Experimental test of the heating and cooling rate effect on blocking temperatures

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Title: Experimental test of the heating and cooling rate effect on blocking temperatures
Authors: Berndt, T
Paterson, GA
Cao, C
Muxworthy, AR
Item Type: Journal Article
Abstract: The cooling rates at which rocks acquire thermoremanent magnetizations (TRMs), affect their unblocking temperatures in thermal demagnetization experiments; similarly the heating rates at which the thermal demagnetization experiments are done also affect the unblocking temperature. We have tested the effects of variable cooling and heating rates on the unblocking temperatures of two natural non-interacting, magnetically uniform (single-domain, SD) (titano)magnetite samples and a synthetic SD magnetoferritin sample. While previous studies have only considered unblocking temperatures for stepwise thermal demagnetization data (i.e. the room-temperature magnetization after incremental heating), in this work we derive an expression for continuous thermal demagnetization of both TRMs and viscous remanent magnetizations (VRMs) and relate the heating rate to an effective equivalent hold time of a stepwise thermal demagnetization experiment. Through our analysis we reach four main conclusions: First, the theoretical expressions for the heating/cooling rate effect do not accurately predict experimentally observed blocking temperatures. Empirically, the relation can be modified incorporating a factor that amplifies both the temperature and the heating rate dependence of the heating/cooling rate effect. Using these correction factors, Pullaiah nomograms can accurately predict blocking temperatures of both TRMs and VRMs for continuous heating/cooling. Second, demagnetization temperatures are approximately predicted by published ‘Pullaiah nomograms’, but blocking occurs gradually over temperature intervals of 5–40 K. Third, the theoretically predicted temperatures correspond to ∼54–82 per cent blocking, depending on the sample. Fourth, the blocking temperatures can be used to obtain estimates of the atomic attempt time τ0, which were found to be 3 × 10−10 s for large grained (titano)magnetite, 1 × 10−13 s for small grained (titano)magnetite below the Verwey transition and 9 × 10−10 s for magnetoferritin (∼8 nm).
Issue Date: 24-Apr-2017
Date of Acceptance: 18-Apr-2017
URI: http://hdl.handle.net/10044/1/48206
DOI: https://dx.doi.org/10.1093/gji/ggx153
ISSN: 1365-246X
Publisher: Oxford University Press (OUP)
Start Page: 255
End Page: 269
Journal / Book Title: Geophysical Journal International
Volume: 210
Copyright Statement: © The Authors 2017. Published by Oxford University Press on behalf of The Royal Astronomical Society. This is an Open Access article distributed under the terms of the Creative Commons Attribution License ( http://creativecommons.org/licenses/by/4.0/ ), which permits unrestricted reuse, distribution, and reproduction in any medium, provided the original work is properly cited.
Sponsor/Funder: Natural Environment Research Council (NERC)
Funder's Grant Number: NE/J020508/1
Keywords: Geochemistry & Geophysics
0404 Geophysics
0403 Geology
0909 Geomatic Engineering
Publication Status: Published
Appears in Collections:Faculty of Engineering
Earth Science and Engineering



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