Physics
Scientific paper
Oct 1988
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988pepi...52..108b&link_type=abstract
Physics of the Earth and Planetary Interiors, Volume 52, Issue 1-2, p. 108-116.
Physics
10
Scientific paper
Fine grain (~ 0.1 μm) synthetic Fe2.4Ti0.6O4 (TM60) was given a thermoremanence (TRM) in a field of 35 A m-1 prior to oxidation in air at temperatures in the range 120-300°C, also in the presence of a magnetic field. Only a small fraction of the moment (~ 10%) was transported into the ambient field direction. The dominant feature was a fall in intensity to ~ 50% of the initial TRM as the degree of oxidation, z (0 < z < 1), increased to a value of ~ 0.4, followed by only a further small decrease as z rose to ~ 0.8.
The fall is too large to be accounted for as being due to the change in the intensity of individual grain moments due to the fall in spontaneous magnetization with composition. A redistribution of grain moments due to unblocking of the initial directions is also ruled out as making a significant contribution to the fall in remanence.
The observed effect can be plausibly modelled as being due to the growth of a high permeability mantle which screens the residual moment of a maghemitized grain. The mantle is formed when high concentration gradients near the surface cause local catastrophic failure of the crystal so that the grain becomes surrounded by a cracked and split layer. As further gradual oxidation takes place at the surface by addition of oxygen and growth of new crystal, it is hypothesized that the porous mantle is gradually reclaimed by the grain and the screening effect reduced.
This is consistent with the final observed remanence at high oxidation levels, which can be accounted for in terms of only the composition-dependent change in spontaneous magnetization.
The implication of these results for the magnetization of the submarine crust is discussed.
Brown Katrina
O'Reilly W.
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