The Persistence of Memory

Michael S. PiercePhysics DepartmentUniversity of Washington

The impact of disorder on magnetic memory and domain configurations.

More on Moore

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?

An X-ray Scattering Experiment

Xray picture

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.

3mTorr 7mTorr 8.5mTorr

10mTorr 12mTorr 20mTorr

MFM Images of the samples

Majorloop Hysteresis Curves 1Major Loop Hysteresis Curves

Majorloop Hysteresis Curves 2Major Loop Hysteresis Curves

Majorloop Hysteresis Curves 3Major Loop Hysteresis Curves

Majorloop Hysteresis Curves 4Major Loop Hysteresis Curves

Majorloop Hysteresis Curves 5Major Loop Hysteresis Curves

Majorloop Hysteresis Curves 6Major Loop Hysteresis Curves

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

Return Points and Conjugate PointsReturn Point Memory and Conjugate Point Memory

3mT pointsReturn Point Memory and Conjugate Point Memory

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?

Domain Configurations at Low Disorder

Eduardo’s Simulation Our ExperimentR

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space

Q-s

pace

Domain Configurations at High Disorder

Eduardo’s Simulation Our ExperimentR

eal-

space

Q-s

pace

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.

Our experiment and Eduardo’s model at the coercive point

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 athatter@u.washington.edu

Thank You!

Papers available viahttp://bragg.phys.washington.edu/papers.html

Or contact via email athatter@u.washington.edu