M A T E R I A L S F O R T O M O R R O W ' S I N N O V A T I V E A N D S U S T A I N A B L E I N D U S T R Y

S C I E N T I F I C H I G H L I G H T S

7 8 H I G H L I G H T S 2 0 2 3 I

X-ray experiments reveal details of magneto-structural, first-order transition in a prototypical alloy

X-ray absorption spectroscopy and X-ray magnetic circular dichroism experiments were carried out to investigate the microscopic mechanisms behind the magneto-structural transition in the prototypical alloy LaFe11.8Si1.2, unravelling the mutual interplay between magnetic, electronic and structural degrees of freedom.

Alloys with a first-order magnetic phase transition play an important role in the manufacturing of advanced refrigeration systems, sensors and transducers, and novel spintronics devices, etc. This transition is quite abrupt, and the magnetic, electronic and structural degrees of freedom in the material are all involved and mutually interrelated. It can be driven by various external stimuli: when an external thermodynamic force is applied to one subsystem of a material; for example, a change in magnetisation caused by a magnetic field, it immediately triggers transformations in other subsystems, such as an expansion of the crystal lattice, a change in electrical resistivity, or an increase in the temperature of the sample. Therefore, it is extremely important to understand which subsystem governs the phase transition and how energy and entropy are exchanged between subsystems during the transition process

With the aim to disentangle the underlying mechanism of such transitions, two new experimental setups were designed to allow simultaneous measurement of magnetisation, longitudinal and transversal magnetostriction and temperature change of the sample combined with element-selective magnetic X-ray spectroscopy. First, simultaneous measurements of the prototypical LaFe11.8Si1.2 alloy were taken, revealing rich details of the first-order magneto-structural transition causing the large magnetocaloric effect. It was found that the transition does not happen all at once; but instead in two separate steps. 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 the hysteresis width (Figure 59a and b).

To complement these findings, X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism (XMCD) experiments were carried out at beamline ID12. The results hold significant importance, since they made it possible to elucidate unprecedented details of the microscopic mechanism of the magneto-structural transition in La(FeSi)13-type alloys. In particular, it was found that the magnetisation 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. This mutual interplay between magnetic, electronic and structural degrees of freedom could only be unravelled with an element-selective approach exploiting XMCD and X-ray absorption near-edge structure (XANES) techniques (Figure 59c and d). Furthermore, the results of Mössbauer spectroscopy and first-principles calculations helped to disclose the full complexity and two-stage nature of the transition.

The two-stage mechanism for the first-order magnetic 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. This 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.