Soft X-Ray Studies of Surfaces, Interfaces and Thin Films: From Spectroscopy to Ultrafast Nanoscale Movies Joachim Stöhr SLAC, Stanford University Work supported by the DOE Office of Basic Energy Science

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

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

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

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

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

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

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

 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

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

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

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)

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

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

 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

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?

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

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, (2003)

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”

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

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, (2007). J. P. Strachan et al., Rev. Sci. Instr. 78, (2007).

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

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, (2006) X-ray image 5  m 100nm

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, (2006) J. P. Strachan et al., Phys. Rev. Lett. 100, (2008) + _ 200ps400ps600ps800ps t=0 100 nm

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

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

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

The End