EFFECT OF CRYSTAL DEFECTS AND INTERNAL-STRESS ON THE DOMAIN-STRUCTUREAND MAGNETIC-PROPERTIES OF MAGNETITE

Domain structures in magnetite are very sensitive to crystal imperfect ions, which play a major role in hysteresis and remanence by hindering the motion of domain walls. Using the Bitter colloid technique, we ha ve observed spike and closure domains of the style predicted by Neel [ 1944] around nonmagnetic inclusions, chemically altered regions, and g rain boundaries in a natural single crystal of magnetite. Isolated inc lusions within body domains have pairs of attached Neel spikes which r educe magnetostatic energy by diluting magnetic poles. In one example we calculated that spikes reduced the energy by a factor of 6-7. In so me cases 71 degrees, 109 degrees and 180 degrees domain walls are pinn ed to defects either through spikes or via chains of subsidiary closur e domains. One example of pinning by a spike gave a calculated microco ercivity of 0.54 mT, similar to the bulk coercive force of 0.5 mT for the crystal. ''Colloid gaps'' in 180 degrees and other walls form line s parallel to a [111] easy axis and are evidence of underlying line de fects, for example, dislocations, whose stress fields deflect the spin s locally, weakening the magnetic field gradient above the walls. We h ave also observed bending of 180 degrees walls anchored at pinning sit es on the grain boundary, the first direct experimental evidence of th e 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 wa ll. Finally, we measured hysteresis curves on a companion magnetite cr ystal at temperatures T from ambient to the Curie point of 585 degrees C. Observed coercivity H-c varies with T as lambda(111)w(0.5)/M-s, in agreement with theoretical predictions of impedance of a wall of widt h w by dislocation stress fields. We therefore propose that the stabil ity of remanence in multidomain magnetite is mainly due to pinning of domain walls by crystal defects.