Magnetic hysteresis as a function of low temperature for deep-sea basalts containing large titanomagnetite grains-inference of domain state and controls on coercivity |
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| Authors: | JP Hodych |
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| Institution: | Departments of Physics and Geology, Memorial University of Newfoundland, St. John''s, Newfoundland A1B 3X7 Canada |
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| Abstract: | Hysteresis loops to 1200 oersteds (9.55×104 A m?1) are measured between 295 K and 105 K for two deep-sea basalts (DSDP, Leg 34 and 37) containing large (～200 μm) unexsolved titanomagnetite grains. The Curie points, electron microprobe analyses and saturation magnetizations of the magnetic grains are the same as for unoxidized synthetic titanomagnetite (xFe2TiO4·(l ? x)Fe3O4) with x=0.6.As temperature is lowered from 295 to 190 K, coercive force Hc slowly rises from ～40 Oe to ～95 Oe approximately in proportion to the rise in the magnetostriction constant λ. Presumably, Hc is controlled by λ through internal stresses impeding domain wall motion. As expected of multidomain grains, the ratio of saturation remanence to saturation magnetization (in 1200 oersted cycles) jR/jS rises approximately in proportion to Hc, with a constant of proportionality consistent with titanomagnetite (x=0.6).As temperature is lowered from 190 to 120 K, Hc rises rapidly to ～400 Oe as a roughly linear function of the magnetocrystalline anisotropy constant K1. Perhaps Hc is now controlled by K1 through non-magnetic inclusions impeding domain wall motion.As temperature is lowered from 120 to 105 K, Hc rises even more rapidly to ～600 Oe. The control over Hc seems to have changed again, though most of the titanomagnetite is in grains large enough to still contain a few domains. The ratio jR/jS reaches 0.7 by 105 K and appears to be saturating towards the theoretical limit of 0.83. |
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