Low-temperature demagnetization of saturation remanence in rocks bearing multidomain magnetite

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Abstract Saturation remanence is given near room temperature to eight magnetite-bearing samples whose coercive force HC ranges from 11 to 152 Oe ((11–152) × 103/4π A m−1)). Intensity of remanence JR is measured while cooling the samples to about −177°C in zero field and then reheating to room temperature. On cooling to the temperature at which HC is a minimum (− 146 ± 6°C), JR decreases (by half on average) in approximate proportion to HC in most samples, as expected of multidomain grains. Because HC for these samples has been shown to decrease in approximate proportion to the polycrystalline saturation magnetostriction λ S during this cooling, much of the demagnetization of saturation remanence is probably the result of decrease in λ S, which presumably unblocks domain walls pinned by internal stresses. (In contrast, change in the magnetocrystalline anisotropy constant K1 on cooling is probably not a major contributor to this demagnetization as only in the samples of lowest HC does the saturation remanence given near − 148°C show much change on warming through the temperature TK at which K1 = 0). On cooling further to − 162 ± 5°C, there is a further more rapid remanence loss (averaging a quarter of the initial remanence). This further demagnetization is centred on the Verwey temperature TV (= − 152 ± 4°C) and is presumably the result of domain restructuring forced by the crystallographic transition at TV. On warming back to room temperature, remanence loss is partially recovered (recovery averaging a sixth of the initial remanence). Most of the recovery in remanence is centered on TV rather than TK and is thus probably a result of the crystallographic transition at TV.