Other
Scientific paper
Dec 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008agufmgp31a0786o&link_type=abstract
American Geophysical Union, Fall Meeting 2008, abstract #GP31A-0786
Other
1512 Environmental Magnetism, 1519 Magnetic Mineralogy And Petrology, 1540 Rock And Mineral Magnetism, 1595 Planetary Magnetism: All Frequencies And Wavelengths, 3672 Planetary Mineralogy And Petrology (5410)
Scientific paper
In multidomain hematite, crystal defects lay a major role in hysteresis and coercivity by hindering the motion of domain walls. The kinds of defects that can pin walls are dislocations and growth and deformational twins and twin boundaries. Multidomain hysteresis is also affected by wall nucleation which generally occurs at irregular surfaces such as voids, cracks or at growth steps in the crystal surface. The temperature dependence of Hc is different for nucleation and for various types of defect pinning. The most rapid variation is Hc(T) varying as K(T)/Ms(T), where K is magnetocrystalline anisotropy constant, due to domain nucleation or magnetocrystalline controlled domain wall pinning. Pinning due to the stress field of dislocations or planar defects results in a slower temperature variation: Hc(T) varying as lambda(T)/Ms(T), where lambda is magnetostriction constant. Hc and Ms were measured as a function of temperature in mm-size single crystals of hematite using a PMC MicroVSM. The experimental Hc(T) data varied as the power 1.8-2.4 of Ms(T) between 400 and 625°C. Flanders and Schuele (1961) reported that K(T) varied as the 10th power of Ms in a large single crystal between 20 and 500°C. The magnetostriction constant of hematite has not been measured directly as a function of temperature. Nevertheless, it is reasonable to expect that the magnetostriction of hematite, as in other materials, should have a much weaker power-law dependence on Ms than does the magnetocrystalline anisotropy constant. In the present multidomain hematite crystals, the observed weaker power-law index of 1.8-2.4 indicates that the coercivity is mainly magnetoelastic in origin.
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