Magnetic Secrets of Vortex State Grains: Insights into Mineral Properties That Store Ancient Magnetic Records

Understanding the microscopic properties of magnetic minerals in rocks is key to interpreting ancient planetary processes. By imaging nanoscopic minerals using cutting-edge technology at a particle accelerator, we have combined non-destructive nanotomography with micromagnetic modeling.
Magnetic Secrets of Vortex State Grains: Insights into Mineral Properties That Store Ancient Magnetic Records
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“We are to examine the construction of the present Earth, in order to understand the natural operations of time past.” This iconic quote by James Hutton is almost a mantra among Earth scientists. It reflects a philosophy that has guided our attempts to uncover hidden clues in rocks and minerals to understand the processes that have shaped our planet—and, by extension, ourselves. But how do we know that Earth is nearly 4.5 billion years old, or that the continents were once drifting around to form today’s landscapes? In a sense, examining rocks and minerals to find these answers is like looking into a crystal ball—but instead of peering into the future, we are uncovering the past.

In this context, paleomagnetists and rock magnetists study rocks to learn how the Earth’s magnetic field has changed over the ages. Rocks contain magnetic minerals that can record the Earth’s magnetic field at the time they formed. If these rocks have not been subjected to extreme heat or pressure, they preserve that magnetic signature (called remanence) for billions of years, and we can retrieve it in our labs. So, it seems like a straightforward process, right? Not quite.

The tricky part is that not all magnetic minerals can record the Earth’s magnetic field, and even those that can might not preserve it over long periods. What we need are tiny mineral grains—nanometers in size, hundreds of times smaller than the thickness of a human hair. At this size, their internal magnetization is uniform, in what we call the “single-domain” (SD) state. If grains get too large, the internal magnetic moments start to rearrange, creating new magnetic domains. The larger the grain, the more complex the magnetic domains become, leading to what’s called a “multidomain” (MD) state.

For decades, scientists have relied on the very stable SD minerals to extract paleomagnetic information from rocks using a range of experimental, analytical, and statistical methods. However, most ancient magnetic records are thought to be stored not in SD grains, but in slightly larger grains that exist in an intermediate state between SD and MD, known as vortex state grains.

But what is a vortex state, exactly? Think of those tiny SD grains, where the internal magnetization all points in the same direction. As these grains increase slightly in size, the magnetic moment starts to curl around an axis instead of remaining uniform (Figure 1). This is the vortex state. The problem is that we often assume the same physical principles that govern SD grains also apply to vortex state grains. We then usually apply the classical paleomagnetic methods to rock samples in order to recover paleomagnetic data, interpreting the results as we would for SD grains, but studies have shown that vortex state minerals might behave very differently.

Figure 1 - Micromagnetic model of an irregular grain viewed from different perspectives. The arrows represent the helicity of the magnetic moment within the grain, while the blue isosurface highlights the structure of the vortex core. Source:  the author.

Suppose we rephrase Hutton’s quote to a different scale. In that case, we might say that we need to understand the microscopic properties of the magnetic minerals in rocks to interpret ancient planetary processes properly. That’s what we aimed to do in this study. By imaging vortex state minerals in rocks that are over 500 million years old, using cutting-edge imaging technology at a particle accelerator, we’re one of the first to apply a non-destructive technique called Ptychographic Computed X-ray Tomography (PXCT) to the study of magnetic minerals.

We conducted our experiments in the Swiss Light Source (SLS, Figure 2A), in Switzerland, at the cSAXS beamline (Figure 2B) . The tomographic scan of our microscopic sample, just a few dozen microns in size, took over 24 hours to acquire, and then several months to process and model.

Figure 2 - A) Swiss Light Source (SLS) at the Paul Scherrer Institute (PSI, Switzerland). PXCT experiments were performed at the cSAXS beamline (B). Source: the author. 

Our research is the first to calculate the energy barriers of these particles to understand their temporal stability. We found that while most grains are stable over time, some near the boundary between vortex and SD states are unstable and lose their magnetization within hours, not eons. To complicate things further, we discovered that irregularly shaped grains can have multiple magnetic states at varying temperatures because the energy levels associated with these states are so similar. This explains why many paleomagnetic experiments fail to accurately measure the intensity of ancient magnetic fields in volcanic rocks, which are rich in vortex-state particles. Heating these samples can trigger changes in their magnetic states, causing the original remanence to be lost.

Our study presents a methodology that can be applied to precious, irreplaceable samples—such as those from space—that cannot be destroyed. It also underscores the need for paleointensity methods that do not involve heating, to prevent the loss of vital magnetic information.

 

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