Disorder-induced magnetic memory: Experiments and theories

TL;DR

This study combines precise experiments and new theoretical models to investigate how disorder affects magnetic memory in thin multilayer materials, revealing partial and imperfect return-point and complementary-point memory behaviors.

Contribution

The paper introduces a novel application of coherent x-ray speckle metrology to directly observe disorder effects on magnetic domain configurations and develops theoretical models that align with experimental results.

Findings

Disorder causes partial and imperfect magnetic memory effects.

Return-point memory is slightly larger than complementary-point memory.

Theoretical models successfully fit experimental speckle pattern data.

Abstract

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. 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 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. 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? The microscopic return-point memory(RPM) is partial and imperfect in the disordered samples, and completely absent when the disorder was not present. We found the complementary-point memory(CPM) is also partial and imperfect in the disordered samples and completely absent when the disorder was not present. 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. We developed new theoretical models that do fit our experiments.