The stability of single-vortex domain state in the context of paleomagnetism by Wyn Williams

19/09/2019

Image of magnetic domains by R.Harryson 2005

The stability of single-vortex domain state in the context of paleomagnetism by Wyn Williams

In short:

On Thursday 19th of September 2019, Wyn Williams presented a new model in order to understand the state of the domain that retain paleomagnetism. His study tries to improve the previously accepted model of magnetic domain states relative to grain size. His model suggest replacing the Pseudo-Single Domain state (PSU) by the Single-Vortex state (SV), which may well be the main recorder of paleomagnetism. The main goal for this model is to improve the accuracy of paleomagnetism studies. As change in accepted models is often faced by resistance, his research may get controversially disputed once it will get published.

Main:

Figuring out how magnetism is recorded is nowadays more important than ever as it is used in a lot of different technologies. For example computer hard-drives already use magnetic domains similar to single-vortex domains to record information, or as carriers in the biomedical industry. The problem when looking at paleomagnetism is that it is an open system with the risk of time overprint. So how do rock actually record geomagnetic data?


The models follows the principle that above the closure temperature the ferromagnetic materials will align themselves with the main magnetic field, here assumed to be the Earth's magnetic field. Then as the rock cools down and the and goes below the closing temperature the geomagnetic information would be locked in the rock and its intensity will linearly proportional to the strength of the original magnetic field. This capacity of recording a magnetic field, the magnetic remanence, is strongly linked to grain size (for the same mineral here mostly magnetite).

In the accepted model the grain size effect is classified in 4 major categories: the very small grain sizes, the Single Domains (SD), the Pseudo-Single Domains (PSD) and the Multi-Domains (MD). For the very small grain size, the thermal excitation reorients the grains so their magnetic remanence is nonexistent. Going up in grainsize the SD have a very good magnetic remanence and correspond the the simplest of the magnetic domain theory. The grain is to small to contain multiple domains so it only has only one domain recording one direction of magnetic field. Then comes the less-well understood PSD which would contain a small amount of domains (typically 4 or less) with an overall magnetic vector very similar to a weaker single domain grain (as domains in opposite directions cancel each other out). Finally the MD are grains containing an great number of domains where most of them cancel each other out leading to low total magnetic remanence.

The SD and PSD having the most magnetic remanence it became necessary to understand them better. The SD being well understood the question was to research the PSD. By making a computer model of the magnetic field in magnetite grains relative to grain size and grain shape the study found that above 200nm and vortex structure in the magnetic field starts to appear.
This vortex grows and seem to link the uniformly magnetized domains. Above a grainsize of about 2µm it flips to multi-domain state. Here came the first suggestion to change the PSD into the Single-Vortex (SV) state.

The SV state model would be better at explaining the gradual drop of magnetic remanace observed relative to grainsize compared to the PSD. But then comes the question of grainshape and how it might affect the magnetic remanance. Wyn Williams and his team found that up to 30% elogation no real change was observed and that only small changes were observed between 50% to 70% elongation where the vortex seemed to twist as it grew bigger.

On average the magnetic remanence per particle of SV if about 1% of the magnetic remanence of SD so their contribution to paleamagnetism recording might seem smal at first glance. But as SV are way more abundant than SD the magnetic remanence per volume of rock of SV is about 100x the one of SD making it the main paleomagnetic signal.

Then came the big challenge of confronting the model with experimental data. The aim was to match the curve of the model with the experimental data, which at first failed meaning the model was still lacking in some important factors yet to be considered. But including a grainsize distribution factor the curve got a bit better but was still far off. Then adding a factor for the interaction between the magnetic particles improved the model a lot, but it still did not match. The main interaction is the habit of SV to form chains making them more SD like, increasing their remanance while keeping the SV in their original orientations. Finally when adding the complex interactions in 3D (crossing chains) the vortex behaves as in an elongated grain, getting twisted and thus increasing it's magnetic remanance. At this point the model curve fitted the experimental data very well.

Wyn Williams and his team hope that this model adding to the understanding of the magnetic remanence will help distinguish noise from signal in paleomagnetism measurements, thus increasing their accuracy.