The dependence of magnetic domain structure upon magnetization state in polycrystalline pyrrhotite

Abstract

The dependence of domain pattern on magnetization state is studied in polycrystalline pyrrhotite, using the Bitter technique. Domain patterns for thermal demagnetization and thermoremanent magnetization (TRM) acquired in a 0.5 Oe field are characterized by undulating, non-planar domain walls, portions of which can be moved reversibly by DC fields of less than 10 Oe. Upon alternating field (AF) demagnetization in progressively higher peak fields, this complex pattern is gradually transformed into a simple pattern of parallel, 180° walls. A similarly simple structure is observed for saturation isothermal remanent magnetization (SIRM) and anhysteretic remanent magnetization (ARM) acquired in an AF field of 1000 Oe with a DC bias field of 0.5 Oe. In contrast to the magnetically soft behavior of domain walls after thermal demagnetization and TRM, the domain walls present after AF demagnetization, SIRM, and ARM require several tens of Oe to move visibly. In addition, the weak-field susceptibility is significantly lower after AF demagnetization than after thermal treatment in a low or null field. These observations indicate that the domain wall geometry of highly multidomain material can be critically dependent upon magnetization state and therefore should be considered in models of magnetization processes such as TRM.