the persistence of memory
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The Persistence of Memory. The impact of disorder on magnetic memory and domain configurations. Michael S. Pierce Physics Department University of Washington. More on Moore. Experimental Collaborators. University of Washington Larry Sorensen Conor Buechler Bo Hu Robert Moore Paul Unwin - PowerPoint PPT PresentationTRANSCRIPT
The Persistence of Memory
Michael S. PiercePhysics DepartmentUniversity of Washington
The impact of disorder on magnetic memory and domain configurations.
Experimental Collaborators
•HitachiOlav HellwigEric Fullerton
•MAX LabJ. Hunter-Dunn
•LBNLJeff KortrightKarine Chesnel
•University of WashingtonLarry SorensenConor BuechlerBo HuRobert MoorePaul Unwin
•University of OregonSteve KevanJosh Turner
•U.C. DavisKai LiuJoe Davies
Theoretical Collaborators
•Abdus Salam International Centre for Theoretical PhysicsEduardo Alberto Jagla
•University of California Santa CruzJosh DeutschTrieu MaiOnuttom Narayan
•University of California DavisChristopher PikeRichard ScalettarGergely Zimanyi
•University of WashingtonConor Buechler
Major Loop Return Point Memory
The net magnetization repeats, but what about the microscopic magnetic domains?
Major Loop Conjugate Point Memory
What about the microscopic magnetic domains on different sides of the major loop?
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.
Domain reversal
The applied fieldis decreased, takingthe sample fromsaturation to past the coercive point.
What can we learn?
• Domain Widths• Domain Correlations• General Configuration
Diffuse scattering tellsus about:
What about the speckles?
• Specific, MicroscopicConfiguration
For quantitative comparison of two speckle patterns take the standard correlation coefficient
And write it in terms of auto and cross-correlation functions
= 1 for perfect correlation and = 0 for no correlation
a
All our measured values are consistent with zero.
No RPM or CPM!
RPM & CPM in Low Disorder 3mTorr sample
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 !
Measured memory at the coercive point
At low disorder, there is little-to-no memory thenfollowed by rapid growth and apparent saturation as the disorder grows.
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?
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?
Eduardo’s Simulation at the coercive point
At low disorder, there is little-to-no memory thenfollowed by rapid growth and apparent saturation as the disorder grows.
How is it that the addition of random fields and random coercivity cause RPM > CPM ?
The addition of static random fields is an excellent idea. But maybe there is a more fundamental explanation…
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.
At about the same time…In another, closer part of the world…
Josh, Trieu and Onuttom were working along similar lines.
Future Possibilities
DynamicsIs Barkhausen Noise observable through dynamic light scattering?Can we observe the speckles as they twinkle?
RPM and CPM PropertiesWhat are the memory properties inside the major loop?Different samples may have different properties.
Theory & ModelingCan we distinguish between the new theories of how our magnetic systems behave?
Real-space and ImagingXRM study is of great interest.Can we invert a speckle pattern to obtain the domain configuration?
FORCsWhat information do XFORCs provide?How can our speckle patterns be compared to FORC diagrams?
Where to find more information:
This work is supported by the DOE.
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)
Disorder-induced microscopic magnetic memory. Pierce, M.S., et al. Phys. Rev. Lett. In Limbo. (2004)
Papers available viahttp://bragg.phys.washington.edu/papers.html
Or contact via email [email protected]
Thank You!
Papers available viahttp://bragg.phys.washington.edu/papers.html
Or contact via email [email protected]