The Persistence of Memory

1. The Persistence of Memory The impact of disorder on magnetic memory and domain configurations. Michael S. Pierce Physics Department University of Washington

2. More on Moore

3. Experimental Collaborators • University of Washington • Larry Sorensen • Conor Buechler • Bo Hu • Robert Moore • Paul Unwin • University of Oregon • Steve Kevan • Josh Turner • U.C. Davis • Kai Liu • Joe Davies • Hitachi • Olav Hellwig • Eric Fullerton • MAX Lab • J. Hunter-Dunn • LBNL • Jeff Kortright • Karine Chesnel

4. Theoretical Collaborators • Abdus Salam International Centre for Theoretical Physics • Eduardo Alberto Jagla • University of California Santa Cruz • Josh Deutsch • Trieu Mai • Onuttom Narayan • University of California Davis • Christopher Pike • Richard Scalettar • Gergely Zimanyi • University of Washington • Conor Buechler

5. Major Loop Return Point Memory The net magnetization repeats, but what about the microscopic magnetic domains?

6. Major Loop Conjugate Point Memory What about the microscopic magnetic domains on different sides of the major loop?

7. An X-ray Scattering Experiment

8. Xray picture

9. Multilayer Ferromagnetic Films(grown by Olav Hellwig and Eric Fullerton) Magnetization driven by the interfacial roughness. Films grown via magnetron sputtering. Changes in sputtering pressure change the interfacial roughness.

10. MFM Images of the samples 3mTorr 7mTorr 8.5mTorr 10mTorr 12mTorr 20mTorr

11. Majorloop Hysteresis Curves 1 Major Loop Hysteresis Curves

12. Majorloop Hysteresis Curves 2 Major Loop Hysteresis Curves

13. Majorloop Hysteresis Curves 3 Major Loop Hysteresis Curves

14. Majorloop Hysteresis Curves 4 Major Loop Hysteresis Curves

15. Majorloop Hysteresis Curves 5 Major Loop Hysteresis Curves

16. Majorloop Hysteresis Curves 6 Major Loop Hysteresis Curves

17. Domain reversal The applied field is decreased, taking the sample from saturation to past the coercive point.

18. What can we learn? Diffuse scattering tells us about: • Domain Widths • Domain Correlations • General Configuration What about the speckles? • Specific, Microscopic Configuration

19. Return Point Memory and Conjugate Point Memory Return Points and Conjugate Points

20. Return Point Memory and Conjugate Point Memory 3mT points

21. For quantitative comparison of two speckle patterns take the standard correlation coefficient = 1 for perfect correlation and  = 0 for no correlation And write it in terms of auto and cross-correlation functions a

22. RPM & CPM in Low Disorder 3mTorr sample All our measured values are consistent with zero. No RPM or CPM!

23. RPM & CPM in Disordered 8mTorr sample • RPM > CPM ! • Neither are zero or one ! • Neither depend upon the number of loops ! • Both start at large values and decrease !

24. Measured memory at the coercive point At low disorder, there is little-to-no memory then followed by rapid growth and apparent saturation as the disorder grows.

25. At about the same time… In beautiful Trieste Italy… Eduardo Alberto Jagla: Numerical Simulations of two dimensional magnetic domain patterns. cond-mat/0402406 Is there a way to vary the disorder in Eduardo’s Model?

26. Eduardo Jagla’s Model Important points: • Continuous, Not Discrete Site Magnetization • Scalar Field Theory • Long-range Interactions Basically it comes down to: H = (4 Theory) + (Dipole Interaction) + (External Field) So what would happen if a small, static random field and/or coercive random field were included in Eduardo’s model?

27. Domain Configurations at Low Disorder Eduardo’s Simulation Our Experiment Real-space Q-space

28. Domain Configurations at High Disorder Eduardo’s Simulation Our Experiment Real-space Q-space

29. Eduardo’s Simulation at the coercive point At low disorder, there is little-to-no memory then followed by rapid growth and apparent saturation as the disorder grows.

30. Our experiment and Eduardo’s model at the coercive point

31. How is it that the addition of random fields and random coercivity cause RPM > CPM ? The random fields do not change sign under spin-reversal. They introduce a component which is not symmetric about conjugate points on the major loop. The random coercivity do change sign under spin-reversal. They are symmetric about conjugate points on the major loop. The addition of static random fields is an excellent idea. But maybe there is a more fundamental explanation…

32. At about the same time…In another, closer part of the world… Josh, Trieu and Onuttom were working along similar lines.

33. Future Possibilities RPM and CPM Properties What are the memory properties inside the major loop? Different samples may have different properties. FORCs What information do XFORCs provide? How can our speckle patterns be compared to FORC diagrams? Dynamics Is Barkhausen Noise observable through dynamic light scattering? Can we observe the speckles as they twinkle? Theory & Modeling Can we distinguish between the new theories of how our magnetic systems behave? Real-space and Imaging XRM study is of great interest. Can we invert a speckle pattern to obtain the domain configuration?

34. Where to find more information: Disorder-induced microscopic magnetic memory. Pierce, M.S., et al. Phys. Rev. Lett. In Limbo. (2004) Quasistatic X-ray Speckle Metrology of Microscopic Magnetic Return Point Memory. Pierce, M.S., R.G. Moore, L.B. Sorensen, S.D. Kevan, J.B. Kortright, O. Hellwig, E. Fullerton. Phys. Rev. Lett. 90, 175502 (2003) Papers available via http://bragg.phys.washington.edu/papers.html Or contact via email at hatter@u.washington.edu This work is supported by the DOE.

35. Thank You! Papers available via http://bragg.phys.washington.edu/papers.html Or contact via email at hatter@u.washington.edu