Common magnetic mineral is reliable witness to Earth's history
|Electron microscopy image of a magnetite nanocrystal (left) and the magnetic vortex |
structure (right), made visible for the first time by researchers from Jülich
and the United Kingdom using electron holography
[Credit: Imperial College London]
Scientists in the area of paleomagnetism use magnetic minerals to investigate the history of the earth's magnetic field and its formation from molten metal flowing within the earth's core, the so-called geodynamo. Furthermore, the movement of continental plates can be monitored with the aid of such rocks.
In the course of millions of years, these minerals could often have been exposed to immense temperature fluctuations, due to extreme climate change or volcanic activity, for instance. How well do the magnetic structures survive such temperature fluctuations and how reliable is the information gained from them?
An international research team has now studied this question for the first time at ultra-high resolution on samples of magnetite, the mineral dominating the magnetic properties in the earth's crust.
|This micromagnetic model shows the three-dimensional vortex structure |
of magnetite nanocrystals [Credit: University of Edinburgh]
Together with colleagues from Forschungszentrum Julich, the University of Edinburgh and the University of Nottingham, Almeida has studied the magnetic vortices in magnetite nanocrystals. As the structures are so tiny -- each grain is only about the size of a virus -- there is only one method with which the nanovortices can directly be observed while they are heated up and cooled down: "A special high-resolution electron microscope at the Ernst Ruska-Centre (ER-C) in Julich is capable of making magnetic fields on the nanoscale holographically visible," explains Almeida. "In this way, images of field lines are produced almost like using iron filings around a bar magnet to make its magnetic field visible, but with a resolution in the nanometre range."
The experiments in Julich showed that although the magnetic vortices alter in strength and direction when heated up, they go back to their original state as they cool down. "Therefore magnetite rocks, which carry signs of temperature fluctuations, are indeed a reliable source of information about the history of the earth," enthuses Almeida.
As an expert in electron holography, he works with his Julich team on further improving the resolution of this technique and in providing German and international scientists the necessary infrastructure to perform this type of study.
"Weak magnetic fields in nanocrystals don't just play a role in paleomagnetism. In information technology, for instance, electron holograms can also be of use to help to push back the physical limits of data storage and processing."
The study has been published in Science Advances.
Source: Forschungszentrum Juelich [April 18, 2016]