Quantification of electronic and magnetoelastic mechanisms of first-order magnetic phase transitions from first principles: application to caloric effects in La(FexSi1-x)(13)

La(FexSi1−x)13 and derived quaternary compounds are well-known for their giant, tunable,
magneto- and barocaloric responses around a first-order paramagnetic-ferromagnetic transition
near room temperature with low hysteresis. Remarkably, such a transition shows a large
spontaneous volume change together with itinerant electron metamagnetic features. While
magnetovolume effects are well-established mechanisms driving first-order transitions, purely
electronic sources have a long, subtle history and remain poorly understood. Here we apply a
disordered local moment picture to quantify electronic and magnetoelastic effects at finite
temperature in La(FexSi1−x)13 from first-principles. We obtain results in very good agreement with
experiment and demonstrate that the magnetoelastic coupling, rather than purely electronic
mechanisms, drives the first-order character and causes at the same time a huge electronic entropy
contribution to the caloric response.