The origin of thermoremanent magnetization: Contribution of pseudo-single-domain magnetic moments

The pseudo-single-domain (PSD) intensity and stability of thermoremanent magnetization (TRM) in multidomain magnetite grains 0.05–15 μm in size are attributed to residual magnetic moments not removed by demagnetization. Barkhausen discreteness in domain wall positions is a possible source of such moments, but the observed grain size and applied field dependences of TRM in the lower PSD range are more convincingly explained by a new theory (F.D. Stacey and S.K. Banerjee, 1974) in which the moments of domain walls and the surface terminations of domain walls play a central role. The magnitudes of PSD moments and the average number of moments per grain required by this theory are deduced from low-field (0–25 Oe) TRM measurements on magnetite grains of controlled sizes between 0.04 and 0.22 μm. The predicted maximum PSD moment is about equal to the saturation moment in grains ≤ 0.1 μm in size but is only 10% of the saturation moment in 0.22-μm grains. Since blocking temperature and hysteresis data independently suggest two-domain structure in 0.22-μm grains and wall-like domain structure in smaller grains, the predicted PSD moments are quantitatively reasonable.