Theoretical effects of mechanical grain-size reduction on GEM domain states in pyrrhotite

Abstract

Recent laboratory experiments by Halgedahl and Ye (1999) show that domain widths in pyrrhotite change very little, or not at all, as a grain is mechanically thinned along one or two directions. In their experiments, particles were initially demagnetized in an alternating field until a global energy minimum (GEM) domain state was attained. Surprisingly, the overall positions of surviving walls and many small-scale details in the shapes of curved walls were remarkably insensitive to thinning. Thus, domain states that survived thinning were interpreted to be local energy minimum (LEM) states. As a first step toward providing a theoretical reference frame for the thinning results, GEM domain widths in pyrrhotite have been calculated here as grains are thinned to one-fourth or less of their original size. Nine models assume one-dimensional (1D) thinning, which greatly changes both particle size and shape. Two other models address the effects of three-dimensional (3D) thinning, in which particles retain a cubic shape as their sizes are reduced. If a particle can maintain a GEM state while it is thinned, seven of the nine 1D models and both 3D models predict that domain widths will adjust by amounts that are readily detected experimentally. Thus, results of these calculations support the interpretation that LEM states in pyrrhotite can be stable over a broad range of grain sizes and shapes. The primary origin of this stability remains an unsolved problem, however. If this stability is intrinsic to the pure material, then future micromagnetic models for pyrrhotite are required to investigate LEM states and their stability as functions of grain size and grain shape. On the other hand, this stability could originate from the pinning of preexisting walls by defects [Halgedahl and Ye, 1999]. Whatever their origins, the energy barriers that inhibit LEM–LEM transitions could play a significant role in the acquisition of remanence and the temporal stability of the paleomagnetic signal in rocks.