HR: 16:30h
AN: GP24A-03    [Abstracts]
TI: Ferrian Ilmenites: Investigating the Magnetic Phase Diagram
AU: * Lagroix, F
EM: lagroix@ipgp.jussieu.fr
AF: Institut de Physique du Globe de Paris (CNRS-Univ Paris 7), 4 place Jussieu, Paris, 75252, France
AB: The main objective of this study is to investigate the magnetic phase changes within the hematite-ilmenite solid solution, yFeTiO3·(1-y)·Fe2O3. Two sets of synthetic ferrian ilmenites of y-values equal to 0.7, 0.8, 0.9, and 1.0 were available for this study. As currently drawn, the magnetic phase diagram, proposed by Ishikawa et al. [1985, J. Phys. Soc. Jpn. v.54, 312-325], predicts for increasing y values (0.5<y<1.0) magnetic odering from paramagnetism to (1) ferrimagnetism, or (2) to superparamagnetism then ferrimagnetism, or (3) to superparamagnetism then antiferromagnetism. Moreover, for y values ranging between 0.65 and 1.0 a transition into a spin glass state is expected at 100K or below. Ilmenite, y=1.0, is antiferromagnetic. Various low tempreature experiments including temperature dependance of remanence and induced magnetizations and AC susceptibility were conducted in order to characterize the magnetic behaviour and changes of magnetic states. In general, the data confirms the predicted phase changes for the different compositions investigated. The y=1.0 sample, pure ilmenite, is antiferromagnetic below 57K, the measured Néel temperature. The y=0.9 sample magnetically orders at about 100K in a superparamagnetic state. Hysteresis loops remain effectively closed down to 60K below which an antiferromagnetic order prior to reaching the spin glass state is ambiguous. The y=0.8 sample magnetically orders at about 270K in an initially superparamagnetic states before entering a ferrimagnetic state below about 250K. Lastly, as previously demonstrated in Lagroix et al. [2004, JGR-B, v.109, doi:10.1029/2004JB003076], the y=0.7 samples order ferrimagnetically at 380K. However, like the y=0.7 samples which also demonstrated an antiferromagnetic state at temperature above the Curie temperature, hysteresis loops for y=0.9 and y=0.8 only achieve perfect linearity at 190K and 340K respectively. All samples (except y=1.0) show a frequency dependent amplitude non-dependent quadrature susceptbility peak at 30K which is most likely associated with the spin glass like state. Time-dependant DC and AC induced magnetization experiments provide convincing evidence and help define the range of temperatures over which the behaviour persists.
DE: 1519 Magnetic mineralogy and petrology
DE: 1540 Rock and mineral magnetism
DE: 1594 Instruments and techniques
DE: 3929 NMR, Mossbauer spectroscopy, and other magnetic techniques
SC: Geomagnetism and Paleomagnetism [GP]
MN: 2007 Fall Meeting