Persistence of memory – how does a magnet “remember” it’s state? Does it remember what it used to look like if you cycle the magnetic field? It appears the answer is yes, and disorder is the driving force behind the “persistence” factor.
Mike Pierce and collaborators have studied CoPt magnetic films using soft x-rays tuned to Co L2-3 adsorption edge, where circular dichroism provides a significant contrast between different orientations of magnetisation in the film. The speckle resulting in small-angle x-ray scattering pattern codes the information about specific magnetic domain orientation within the sample. By cycling the magnetic field and comparing the speckles one can calculate the “persistence” of memory or correlation of magnetic domain configurations between the cycles, as the fingerprint-like domains are forced to nucleate, grow and disappear.
The correlations increase for rougher films, because of stronger “pinning” due to disorder, below are several excellent papers by Washington/Oregon/Hitachi/UCSC collaboration lead by Larry Sorensen and his former student Mike Pierce (who is now a postdoc at Argonne).
Development of such precise x-ray speckle metrology is (experimentally) a truly remarkable feat and something that cannot be easily accomplished with local microscopy probes, such as MFM, for example, since presence of magnetic tip is likely to nucleate new domains.
For details see:
M.S. Pierce et al,