Astronomy and Astrophysics – Astronomy
Scientific paper
May 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000geoji.141..351o&link_type=abstract
Geophysical Journal International, Volume 141, Issue 2, pp. 351-356.
Astronomy and Astrophysics
Astronomy
19
Coercive Force, Magnetic Anisotropy, Magnetic Hysteresis, Magnetite, Verwey Transition
Scientific paper
The temperature dependence of coercive force Hc was studied on well-characterized and stoichiometric millimetre-sized single crystals of magnetite at a series of 16 temperatures from 300 to 10K using a SQUID magnetometer. Hc decreases gradually with cooling to the isotropic temperature, Ti=130K, where the first magnetocrystalline anisotropy constant K1 becomes zero. Hc exhibits a sharp increase at the Verwey transition, Tv=120K, where the structure changes from cubic to monoclinic. In crossing the Verwey transition, Hc increases by more than two orders of magnitude, from 20μT to 2.4mT, and the shape of the hysteresis loops becomes wasp-waisted. Observed coercivity between 300K and 170K varies with temperature as λs/Ms, where λs is the magnetostriction constant and Ms is the saturation magnetization, indicating that the coercivity in MD magnetite is controlled mainly by internal stress associated with dislocations or other crystal defects. It seems likely that the stable single-domain-like magnetic memory observed in large MD magnetite crystals is due to magnetoelastically pinned domain walls. The discontinuous change in Hc at the Verwey transition is controlled by abrupt changes in magnetocrystalline and magnetostriction constants due to crystal deformation from cubic to monoclinic structure.
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