New approach to study alloys with a first-order magnetic transition

Alloys with a first-order magnetic transition play an important role in the manufacturing of advanced refrigeration systems, sensors, and devices that use the properties of electrons for various purposes. This magnetic transition is unique because it happens suddenly and is connected to how the magnetic part, the electronic part, and the structural part of the material all work together. Sometimes, this change can happen in different parts of the material.

Such transition can, in principle, cross several metastable states, where at one point, the transition takes place within the magnetic subsystem, while at another, the changes occur in the structural or electronic subsystems.

With the aim to disentangle the nature of such transition, researchers led by the University of Darmstadt designed two new experimental set-ups, allowing simultaneous measurement of magnetization, longitudinal and transversal magnetostriction and temperature change of the sample. They carried out simultaneous measurements of the prototypical LaFe11.8Si1.2 alloy in the lab and revealed rich details of the magneto-structural, first-order transition.

“What we found was that this magnetic change doesn't happen all at once; it actually takes place in two separate steps. The middle step, where things are in between, affects how the change behaves, like how wide the ‘hysteresis’ is, meaning how much it goes back and forth”, explains Konstantin Skokov, corresponding author of the paper, from Darmstadt University.

At the first stage, only the magnetic subsystem contributes to the magnetocaloric effect and, for this reason, the hysteresis losses are insignificant. In contrast, in the second stage, both magnetic and structural subsystems participate in the transformation, leading to an increase in hysteresis width.

The team then came to the ESRF to complement these findings with X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) experiments on ID12 beamline. “The experiments' outcomes hold significant importance, since we were able to see with unprecedented detail the microscopic mechanism of the magneto-structural transition in La(FeSi)13-type alloys. In particular, we found that the magnetic moment of Fe exhibits a transition from a low-spin state to a high-spin state, a direct consequence of which is a rearrangement of the local electronic structure and the volume expansion. The mutual interplay between magnetic, electronic and structural degrees of freedom plays a key role in the phase transition and could only be unraveled with element selective approach exploiting XMCD/XANES techniques”, says Skokov.

They also used Mössbauer spectroscopy, and first-principles calculations to reveal the full complexity and two-stage nature of the transition.

The results suggest that the two-stage mechanism for the first-order transition, proposed here for La(FeSi)13-type alloys, can be extended to a large class of advanced magnetic materials with itinerant moments exhibiting comparable field- and temperature-induced transformations, for example, Mn-Fe-P-Si, and Heusler-type alloys.

“Our work shows a new pathway to disentangle the interplay between the structural, magnetic, and electronic degrees of freedom, and serves as the next step toward a complete understanding of the driving forces of the transition, along with the origin of thermal hysteresis in magnetic phase-change materials”, concludes Skokov.

Reference:

K. P. Skokov, et al, Applied Physics Reviews 10(3), 031408 (2023). https://doi.org/10.1063/5.0133411

Top image: Schematics of the two-stage transition in LaFe11.8 Si1.2 alloy.