For decades, metallurgists have known that applying a magnetic field during heat treatment can subtly change the properties of steel. The observation dates back to the 1970s, but until now, the underlying mechanism remained largely speculative. A new study led by researchers at the University of Illinois Urbana-Champaign provides the first quantitative explanation for how magnetic fields influence the movement of carbon atoms through iron, a process central to steel manufacturing.
The research team, led by Dallas Trinkle, Ivan Racheff Professor of Materials Science and Engineering, used atomistic simulations to examine how magnetic ordering in iron alters the energy landscape that carbon atoms must traverse as they move through the metal. Their findings and build on earlier experimental observations that magnetic fields can slow carbon diffusion, particularly near iron’s Curie temperature.
Wirth, L. J., & Trinkle, D. R. (2025). External Magnetic Field Suppression of Carbon Diffusion in Iron. Physical Review Letters, 135(25), 256302. https://doi.org/10.1103/j4sg-qmg7
Steel is fundamentally an iron–carbon alloy, and its mechanical properties depend strongly on how carbon atoms diffuse through the iron lattice during heating and cooling. This diffusion typically requires high temperatures, which makes steel production one of the most energy-intensive industrial processes worldwide. Understanding how to control atomic motion more precisely could offer new ways to reduce both energy use and emissions.
Dallas Trinkle, Ivan Racheff from University of Illinois Grainger College of Engineering stated,
“It takes an extremely strong field to switch magnetic moments. If you’re near the Curie temperature, the magnetic field has a strong effect… When the spins are more random, the octahedron (cage) actually gets more isotropic: the whole thing kind of opens up and has more space to move.”
At the atomic level, carbon atoms in iron occupy interstitial octahedral sites, often described as cages formed by surrounding iron atoms. To diffuse, a carbon atom must pass through a higher-energy transition state, typically associated with a tetrahedral configuration. The ease of this movement depends on the energy barrier between these sites.
Trinkle and his colleagues modeled this process using a technique known as spin-space averaging, which allows simulations to capture how thermal effects and magnetic fields influence the alignment of atomic magnetic moments. In iron, these moments can be ferromagnetically ordered, with spins aligned, or paramagnetic, where spins are more disordered due to thermal agitation.
The simulations revealed that magnetic ordering changes the shape and stiffness of the atomic cages surrounding carbon. When iron atoms exhibit stronger magnetic alignment, the local lattice becomes less accommodating, increasing the energy barrier for carbon migration. In contrast, when magnetic order weakens near the Curie temperature, the cages become more isotropic, effectively opening pathways that make diffusion easier.
This provides a concrete explanation for why strong magnetic fields can suppress carbon diffusion under certain conditions. Rather than acting directly on carbon atoms, the magnetic field influences the electronic and magnetic structure of iron, which in turn reshapes the atomic-scale energy landscape governing diffusion.
Importantly, the study moves beyond qualitative descriptions. Previous explanations relied on phenomenological arguments that lacked predictive power. By quantifying how magnetic order modifies diffusion barriers, the researchers have created a framework that can be used to model real processing conditions, including temperature and field strength.
The implications extend beyond academic interest. Heat treatment accounts for a significant fraction of steel’s carbon footprint, and even modest reductions in processing temperatures or times could translate into substantial energy savings at industrial scale. Magnetic-field-assisted processing, guided by predictive models, may offer one route toward more efficient steel production.
The authors also note that the approach developed here could be applied to other alloy systems where diffusion plays a critical role. While iron and carbon were the focus of this study, the broader methodology opens the door to designing materials where magnetic effects are deliberately used to control atomic motion.
As materials engineering increasingly relies on computational tools to guide design decisions, this work demonstrates how long-standing empirical observations can be translated into quantitative, atom-level understanding. In doing so, it offers a clearer path toward engineering alloys with tailored properties while addressing the growing pressure to reduce industrial energy consumption.
Adrian graduated with a Masters Degree (1st Class Honours) in Chemical Engineering from Chester University along with Harris. His master’s research aimed to develop a standardadised clean water oxygenation transfer procedure to test bubble diffusers that are currently used in the wastewater industry commercial market. He has also undergone placments in both US and China primarely focused within the R&D department and is an associate member of the Institute of Chemical Engineers (IChemE).
