Astronomy and Astrophysics – Astronomy
Scientific paper
Jun 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004geoji.157.1017h&link_type=abstract
Geophysical Journal International, Volume 157, Issue 3, pp. 1017-1026.
Astronomy and Astrophysics
Astronomy
4
Coercivity, Magnetostriction, Oceanic Basalt, Titanomaghemite
Scientific paper
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 JRS to saturation magnetization JS approaches 0.50 (JRS/JS= 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 JRS 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 ΔJRS in JRS is measured. Combining equations from single-domain theory for this piezomagnetic effect and for the sample's coercive force HC gives (using cgs units, or with HC in mT, JS in 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 HCJS for each oceanic basalt to vary in approximate proportion to with n between 1.9 and 2.0 (where T is temperature and TC 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 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 HCJS of such basalts to vary in approximate proportion to with n near 1.9 or 2.0.
Hodych Joseph P.
Matzka Jürgen
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