Astronomy and Astrophysics – Astronomy
Scientific paper
Aug 1999
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1999geoji.138..319k&link_type=abstract
Geophysical Journal International, Volume 138, Issue 2, pp. 319-333.
Astronomy and Astrophysics
Astronomy
3
Hysteresis Properties, Rock Magnetism, Small Particles, Titanomagnetite
Scientific paper
Hysteresis parameters as a function of temperature were measured for milled and etched synthetic 2.4, 12.5 and 165 μm titanomagnetite (TM60) particles. The results show characteristics that appear to be dictated by microstresses, generated by a high density of dislocations and other lattice defects, that were introduced in the spinel lattice by the milling procedure. The following features were found. (1) 2.4 μm particles are single domain (SD) with a saturation remanent magnetization ratio sigma_rs/sigma_s(20 degC)~0.5 and coercive force H_c(20 degC)~75 kA m^-1 the latter value can only be interpreted in terms of stress-induced anisotropy because shape and magnetocrystalline anisotropies are too low by far. The nearly temperature-independent coercivity ratio H_cr/H_c~1.5 up to ~200 degC is higher than expected for Stoner-Wohlfarth SD particles with uniaxial anisotropy; this behaviour is attributed to irregularly directed microstresses as a consequence of lattice defects. By contrast, a Henkel plot shows almost the behaviour expected for Stoner-Wohlfarth particles. Due to fracturing processes during milling, partial low-temperature oxidation has taken place, resulting in an enhanced Curie temperature T_c~230-260 degC, as extrapolated from data of saturation magnetization. (2) 12.5 μm particles (T_c=205 degC) show typical pseudo-single-domain (PSD) behaviour, with sigma_rs/sigma_s(20 degC)~0.18, H_c(20 degC)~11 kA m^-1, both values being much higher than for multidomain (MD) particles, and H_cr/H_c(20 degC)~1.9. Magnetization processes appear to be dictated largely by reversible and irreversible domain rotations; that is, domain wall displacements play a minor role because of strong pinning forces for domain walls at lattice defects. As the temperature rises, a comparatively weak decrease in sigma_rs/sigma_s takes place, whence it follows that the PSD character persists up to the Curie point. H_c falls with temperature steadily to a minimum of H_c(160 degC)~4 kA m^-1 a further increase towards 200 degC may be associated with an inhomogeneous magnetic behaviour throughout a particle because of the formation of some sort of SD regions, related in part to the microcrystalline particle structure; furthermore, thermal fluctuations may play a role. (3) 165 μm particles (T_c=200 degC) exhibit MD behaviour with sigma_rs/sigma_s(20 degC)~0.025, H_c(20 degC)~1 kA m^-1 and H_cr/H_c(20 degC)~8. While H_c shows a smooth variation with temperature up to 160 degC, H_cr/H_c falls with temperature to a minimum H_cr/H_c(160 degC)~2 and an increase occurs again to H_cr/H_c(200 degC)~4.5.
Keller Rainer
Schmidbauer E.
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