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Microscopic Memory Print
Tuesday, 22 November 2011 17:16

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Researchers at the University of Oregon and the ALS have used the high coherence and brightness of the ALS to examine the amount of microscopic memory in a magnetic system as a function of both length scale and applied field. They observed a large memory as domains begin to nucleate near saturation, indicating that nucleation sites are highly similar across field cycles.

In this work, magnetic thin films of Co/Pd were studied using coherent x-ray scattering on ALS Beamline 12.0.2 . The coherent beam leads to speckle in the diffraction pattern that provides a unique fingerprint specific to the illuminated lateral magnetic domain configuration. The speckle pattern is divided into annular regions of constant scattering wave vector magnitude, each of which probes a particular real-space length scale.  Cross-correlating the same annular region extracted from two speckle patterns separated by a field cycle quantifies how well the microscopic magnetization at the corresponding length scale is reproduced from loop to loop. Analysis is performed as a function of both length scale and applied field/average magnetization to produce a detailed measure of the spatial evolution of magnetic memory.

 

magnetic memory

 

Coherent scattering patterns collected at H = 64 mT (left) as the magnetic system begins to reverse and H = 256 mT (right) as the reversal proceeds. The central beam stop blocks the strong direct beam and allows for longer sampling of the diffuse magnetic scattering.The ring of magnetic scattering due to the labyrinthine configuration of domains can be seen.  The magnetic scattering is modulated by speckle, which is due interference of wavefronts scattered from the coherent beam.

Results from this analysis show that the memory of this system is largest near initial magnetization reversal at a length scale that is twice that of the dominant magnetic scattering ring.  This memory peak decreases as the magnetization in the system is further reversed, indicating that the system becomes less-similar as the field is increased. Taken together, the location and field dependence of the memory peak indicate that bubble domain formation is the cause. At nucleation, both the size and the spatial distribution of the bubble domains show reproducibility; as the field increases, the bubble domains grow stochastically into a labyrinthine configuration, and the memory signature consequently decreases.



Work performed on ALS Beamline 12.0.2

Citation: Keoki A. Seu, S. Roy, R. Su, D. H. Parks, E. Shipton, E. E. Fullerton, and S. D. Kevan, “Momentum transfer resolved memory in a magnetic system with perpendicular anisotropy,” Appl. Phys. Lett. 98 122505 (2011).