Disorder-induced magnetic memory: experiments and theories
Beautiful theories of magnetic hysteresis based on randommicroscopic disorder have been developed over the past ten years. Ourgoal was to directly compare these theories with precise experiments. Todo so, we first developed and then applied coherent x-ray specklemetrology to a series of thin multilayer perpendicular magneticmaterials. To directly observe the effects of disorder, we deliberatelyintroduced increasing degrees of disorder into our films. We usedcoherent x rays, produced at the Advanced Light Source at LawrenceBerkeley National Laboratory, to generate highly speckled magneticscattering patterns. The apparently "random" arrangement of the specklesis due to the exact configuration of the magnetic domains in the sample.In effect, each speckle pattern acts as a unique fingerprint for themagnetic domain configuration. Small changes in the domain structurechange the speckles, and comparison of the different speckle patternsprovides a quantitative determination of how much the domain structurehas changed. Our experiments quickly answered one longstanding question:How is the magnetic domain configuration at one point on the majorhysteresis loop related to the configurations at the same point on theloop during subsequent cycles? This is called microscopic return-pointmemory "RPM". We found that the RPM is partial and imperfect in thedisordered samples, and completely absent when the disorder is below athreshold level. We also introduced and answered a second importantquestion: How are the magnetic domains at one point on the major looprelated to the domains at the complementary point, the inversionsymmetric point on the loop, during the same and during subsequentcycles? This is called microscopic complementary-point memory "CPM". Wefound that the CPM is also partial and imperfect in the disorderedsamples and completely absent when the disorder is not present. Inaddition, we found that the RPM is always a little larger than the CPM.We also studied the correlations between the domains within a singleascending or descending loop. This is called microscopic half-loop memoryand enabled us to measure the degree of change in the domain structuredue to changes in the applied field. No existing theory was capable ofreproducing our experimental results. So we developed theoretical modelsthat do fit our experiments. Our experimental and theoretical results setbenchmarks for future work.
- Research Organization:
- COLLABORATION - University ofWashington
- DOE Contract Number:
- DE-AC02-05CH11231
- OSTI ID:
- 901824
- Report Number(s):
- LBNL-62565; R&D Project: 504103; TRN: US0702670
- Journal Information:
- Physical Review B, Vol. 75, Issue 14; Related Information: Journal Publication Date: 04/05/2007
- Country of Publication:
- United States
- Language:
- English
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