  Alloys with a first-order magnetic transition are central to solid-state
refrigeration technology, sensors and actuators, or spintronic devices. The
discontinuous nature of the transition in these materials is a consequence of
the coupling between the magnetic, electronic and structural subsystems, but in
a real experiment, it is difficult to observe and analyze the simultaneous
evolution of all the subsystems. As a result, it is very hard to determine the
main mechanisms of the transition and purposefully develop these advanced
magnetic materials. To resolve this issue, we changed the existing paradigm and
conducted simultaneous measurements of the macroscopic properties -
magnetization, temperature change of the sample, longitudinal and transversal
magnetostrictions - to reveal the rich details of the magneto-structural,
first-order transition occurring in the prototypical alloy LaFe11.8Si1.2. We
complement these findings with experiments on the atomistic scale, i.e., x-ray
absorption spectroscopy (XAS), x-ray magnetic circular dichroism (XMCD) and
M\"ossbauer spectroscopy, and then combine them with first-principles
calculations to reveal the full complexity and two-stage nature of the
transition. This new approach can be successfully extended to a large class of
advanced magnetic materials that exhibit analogous transformations.

Materials Science