SATURATION MAGNETOSTRICTION AND ITS LOW-TEMPERATURE VARIATION INFERRED FOR NATURAL TITANOMAGHEMITES: IMPLICATIONS FOR INTERNAL STRESS CONTROL OF COERCIVITY IN OCEANIC BASALTS

Abstract

Highly oxidized titanomaghemite in oceanic basalts often carries remanent magnetization of high coercivity (stability), helping preserve the oceanic magnetic anomaly pattern. We study the source of this high coercivity in four oceanic basalts (from ODP sites 238, 572D, 470A and 556) containing highly oxidized titanomaghemite (titanium content parameter $x ≈ 0.55$ and oxidation parameter $z ≈ 0.9$ on average). Most of the titanomaghemite is likely in single-domain grains with uniaxial anisotropy because the ratio of saturation remanence $J RS $ to saturation magnetization $J S $ approaches 0.50 ($J RS /J S = 0.46$ on average). We show that the uniaxial anisotropy is very likely magnetostrictively controlled through internal stresses $σi $ in the titanomaghemite grains. This allows us to use a novel indirect method to estimate the saturation magnetostriction $λS $ of the titanomaghemite. A saturation remanence $J RS $ is given along the axis of a cylindrical sample of each basalt. Then a small compression σ is applied repeatedly along this axis and the reversible change $ΔJ RS $ in $J RS $ is measured. Combining equations from single-domain theory for this piezomagnetic effect and for the sample's coercive force $H C $ gives &formmu12; (using cgs units, or with $H C $ in mT, $J S $ in &formmu15; and σ in Pa). This yields four $λS $ estimates (with ca 50 per cent expected error) ranging from $3 × 10-6 $ to $10 × 10-6 $ and averaging $6 × 10-6 $. Theory for the piezomagnetic effect yields four $σi $ estimates averaging $2 × 108 Pa$. This is similar to the internal stress magnitude thought to be responsible for the high coercivity of ball-milled single-domain titanomagnetite ($x ≈ 0.6$) and natural single-domain haematite. We also show that cooling to 120 °K causes $H C J S $ for each oceanic basalt to vary in approximate proportion to &formmu24; with n between 1.9 and 2.0 (where T is temperature and $T C $ is Curie point, both in °K). This implies that $λS $ of titanomaghemite with $x ≈ 0.55$ and $z ≈ 0.9$ also varies in approximate proportion to &formmu29; with n near 1.9 or 2.0 on cooling to 120 °K (assuming that $σi $ remains constant on cooling). Our results support the hypothesis that coercivity (magnetic stability) is often magnetostrictively controlled by internal stresses in the highly oxidized titanomaghemites typical of oceanic basalts older than ca 10 Myr. We suggest that this hypothesis can be further tested by more extensive observation of whether cooling to 120 °K often causes $H C J S $ of such basalts to vary in approximate proportion to &formmu32; with n near 1.9 or 2.0.