Ultrafast photoinduced phase separation dynamics in Pr0.6Ca0.4MnO3 thin films

The time resolved Magnetooptical Kerr effect in the substrate-strain-induced insulating ferromagnetic phase in (Pr0.6Ca0.4)MnO3 thin films on two different substrates was measured in a magnetic field up to 1.1T. The photoinduced Kerr rotation and ellipticity show remarkably different magnetic-field dependence. From the comparison of the magnetic field dependencies of the photoinduced and static Kerr rotation and ellipticity we conclude that a transient ferromagnetic metallic phase, embedded within the insulating ferromagnetic phase, is created upon the photoexcitation at low temperatures. A comparison of temporal dependence of the photoinduced Kerr signals with the photoinduced reflectivity indicates the change of the fractions of the phases takes place on a timescale of ten picoseconds independent of the substrate. Introduction. – Photoinduced insulator-metal (IM) transition in (Pr,Ca)MnO3 [1] and other related perovskite manganites is interesting not only due to its potential practical application, but also because of intimate connection between the metallic state and ferromagnetism in the manganites [2–4]. Experiments indicate that a photoexcited metallic state forms on a sub-ps scale [5, 6] and that the ultrafast transition to the metallic state can be triggered by a direct excitation of the lattice degrees of freedom [7]. It is not clear, however, whether on the sub-ps timescale the transient photoinduced metallic phase is the same as the static ferromagnetic metallic percolating phase. [8] There are some indications from time resolved magnetooptical Kerr effect (TRMOKE) measurements that the short range ferromagnetic order might be created simultaneously with the metallic state on the sub-ps timescale. [9, 10] However, the difficulty is that in the manganites the TRMOKE response might not directly reflect magnetization dynamics [11] as in the transition-metal ferromagnets [12,13] due to relatively slow charge dynamics [14,15] and/or presence of phase separation [8,16–18], which both can result in nonmagnetic contributions to TRMOKE. The fundamental questions how and on which timescale the transient metallic phase transforms into the metastable ferromagnetic metallic phase are therefore still open. The local ferromagnetic order can develop simultaneously with the metallic phase on a sub-ps timescale followed by slower reorientation of the local magnetization towards effective magnetic field. Alternatively, the transient metallic phase can be nonmagnetic on the sub-ps timescale nad the metastable ferromagnetic phase grows in form of clusters on a slower timescale. In order to answer these questions we investigate the photoinduced-phase formation dynamics in the straininduced ferromagnetic insulating state of Pr0.6Ca0.4MnO3 thin films [19, 20] by means of ultrafast TRMOKE. By comparing the photoinduced reflectivity with TRMOKE and analyzing magnetic field dependence of both responses in addition to their time evolution at two different photon energies we are able to disentangle magnetic and nonmagnetic contributions to TRMOKE and detect a transient modulation of the individual fractions of coexisting phases which appears on a timescale of ten picoseconds. We find no clear evidence that the magnetization is modified on the sub-picosecond scale. This suggests that the conducting ferromagnetic phase similar to the static one is not formed immediately on the sub-ps timescale.