Research Area: micromagnetic sciences
- ualissonbellon
- Sep 26
- 4 min read
“We are to examine the construction of the present Earth, in order to understand the natural operations of time past.”
This legendary quote by James Hutton is practically a rallying cry for Earth scientists! It embodies a philosophy that has driven our quest to uncover the hidden secrets in rocks and minerals, helping us grasp the processes that have shaped our planet - and ultimately, ourselves. But how do we know Earth is nearly 4.5 billion years old, or that the continents once roamed to create today’s landscapes? In a way, examining rocks and minerals to find these answers is like gazing into a crystal ball - but instead of glimpsing the future, we’re unveiling the past.
The microscopic magnetic recorders
In this thrilling journey, paleomagnetists and rock magnetists delve into rocks to discover how Earth’s magnetic field has evolved over the ages. Rocks contain magnetic minerals that capture the Earth’s magnetic field at the time they formed. If these rocks haven’t faced extreme heat or pressure, they preserve that magnetic signature (called remanence) for billions of years, and we can retrieve it in our labs. Sounds simple, right? Not exactly!
The challenge is that not all magnetic minerals can record 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 a human hair’s thickness. 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 incredibly stable SD minerals to extract paleomagnetic information from rocks using a variety of experimental, analytical, and statistical methods. However, most ancient magnetic records are believed 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 particles.
But what is a vortex state, exactly? Picture 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 challenge is that we often assume the same physical principles that govern SD grains also apply to vortex state grains. We then typically apply the classical paleomagnetic methods to rock samples 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.
The mission (goal)
Imagine rephrasing 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 accurately.
Our mission is to establish a new framework for understanding magnetic mineral stability and its role in paleomagnetism. By combining advanced synchrotron-based tomographic imaging with micromagnetic modeling, we can infer the internal magnetic configurations of minerals and quantify their stability over geological timescales. This reveals when and why certain grains faithfully preserve magnetic information, while others lose it, helping to resolve long-standing challenges in interpreting ancient magnetic fields.
A key advantage of synchrotron-based X-ray ptychographic computed tomography (PXCT) is its ability to image the three-dimensional internal structure of magnetic minerals at nanometer-scale resolution without destroying the sample. Unlike traditional destructive techniques such as focused ion beam sectioning, PXCT preserves the integrity of rare or unique specimens while still revealing the detailed morphologies that govern magnetic behavior. This capability is especially valuable for extraterrestrial materials, where each grain is irreplaceable. In addition, because PXCT recovers the electron density of the studied samples, it allows us to directly link the imaged structures to bulk densities and confidently match the observed phases with known minerals. For our studies, we are currently performing experiments at the Sirius synchrotron in Brazil (Figure 2), one of the world’s brightest light sources, which provides the long, high-resolution scans necessary to capture such fine-scale structures.
A vital and emerging tool in this effort is quantum diamond microscopy (QDM). QDMs (Figure 3) are capable of measuring magnetic fields from individual particles at the micron to nanometer scale. Unlike bulk measurements, QDM provides spatially resolved maps of magnetic anomalies, making it possible to directly link magnetic signals to the physical grains that carry them.
By integrating our tomographic and micromagnetic approach with QDM measurements, we aim to build a more robust paleomagnetic toolkit. This combination has the potential to overcome the limitations of traditional bulk methods by enabling direct investigation of the magnetic responses of individual particles (or clusters of particles) while simultaneously constraining their physical limits of magnetic stability.
Ultimately, this integrated strategy paves the way for a new micropaleomagnetic method to be applied in both Earth and extraterrestrial materials - including the invaluable samples that will one day be returned from Mars!
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