Stability of Single-Vortex Domain States
This seminar hosted by Wyn Williams (Professor of Rock and Mineral Magnetism,UofE), coincidentally, dealt with new breakthrough research in geomagnetism.
Geomagnetism - Explained
Geomagnetism (for those not in the know) is simply the study of the magnetic properties of the Earth. Perhaps the simplest and most famous use of geomagnetism is it's use as a dating tool.
If you are thinking "Magnets aren't clocks"... you would in fact be right, but lets see this out together.
When a lava cools, the liquid hardens into crystals. The crystals that solidify can contain a magnetic direction. These crystals are like if you froze liquid inside a compass, freezing the needle in place. So as these crystals cool they keep their magnetic direction but also their magnetic intensity. As the earth tends to flip the poles occasionally (deep geophysics is needed to explain it, so lets just accept it as a fact) we can correlate the magnetic direction with when the crystals cooled. This simply boils down to if the crystal's magnetism has been orientated north (current north pole) or south (earlier north pole).
Therefore it is crucial for research to find the crystals that are the best carriers of paleomagnetism (i.e. fossil/preserved magnetism).
Applying the Science
The traditional view on the best candidate crystal to study for this was a single, small crystal. Any smaller and the reading would be too weak and/or lose it's direction (a relatable issue I'm sure). A larger crystal can hold multiple magnetic directions (like if you melted compasses together to make a bigger compass) which unfortunately interact and spoil the sample. However it is in this seminar where we learn that everything we ever knew was wrong! Well, at least for what is the best crystal to use.
The reason larger crystals begin to segment into different magnetic directions (domain states) is due to the rotation of particles around and (crucially) through the magnetic field. This is known as the vortex core, where crystals that present a vortex core are referred to being in a 'vortex state'.Where a crystal's vortex core begins to become more complex, it forms multiple domain states.
Following? No? Don't worry, this stuff is complicated. In layman's terms, the larger a crystal grows the more complicated it's magnetic field becomes due to multiple magnetic (domain states) and rotation directions (vortex states) forming.
As it turns out, it is this 'Vortex-state' may be a far more powerful record than the single domain state crystals.
Findings
We used to think that there was a curve for preserved magnetic intensity, with the crest of the curve peaking at a crystal size of slightly under 1500nm (e.g. just before a vortex core forms), with magnetic intensity trailing off with crystal size. However what Prof. William's work has shown us is that this is not strictly true and in fact the best samples for use are in the single-vortex state and not the single domain state.
This phenomena is a result of how we process and prepare samples rather than a natural phenomena. In order to prepare a sample, we must powderise a sample before measuring. However, much like a chain of magnets these particles begin to interact in complex ways that overall reduce the remnant magnetism (the very thing we're trying to measure). Single-domain states are not well equipped to deal with this as we are essentially increasing the grain size, therefore reducing the quality of measurements. However, single-vortex states deal with this problem quite effectively. With large magnetic chains, the vortex core is warped and increases in intensity, therefore increasing the quality of our measurement.
Another finding was that the transition zone between the two states is an incredibly weak zone where a new magnetic direction can quite easily be given to the crystal. Usually very high temperatures are needed to change a crystal's magnetic direction, but in this zone it is only a couple hundred degrees (a temperature the Earth can easily get to). It was this previously unknown zone of weakness that gave geophysicists the idea that any crystal larger than holding a single domain state was not useful.
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