Coercive forces and coercivity spectra of submicron magnetites

Abstract

Remanent coercivity spectra have been determined for four dispersions of magnetite with mean particle sizes of 0.04–0.22 μm using four different methods: (1) acquisition of isothermal remanent magnetization (IRM) in continuous fields; (2) stepwise reversal of saturation IRM by opposite-polarity continuous fields; (3) stepwise alternating-field (AF) demagnetization of saturation IRM; and (4) AF demagnetization of weak-field thermoremanent magnetization (TRM). Alternating-field methods gave narrower distributions with lower median coercivities than continuous-field (CF) methods. The two CF spectra are offset: the remanent coercive force, H R , which is the median coercivity determined by CF reversal of saturation remanence, is always less than the median coercivity, H′ R , determined from IRM acquisition. The median destructive field of saturation IRM, H˜ 1/2 , the median coercivity determined by AF demagnetization, is less than either H R or H′ R . The relation reported by Dankers [1], H′ R +H˜ 1/2 ≈ 2H R , is obeyed for these samples and for a companion suite of single-domain magnetites and maghemites. Ordinary coercive force. H C , is substantially affected by reversible magnetization and is a poor indicator of the median coercivity of remanence. H C is much smaller than H˜ 1/2 , H R or H′ R and varies in a different way with particle size.