First-order magnetic transitions are of both fundamental and technological interest given that a number of emergent phases and functionalities are thereby created. Of particular interest are giant magnetocaloric effects, which are attributed to first-order magnetic transitions and have attracted broad attention for solid-state refrigeration applications. While the conventional wisdom is that atomic lattices play an important role in first-order magnetic transitions, a coherent microscopic description of the lattice and spin degrees of freedom is still lacking. Here, we present a comparative neutron scattering study on the lattice and spin dynamics in intermetallic $\mathrm{La}{\mathrm{Fe}}_{11.6}{\mathrm{Si}}_{1.4}$ and $\mathrm{La}{\mathrm{Fe}}_{11.2}{\mathrm{Si}}_{1.8}$, which represent one of the most classical giant magnetocaloric systems and undergo first-order and second-order magnetic transitions, respectively. While their spin-phonon coupling effects are quite similar, $\mathrm{La}{\mathrm{Fe}}_{11.6}{\mathrm{Si}}_{1.4}$ exhibits a much stronger magnetic diffuse scattering in the paramagnetic state preceding its first-order magnetic transition, corresponding to intense ferromagnetic fluctuations. These dynamic insights suggest that the magnetic degree of freedom dominates this magnetoelastic transition and ferromagnetic fluctuations might be universally relevant for this kind of compounds.

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• Magnetic order
• Magnetic phase transitions
• Magnetism
• Magnetocaloric effect
• Spin dynamics