Beautiful theories of magnetic hysteresis based on random microscopic disorder have been developed over the past ten years. Our goal was to directly compare these theories with precise experiments. To do so, we first developed and then applied coherent x-ray speckle metrology to a series of thin multilayer perpendicular magnetic materials. To directly observe the effects of disorder, we deliberately introduced increasing degrees of disorder into our films. We used coherent x-rays, produced at the Advanced Light Source at Lawrence Berkeley National Laboratory, to generate highly speckled magnetic scattering patterns. The apparently “random” arrangement of the speckles is due to the exact configuration of the magnetic domains in the sample. In effect, each speckle pattern acts as a unique fingerprint for the magnetic domain configuration. Small changes in the domain structure change the speckles, and comparison of the different speckle patterns provides a quantitative determination of how much the domain structure has changed. Our experiments quickly answered one longstanding question: How is the magnetic domain configuration at one point on the major hysteresis loop related to the configurations at the same point on the loop during subsequent cycles? This is called microscopic return-point memory (RPM). We found the RPM is partial and imperfect in the disordered samples, and completely absent when the disorder was not present. We also introduced and answered a second important new question: How are the magnetic domains at one point on the major loop related to the domains at the complementary point, the inversion symmetric point on the loop, during the same and during subsequent cycles? This is called microscopic complementary-point memory (CPM). We found the CPM is also partial and imperfect in the disordered samples and completely absent when the disorder was not present. In addition, we found that the RPM is always a little larger than the CPM. We also studied the correlations between the domains within a single ascending or descending loop. This is called microscopic half-loop memory (HLM) and enabled us to measure the degree of change in the domain structure do to changes in the applied field. No existing theory was capable of reproducing our experimental results. So we developed new theoretical models that do fit our experiments. Our experimental and theoretical results set new benchmarks for future work.
Department, Program, or Center
Center for Materials Science and Engineering
Pierce, Michael; Buechler, C.; and Sorensen, L., "Disorder-induced magnetic memory: Experiments and theories" (2006). American Physics Society,Accessed from
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