Quasistatic X-Ray Speckle Metrology of Microscopic Magnetic Return-Point Memory

We have used coherent, resonant, x-ray magnetic speckle patterns to measure the statistical evolution of the microscopic magnetic domains in perpendicular magnetic films as a function of the applied magnetic field. Our work constitutes the first direct, ensemble-averaged study of microscopic magnetic return-point memory, and demonstrates the profound impact of interfacial roughness on this phenomenon. At low fields, the microscopic magnetic domains forget their past history with an exponential field dependence.

Figure 1

Kerr measurements of the major loops for the samples grown at different argon sputtering pressures.

Figure 2

X-ray metrology for the 3 mT sample. Top: the measured speckle pattern at zero field. Bottom: the calculated autocorrelation function. Our CCD has 1024 by 1024 pixels; the autocorrelation function is plotted in terms of 2047 by 2047 pixels. The very sharp spike in the center is a few pixels wide and is due to the speckles produced by the coherent scattering; the broad mountain under the coherent spike is hundreds of pixels wide and is due to the smooth average (incoherent) scattering.

Figure 3

The measured minor loop microscopic correlation coefficients for the 3 mT sample. The solid lines represent fits to exponential deviations from unity. The dashed lines are guides to the eye.

Figure 4

The measured major loop microscopic correlation coefficients for the 12 mT sample. The circles and squares (the triangles) represent the correlations between the first and the second (eleventh) loop.