Measuring the tiniest magnets

In order to make accurate predictions using the Earth's magnetic field, it is important to be able to measure the magnetic field of individual grains in rocks, but up to now there was no good method for doing this. During the Fall Meeting of the AGU, Utrecht University student Annemarieke Béguin demonstrated a method that makes this possible, winning her a prestigious outstanding student paper award.

Individual grains

Just like every magnet, the Earth has a magnetic field with two poles. This magnetic field is generated in the Earth's core and a record of it is locked in rocks as they are formed. This takes place in billions of minuscule magnetic grains measuring just a few micrometres (thousandths of a millimetre) in length. To understand how the Earth's magnetic field behaved in the past and to be able to more accurately predict how it will develop in the future, scientists measure the direction and strength of the magnetic field in these grains. At present this is done per drilled core sample in which millions of grains are measured together, but because many grains have different magnetic properties, the sum of all these properties deviates too much from the individual and a large proportion of the core samples are unusable.

For this reason Master's student Annemarieke Béguin worked with her supervisor Dr Lennart de Groot (from the Paleomagnetic Laboratory in Fort Hoofddijk) and others to develop a method to measure the magnetic properties of individual grains without destroying the core samples.

Stones in the CT scanner

For some time now it has been possible to measure the magnetisation of individual grains in a rock, but only using destructive methods. By shaving off a small piece of the rock each time, it was possible to make a 3D reconstruction of the grains in the rock, but using this method it is not possible to measure changes in the magnetic field. Béguin and De Groot came up with the idea that it would be easier to first make a CT scan of the structure of the rock at microscale and then, using the same tiny scale, to measure the magnetisation at the surface of the rock. Combining the two resulting datasets enabled them to reconstruct the alignment and strength of the magnetisation in each individual grain and to map changes in the magnetisation by repeatedly re-analysing a sample.

Collaboration

Measuring the magnetisation and structure of the rock at this minuscule scale requires advanced equipment. This was one of the reasons why no-one had attempted this method previously. Béguin and De Groot got in touch with the University of Twente, where they were able to use a device that measures the magnetic field of the surface of a rock on a microscale. TU Delft has a micro CT-scanner, which can produce a 3D picture of the grains. They were permitted to use this also.

This equipment, coupled with the specific knowledge of paleomagnetism from Utrecht University's Béguin and De Groot, provided the unique opportunity to put the new method into practice.

Presentation prize

Béguin presented this new method during the American Geophysical Union (AGU) Fall Meeting, the largest worldwide conference in geophysical sciences. Her presentation won her a prestigious outstanding student paper award, a prize usually won by PhD candidates. For a Master's student to win this prize is a unique achievement.

Faculty of Geosciences: a sustainable Earth for future generations