Magnetism and X-Rays: Past, Present, and A Vision of the Future

1. Magnetism and X-Rays: Past, Present, and A Vision of the Future Joachim Stöhr Stanford Synchrotron Radiation Laboratory Stanford University Static image Femtosecond single shot image 100 picoseconds dynamics 1993 2003 200X http://www-ssrl.slac.stanford.edu/stohr/index.htm

2. Past: Press release by the Royal Swedish Academy of Sciences, Nobel Prize in Physics: B. N. Brockhouse and C. G. Shull 1994 ``Neutronsare small magnets……(that) can be used to study the relative orientations of the small atomic magnets. ….. the X-ray method has been powerless and in this field of application neutron diffraction has since assumed an entirely dominant position. It is hard to imagine modern research into magnetism without this aid." Present: 2004: It is hard to imagine modern research into magnetism without the aid of x-rays!

3. Some Magnetic Devices in Computers Present: Size > 100 nm, Speed > 1 nsec Future: Size < 100 nm, Speed < 1 nsec Ultrafast Nanoscale Dynamics

5. Experimental X-Ray Methods

6. Non-resonant magnetic x-ray scattering is weak Relative intensity of charge scattering: 1 Relative intensity of spin scattering: 10-4 First experiment: F. de Bergevin, M. Brunel: Phys. Lett. A 39, 141 (1972)

7. Development of X-Ray Techniques for Magnetism Theory: J.L. Erskine, E.A. Stern: Phys. Rev. B 12, 5016 (1975) M. Blume: J. Appl. Phys. 57, 3615 (1985) B.T. Thole, G. van der Laan, G.A. Sawatzky: Phys. Rev. Lett. 55, 2086 (1985) Experiments: X-Ray Magnetic Resonant Scattering: K. Namikawa, M. Ando, T. Nakajima, H. Kawata: J. Phys. Soc. Jpn 54, 4099 (1985) X-Ray Magnetic Linear Dichroism: G. van der Laan, B.T. Thole, G.A. Sawatzky, J.B. Goedkoop, J.C. Fuggle, J.M. Esteva, R. Karnatak, J.P. Remeika, H.A. Dabkowska: Phys. Rev. B34, 6529 (1986) X-Ray Magnetic Circular Dichroism: G. Schütz, W. Wagner, W. Wilhelm, P. Kienle, R. Zeller, R. Frahm, G. Materlik: Phys. Rev. Lett. 58, 737 (1987) X-Ray Magnetic Imaging: J. Stöhr, Y. Wu, B. D. Hermsmeier, M. G. Samant, G. R. Harp, S. Koranda, D.Dunham, B. P. Tonner: Science 259, 658 (1993)

8. Valence Shell Properties and X-Ray Magnetic Circular Dichroism (XMCD) Thole et al., PRL 68, 1943 (1992); Carra, et al., PRL 70, 694 (1993); Stöhr and König, PRL 75, 3748 (1995)

9. Fe metal – L edge Kortright and Kim, Phys. Rev. B 62, 12216 (2000)

10. Magnetic Spectroscopy and Microscopy x-ray "spin" Soft X-Rays are best for magnetism!

11. bulk surface

13. PEEM-2 at ALS • Full Field Imaging • Electrostatic (30 kV) • 20 - 50 nm Resolution • Linear and circular polarization

15. I+ I- Oxygen Fe 9 1.3 1.2 6 Electron Yield Electron Yield 1.1 3 1.0 528 530 532 700 710 720 Photon Energy [eV] Photon Energy (eV) Element Specific Magnetic Imaging: Ferromagnetic Domains in Magnetite – Magnetic Fe and Oxygen Magnetite Fe3O4 12 mm

16. Co XMCD 8 Electron Yield 4 s s [010] 0 2mm 780 776 778 Photon Energy (eV) 15 NiO XMLD 10 Electron Yield 5 0 870 874 Photon Energy(eV) Spectro-Microscopy of Ferromagnets on Antiferromagnets Tune toCo edge – use circular polarization – ferromagnetic domains Tune toNiedge – use linear polarization – antiferromagnetic domains H. Ohldag et al., PRL 86, 2878 (2001).

17. Experimental Results: • Exchange bias • Time resolved imaging of magnetic structures

18. W Exchange bias – a 50 year puzzle A ferromagnet has a preference direction when in contact with an antiferromagnet The spin-valve sensor FM 1 FM 2 AFM Blue layer: direction is fixed by exchange bias Red layer: direction determines resistance Conventional techniques cannot study the magnetic FM-AFM interface

19.  E22Ek The Basic Model – Meiklejohn (~ 1960) Exchange coupling: Bulk FM spins: S1 E12 = J12 S1 S2 Uncompensated spins: S2 E22 = J22S2S2 & anisotropy of AFM EK Bulk AFM spins: S2=S2 Observed loop shift (bias) is 100 times smaller than expected from model ! • 40+ years of theoretical models - reduce bias by: • new effective number of spins S2 • twist of AFM spins – domain wall with energy

20. 50 years of models…need experimental tests…  E22Ek Reduce bias through effective SAFM SAFM: uncompensated spins near AFM surface Origin ? Number ? Size ? Parallel or perpendicular ? Malozemoff model Koon model Reduce bias through domain wall : Domain wall energy Mauri-Siegmann model Domain wall formation ?

21. Co on NiO(001) s s s [010] 2mm NiO after deposition 2nm Co on NiO(001) Bare NiO(001) Co causes Ni spins at NiO surface to rotate into plane AFM and FM spins couple parallel

22. X-Rays-in / Electrons-out - A way to study Interfaces FM Co – tune to Co edge – circular polarization AFM NiO – tune to Ni edge – linear polarization FM Ni(O) – tune to Ni edge – circular polarization

23. Interface Microscopy Co Co Interfacial spins Ni–rich NiO NiO NiO FM: Ni-rich NiO AFM: NiO FM: Co Circular pol. Ni edge Circular pol. Co edge Linear pol. Ni edge Chemically induced interfacial Ni spins provide the magnetic link

24. X-Ray Picture of Exchange Bias The role of interfacial spins: SAFM Co/IrMn Co/NiO Co NiO pinned spins Imaging: Element specific FM loops: • AFM axis is rotated at interface • The interface is not sharp -SAFM • SAFM || SFM • Free spins: 96% of ML – coercivity • Pinned spins SAFM: 4% of ML • Small number determines bias size

25. Nanaoscale Magnetization Dynamics - Smaller and Faster

27. Time resolved x-ray microscopy PEEM2 50 nm / 100 ps resolution Laser pump – x-ray probe synchronization < 1 ps excitation laser pulse < 100 ps observation x-ray pulse t 328 ns

28. 100 m 100 m 10 m 2 m 2 m Production of Magnetic Field Pulses Photoconductive switch H ~ 200 Oe Conducting wire 50  => I = 200 mA, 10 V bias Magnetic Cells Current

29. Magnetic Patterns in 20 nm Co90Fe10 films on waveguide M 3mm Field pulse x-ray "spin"

30. Two pattern with same static structure, but ….. Field response Field response Opposite rotation is caused by direction of vortex core magnetization, i.e. chirality

31. Response to a fast field pulse Instanteneous precession determined by torque: T = H x m H slow "damping" fast (<1ns) T "precession" m H Tiny vortex core determines fast dynamics of the whole domain structure!

32. A Vision of the Future…….. • Improved microscopes – toward atomic resolution • X-ray lasers - ultrafast single shot imaging • ……..

33. Tomorrow: 5 nm spatial resolution with PEEM3 Lenses CCD Deflector Separator Lenses Manipulator High voltage feedthroughs CCD -alignment Electron mirror

34. Spatial Resolution of PEEM3 4-5 nm

35. In 2007: The first x-ray laser - LINAC COHERENT LIGHT SOURCE (LCLS) 0 Km 2 Km 3 Km

36. SASE gives 106 intensity gain • over spontaneous emission • FELs can produce ultrafast • pulses (of order 100 fs) l

37. Free electron lasers We are here Growth of X-Ray Brightness and Magnetic Storage Density each pulse: 1012 photons < 100 fs coherent

38. Lensless Imaging by Coherent X-Ray Scattering Eisebitt et al. (BESSY) Challenge: Inversion from reciprocal to real space image

39. A Glimpse of the Future…….. • Ultrafast magnetic processes

42. Experimental Principle of Ultrafast Field Pulses 100 fs – 10 ps • Relativity allows 1010electrons in short bunch of < 1 ps length • High field pulses up to 5 T = 50,000 Oe C. H. Back, R. Allenspach, W. Weber, S. S. P. Parkin, D. Weller, E. L. Garwin, H. C. Siegmann, Science285, 864 (1999)

43. Torques on Magnetization by Beam Field Maximum torque Minimum Torque

44. The Ultimate Speed of Magnetic Switching tpulse= 3 ps tpulse= 100 fs 90 mm 90 mm Deterministic switching Chaotic switching Under ultrafast excitation the magnetization fractures !

45. Magnetization fracture under ultrafast field pulse excitation Uniform precession chaotic excitation

46. The magnetism "team" – Stanford (SSRL) - Berkeley (ALS) Funded by: DOE-BES and NSF Squaw Valley, April 2003 Missing: Hans Christoph Siegmann

47. Conclusions • X-rays have become an important probe of magnetic materials and phenomena • X-rays offer elemental, chemical and magnetic specificity with nanoscale spatial resolution • Transmission experiments probe bulk, electron yield experiments probe surfaces and interfaces • X-rays allow time-dependent studies, paving the way for picosecond nanoscale technology • Future x-ray sources, new techniques and instrumentation will allow the complete exploration of magnetic phenomena in space and time For more, see: http://www-ssrl.slac.stanford.edu/stohr/index.htm H. C. Siegmann and J. Stöhr Magnetism: From Fundamentals to Nanoscale Dynamics Springer 2004 (to be published)

48. The end