Ultrafast ferromagnetic fluctuations preceding magnetoelastic first-order transitions

TL;DR

This study reveals ultrafast ferromagnetic fluctuations occurring before first-order magnetic transitions in LaFe13-xSix, highlighting the dominant role of magnetic degrees of freedom and providing insights into the microscopic mechanisms of magnetoelastic phase changes.

Contribution

It provides the first direct observation of picosecond ferromagnetic fluctuations preceding first-order magnetic transitions using neutron scattering and ultrafast X-ray diffraction.

Findings

Ferromagnetic fluctuations are observed in the paramagnetic state before the transition.

Photon-excitation suppresses ferromagnetic order in less than 1 ps.

Magnetic fluctuations are likely universally relevant in similar magnetocaloric compounds.

Abstract

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 study the magnetic phase transition dynamics on the intermetallic LaFe13-xSix, which is one of the most classical giant magnetocaloric systems, in both frequency and time domains utilizing neutron scattering and ultrafast X-ray diffraction. We have observed a strong magnetic diffuse scattering in the paramagnetic state preceding the first-order magnetic transition, corresponding to picosecond ferromagnetic fluctuations. Upon photon-excitation, the ferromagnetic state is completely suppressed in 0.9 ps and recovered in 20 ps. The ultrafast dynamics suggest that the magnetic degree of freedom dominates this magnetoelastic transition and ferromagnetic fluctuations might be universally relevant for this kind of compounds.