Ultrafast lattice dynamics of FeRh

FeRh undergoes magnetic and structural phase transitions at $\sim 100^{\circ}$ C where a transition from antiferromagnetic to ferromagnetic orders occurs upon heating. Commensurate with this magnetic transition is a $\sim$1\% expansion in the lattice parameter. Recent optical measurements have shown that the magnetic transition can be quite fast, i.e., on the picosecond or sub-picosecond time scales [1,2]. We have used ultrafast x-ray diffraction techniques at the Advanced Photon Source to probe the speed of the corresponding structural transition. An epitaxial FeRh thin film on a MgO(001) substrate was driven through the phase transition by ultrafast laser excitation, and the response of the lattice was directly observed via picosecond-time-resolved x-ray diffraction. The temporal evolution of the FeRh lattice is reported as a function of laser fluence. [1] J.-U.\ Thiele \textit{et al}., Appl.\ Phys.\ Lett.\ 85, 2857 (2004). [2] G.\ Ju \textit{et al}., Phys.\ Rev.\ Lett.\ 93, 192301 (2004).