Other
Scientific paper
Dec 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007agufmgp24a..02o&link_type=abstract
American Geophysical Union, Fall Meeting 2007, abstract #GP24A-02
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
Morin transition temperatures Tm were measured for submicron (SD) synthetic hematites with grain sizes between 120 and 520 nm and on 0.5 and 6 mm (MD) natural single crystals. The Morin transition temperature is quite variable in both SD and MD hematites. Tm ranged from 250 to 260 K for natural crystals and even more widely, from 241 to 254 K, for the more heterogeneous synthetic hematites. The scatter must be due to different methods of preparation, crystal morphology, strain and crystal imperfections that are common for both synthetic and natural crystals. The present Tm data for SD and MD hematites are compatible with published data by other workers in the 100 nm - 10 mm range. In this interval, Tm decreases slightly with decreasing particle size but is almost size independent. However, in nanoparticles with grain sizes between 30 and 90 nm, the spin-flop transition is strongly dependent on particle size: Tm decreases sharply with decreasing size and the transition disappears below 20 nm. This phenomenon is due to surface effects that lead to spin directions deviating from the easy axis. The present SD and MD hematites exhibit a thermal hysteresis in the Morin transition: the values of Tm in cooling and in heating are different. For the same cooling/warming rate, the width of the Morin transition in the submicron crystals is broader than for the natural single crystals. This broadening in SD hematites is directly attributable to the wide distribution of particle sizes. In natural single crystals the entire width of the transition could be due to crystal imperfections and the internal stresses that result from these defects.
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