFM domains Ni edge – use linear polarization – antiferromagnetic domains Co edge – use circular polarization – ferromagnetic domains H. Ohldag et al., PRL 86, 2878 (2001) [010]  2m2m 2nm

16. X-Rays-in / Electrons-out: A way to study thin film interfaces pure Co/NiO pure Co/NiO pure Interface is mixed CoNiO x layer - is it magnetic?

17. Images of the Ferromagnet-Antiferromagnet Interface Ohldag et al., PRL 87, 247201 (2001) Interface layer contains ferromagnetic NiO x - is it coupled to AFM NiO?

18. Exchange bias model A thin interfacial diffusion layer (1–2 layers) of CoNiO x is formed Interface layer contains ferromagnetic Ni spins from modified NiO About 95% of interfacial Ni spins rotate with FM (not pinned) Only < 5% of interfacial Ni spins are pinned to bulk NiO This tiny fraction is the origin of exchange bias Ohldag et al PRL 91, 017203 (2003)

19. What have we learned so far ? Interface effects play import role in modern nanoscale materials Suble interface properties can lead to important phenomena Soft x-rays are powerful tool to reveal interface-specific effects  elemental specificity  chemical specificity  magnetic specificity  orientational specificity  nanoscale spatial resolution The new frontier: dynamics or “the need for speed”

20. The ultrafast technology gap Drivers of Modern Magnetism Research: Smaller and Faster Fundamental Timescales Operational Timescales The goal

21. Bunch spacing 2 ns Bunch width ~ 50 ps Time Resolution: Pulsed X-Rays from Electron Storage Ring beam line pulsed 50 ps x-rays State-of-the art ultrafast electronics : Y. Acremann et al., Rev. Sci. Instr. 78, 14702 (2007). J. P. Strachan et al., Rev. Sci. Instr. 78, 54703 (2007).

22. From reading to writing information Suggested by J. Slonczewski & L. Berger in 1996 “spin torque switching” – no external magnetic field ! Verified by: F.J. Albert, J.A. Katine, R.A. Buhrman, D. Ralph, Appl. Phys. Lett. 77, 3809 (2000) free fixed

23. Time-Resolved Scanning Transmission X-Ray Microscopy Detector leads for current pulses 2 nm magnetic layer buried in 250nm of metals current ~100 nm Y. Acremann et al., Phys. Rev. Lett. 96, 217202 (2006) X-ray image 5  m 100nm

24. Spin Torque Switching : 180nm x 110nm x 2 nm nanostructure of CoFe switch back current pulse switch Y. Acremann et al., Phys. Rev. Lett. 96, 217202 (2006) J. P. Strachan et al., Phys. Rev. Lett. 100, 247201 (2008) + _ 200ps400ps600ps800ps t=0 100 nm

25. Vortices are important on all length scales ~ 50nm 100 km 100,000 light years = 10 18 km Hurricane Milky Way Nano-element

26. A Glimpse of the Future X-ray snap shots on the fundamental time scales of motion of atoms, electrons and spins ….femtoseconds and faster….

27. The Light Fantastic Birth of the X-Ray Laser …..and a New Era of Science The Light Fantastic Birth of the X-Ray Laser …..and a New Era of Science

28. The End