MAGNETIC HYSTERESIS AS A FUNCTION OF LOW-TEMPERATURE IN ROCKS - EVIDENCE FOR INTERNAL-STRESS CONTROL OF REMANENCE IN MULTIDOMAIN AND PSEUDO-SINGLE-DOMAIN MAGNETITE

Magnetic hysteresis loops are measured as a function of temperature as magnetite-bearing rocks are cooled to about -150°C. For low coercive force samples, coercive force HC and saturation remanence JR are found to vary on cooling in approximate proportion to λS JS, where λS is magnetite's polycrystalline saturation magnetostriction and JS is its saturation magnetization. This is expected of multi-domain grains with domain wall motion opposed through internal stresses σ1 so that HC = C1σ1λS JS and JR = HC N1 (where C1 is a constant and N1 is the average self-demagnetizing factor of the multi-domain grains). The slope of the plot of HC vs. λS JS on cooling gives an estimate of C1σ1 which is found to vary approximately as the inverse of the cube root of grain diameter for crushed natural magnetite. For the higher coercive force samples, plots of JR vs. HC on cooling yield straight lines that often pass above rather than through the origin. It is argued that this is good evidence for the presence of a small fraction f{hook}2 of magnetite in pseudo-single-domain grains with coercive force HC2 controlled through internal stresses σ2 so that HC2 = C2σ2λS JS and with saturation remanence JR2 independent of the self-demagnetizing factor. Assuming that JR2 = JS 2, a minimum estimate of f{hook}2 should be given by the JR-intercept of JR vs. HC on cooling divided by JS 2. A maximum estimate off{hook}2 should be given by the JR-intercept of JR vs. λS JS on cooling divided by JS 2. About a tenth of the magnetite in the highest coercive force samples is estimated to be pseudo-single-domain. It may consist of small elongated grains in which reverse domains are difficult to nucleate. It is shown that estimating internal stresses from reversible work Wrev recovered between saturation and the remanence point of hysteresis loops is probably not reliable for magnetite since Wrev for all our hysteresis loops (to 1200 Oe maximum field) remains approximately constant on cooling rather than varying in proportion to λS. © 1990.