Disorder-Induced Microscopic Magnetic Memory

Using coherent x-ray speckle metrology, we have measured the influence of disorder on major loop return point memory (RPM) and complementary point memory (CPM) for a series of perpendicular anisotropy $\mathrm{C}\mathrm{o}/\mathrm{P}\mathrm{t}$ multilayer films. In the low disorder limit, the domain structures show no memory with field cycling—no RPM and no CPM. With increasing disorder, we observe the onset and the saturation of both the RPM and the CPM. These results provide the first direct ensemble-sensitive experimental study of the effects of varying disorder on microscopic magnetic memory and are compared against the predictions of existing theories.

Figure 1

Measured magnetic hysteresis loops. Starting at the origin, and moving to increasing applied field $|H|$, the loops shown are for samples 3, 7, 8.5, 10, 12, and 20. The inversion symmetry through the origin is shown.

Figure 2

Measured effects of the argon sputtering pressure on the magnetic domains. Top: real space. Three square micron MFM images of the magnetic domain structure at remanence are shown. Note the apparent disappearance of the labyrinthine structure as the disorder grows. Bottom: reciprocal space. The radial profiles measured by coherent x-ray magnetic scattering at the coercive point. The labels (3, 8.5, 10, 12, and 20) indicate the argon sputtering pressure in milliTorr during the growth of that film (see Table ).

Figure 3

Measured RPM and CPM values versus the applied field for the 8.5 milliTorr sample.

Figure 4

RPM and CPM values at the coercive point versus disorder. Top: our measured RPM and CPM values versus the measured rms roughness. Bottom: the RPM and CPM values obtained from our nonzero-temperature numerical simulations that combine the RCIM and the RFIM.