Visit to the Institute of Rock Magnetism

These days, Tom is visiting the Institute of Rock Magnetism (IRM) at the University of Minnesota. His third visit to the IRM, he is working on maintaining his record on the longest-running experiment on a single-sample: Studying the viscous behaviour of single-domain assemblages like magnetoferritin and the infamous Tiva Canyon Tuff in the IRM’s two MPMS.

Blocking temperatures: Are they really as we think they are?

berndt et al 2017 gji
Download the paper “Experimental test of the heating and cooling rate effect on blocking temperatures” (Berndt et al., 2017)

After previously having studied the blocking temperatures of these samples under continuous heating and cooling to an accuracy of a fraction of a degree in the MPMS, it was found that traditional SD theory did not predict these blocking temperatures accurately (Berndt et al., 2017). In other words, the widely used blocking condition given by Néel’s single-domain theory and popularised in the so-dubbed Pullaiah (1975) nomograms did not hold under continuous heating/cooling.

Pullaiah (1973) nomograms showing the blocking temperatures of a Tiva Canyon Tuff sample under continuous heating/cooling. Only after applying an empirical correction factor (right) does the blocking condition accurately predict the experimental data.

This left the question if the theoretical blocking condition fails to predict blocking temperatures only under continuous heating/cooling, or if the condition is inaccurate altogether. On the current visit to the IRM,  Tom aims to answer this question by carefully measuring blocking and unblocking at constant temperatures to extremely high accuracy.

All this is part of Tom‘s greater mission of developing the VRM dating method.

VRM dating involves measuring the unblocking temperature of a viscous remagnetization of a sample and extrapolating the relaxation time to the ambient temperature at the field location (Berndt and Muxworthy, 2017).

VRM dating requires to measure the unblocking temperature of a viscous remagnetization that was acquired by the sample after the event to be dated (e.g. a flood, landslide, tsunami) and extrapolating the relaxation time to the ambient temperature at the field location (Berndt and Muxworthy, 2017). As this approach is based on the accuracy of the blocking condition that relates blocking temperatures to relaxation (i.e. acquisition) times, it critically depends on this relation to hold true.

After the IRM visit we should be a step closer to the answer.

Time-asymmetric FORC diagrams

Download the talk “Time-Asymmetric FORC diagrams” at the Magnetic Interactions Meeting in Oxford January 2018

The second idea Tom is trying out is a new tool to characterise samples: Time-Asymmetric FORC diagrams (TAFORC). TAFORCs make use of thermal fluctuations in FORC diagrams. While traditionally FORC diagrams are interpreted in terms of Preisach theory (i.e. assemblages of elementary square hysteresis loops, called hysterons), this completely neglects thermal fluctuations, and therefore implicitly assumes that magnetic particles only switch as a reaction to the applied field. My idea was that particles may actually switch because of thermal activation during the time of measuring a FORC as well, and this effect could be modelled and used to obtain further information about the properties of a sample. Theoretical considerations (presented at the Magnetic Interactions Meeting in Oxford January 2018) showed that thermal fluctuations can be made visible in FORC diagrams by applying the reversal field Ha for a different time than the measuring field Hb, and that this would allow to distinguish different grain shapes or different magnetic minerals.

Simulation of a sample containing a mixture of magnetite and titanomagnetite: in the traditional FORC diagram (left) the signals of the two minerals completely overlap, but in the Time-Asymmetric FORC diagram (right) the two minerals are clearly distinguishable.

At the IRM these predictions are currently being tested experimentally.

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