Magnetic hysteresis as a function of low temperature for deep-sea basalts containing large titanomagnetite grains—inference of domain state and controls on coercivity

Abstract

Hysteresis loops to 1200 Oe (95 kA∙m⁻¹) are measured between 295 and 105 K for two deep-sea basalts (DSDP, legs 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 synthetic titanomagnetite (xFe₂TiO₄∙(1 −x)Fe₃O₄) with x = 0.6.As temperature is lowered from 295 to 190 K, coercive force H_c slowly rises from ~40 to ~95 Oe (3.2 to 7.6 kA∙m⁻¹) approximately in proportion to the rise in the magnetostriction constant λ. Presumably, H_c 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 Oe (95 kA∙m⁻¹) cycles) j_R/j_S rises approximately in proportion to H_c with a constant of proportionality consistent with titanomagnetite (x = 0.6).As temperature is lowered from 190 to 120 K, H_c rises rapidly to ~400 Oe (32 kA∙m⁻¹) as a roughly linear function of the magnetocrystalline anisotropy constant K₁. Perhaps H_c is now controlled by K₁ through non-magnetic inclusions impeding domain wall motion.As temperature is lowered from 120 to 105 K, H_c rises even more rapidly to ~600 Oe (48 kA∙m⁻¹). The control over H_c seems to have changed again, though most of the titanomagnetite is in grains large enough to still likely contain a few domains. The ratio j_R/j_S reaches 0.7 by 105 K and appears to be saturating towards the theoretical limit of 0.83.