Effect of crystal defects and internal stress on the domain structure and magnetic properties of magnetite

Abstract

Domain structures in magnetite are very sensitive to crystal imperfections, which play a major role in hysteresis and remanence by hindering the motion of domain walls. Using the Bitter colloid technique, we have observed spike and closure domains of the style predicted by Néel [1944] around nonmagnetic inclusions, chemically altered regions, and grain boundaries in a natural single crystal of magnetite. Isolated inclusions within body domains have pairs of attached Néel spikes which reduce magnetostatic energy by diluting magnetic poles. In one example we calculated that spikes reduced the energy by a factor of 6–7. In some cases 71 °, 109° and 180° domain walls are pinned to defects either through spikes or via chains of subsidiary closure domains. One example of pinning by a spike gave a calculated microcoercivity of 0.54 mT, similar to the bulk coercive force of 0.5 mT for the crystal. “Colloid gaps” in 180° and other walls form lines parallel to a <111> easy axis and are evidence of underlying line defects, for example, dislocations, whose stress fields deflect the spins locally, weakening the magnetic field gradient above the walls. We have also observed bending of 180° walls anchored at pinning sites on the grain boundary, the first direct experimental evidence of the effect of internal stresses on the domain structure of magnetite. We determined internal stress magnitudes in the range 7–34 MPa from the observed linear dimension and transverse displacement of each bowed wall. Finally, we measured hysteresis curves on a companion magnetite crystal at temperatures T from ambient to the Curie point of 585°C. Observed coercivity Hc varies with T as λ111W0.5/Ms, in agreement with theoretical predictions of impedance of a wall of width w by dislocation stress fields. We therefore propose that the stability of remanence in multidomain magnetite is mainly due to pinning of domain walls by crystal defects.