Effects of internal stress on remanence intensity jumps across the Verwey transition for multi-domain magnetite

The magnetic properties of magnetite (Fe3O4) are strongly dependent on the internal stress related to stress-controlled regions and to closure domains associated with defects. The contribution of internal stress to the low-temperature magnetic properties of magnetite was tested using annealed and unannealed multi-domain (MD) magnetites. During low-temperature cooling, a room temperature-induced saturation isothermal remanent magnetization (SIRM) increased abruptly at the Verwey transition (Tv ∼ 122 K). In particular, the absolute intensity jump (δVJ, defined as the jump in SIRM at Tv upon cooling) resulted from the high-coercivity fraction of MD grains. We observe that annealing significantly reduces internal stress and thus decreases the average microcoercivity. Comparison of the alternating field (AF) demagnetization spectra of δVJ both for annealed and unannealed magnetites directly links δVJ to the internal stress. It is likely that removal of the closure domain associated with stress-controlled regions was dominant when the peak AF was less than the average micromagnetic coercivity 〈hc〉, resulting in a net increase of δVJ with increasing AF. However, when the AF exceeded the 〈hc〉 threshold, δVJ decreased because the stress-controlled regions were demagnetized. Such observations could therefore be useful for estimating the 〈hc〉 of MD magnetite.