Using ultrafast optical pump-probe spectroscopy to reveal coexisting magnetic orders in epitaxial $R$MnO$_{3}$ films

Recent discoveries of spin-driven ferroelectricity in perovskite manganites, $R$MnO$_{3}$ ($R$=rare-earth ions), have attracted enormous interest in the research of multiferroics. Although extensive experimental and theoretical studies have already been done on single crystal $R$MnO$_{3}$, there are only a few reports describing the properties of $R$MnO$_{3}$ thin films. Here, we choose two typical materials in $R$MnO$_{3}$ manganites as examples: SmMnO$_{3}$ and TbMnO$_{3}$. Previously, ultrafast optical pump-probe spectroscopy has proven to be an ideal technique for unraveling the interplay between different orders in the time domain. In this work, we used this technique to study ultrafast dynamics in epitaxial SmMnO$_{3}$ and TbMnO$_{3}$ films grown on SrTiO$_{3}$ substrates. At low temperatures, we observed an extraordinarily slow rising process, with a timescale of tens of picoseconds, followed by another decay process with a relaxation time of hundreds of picoseconds. Analysis of the time constants associated with these two processes as a function of temperature reveals that antiferromagnetic, ferromagnetic, and ferroelectric orders can coexist in these materials.