Domain wall pinning and dislocations: Investigating magnetite deformed under conditions analogous to nature using transmission electron microscopy

Abstract

In this study, we deformed samples cut from a single magnetite octahedron and used transmission electron microscopy (TEM) and magnetic measurements to experimentally verify earlier computational models of magnetic domain wall pinning by dislocations and to better understand the nature of dislocations in magnetite. Dislocations in magnetite have been of interest for many decades because they are often cited as a likely source of stable thermoremanent magnetizations in larger multidomain (MD) magnetite grains, so a better understanding of dislocation effects on coercivity in MD magnetite is crucial. TEM imaging shows, for the first time, domain walls sweeping through the magnetite sample and being pinned at dislocations. In agreement with theory, these findings demonstrate that domain walls are more strongly pinned at networks of dislocations than at single dislocations and that domain walls pinned at longer dislocations have higher microcoercivities than those pinned at shorter dislocations. This experimentally illustrates the ability of dislocations to increase the coercivity of larger multidomain magnetite grains. The observed values for microcoercivity and bulk coercivity are in reasonable agreement with theoretical calculations. Burgers vectors were determined for some dislocations to verify that they were in keeping with expected dislocation orientations. The dislocations were found to be primarily located on close-packed {111} planes within the magnetite. Deformation caused only a minor change in bulk coercivity, but first-order reversal curve diagrams show populations with increased coercivity not visible in hysteresis loops.