Dynamic pathway of the photoinduced phase transition of ${\mathrm{TbMnO}}_3$

We investigate the demagnetization dynamics of the cycloidal and sinusoidal phases of multiferroic ${\mathrm{TbMnO}}_3$ by means of time-resolved resonant soft x-ray diffraction following excitation by an optical pump. The use of orthogonal linear x-ray polarizations provides information on the contribution from the different magnetic moment directions, which can be interpreted as signatures from multiferroic cycloidal spin order and sinusoidal spin order. Tracking these signatures in the time domain enables us to identify the transient magnetic phase created by intense photoexcitation of the electrons and subsequent heating of the spin system on a picosecond time scale. The transient phase is shown to exhibit mostly spin density wave character, as in the adiabatic case, while nevertheless retaining the wave vector of the cycloidal long-range order. Two different pump photon energies, 1.55 and 3.1 eV, lead to population of the conduction band predominantly via intersite $d-d$ or intrasite $p-d$ transitions, respectively. We find that the nature of the optical excitation does not play an important role in determining the dynamics of magnetic order melting. Further, we observe that the orbital reconstruction, which is induced by the spin ordering, disappears on a time scale comparable to that of the cycloidal order, attesting to a direct coupling between magnetic order and orbital reconstruction. Our observations are discussed in the context of recent theoretical models of demagnetization dynamics in strongly correlated systems, revealing the potential of this type of measurement as a benchmark for such theoretical studies.

Figure 1

Intensity maps of the (0 $q$ 0) reflection recorded at 653 eV with incoming (a) $\pi$- and (b) $\sigma$-polarization as functions of temperature and momentum transfer (in 1/Å on the left and in reciprocal lattice units on the right, using the lattice constants from Blasco et al. [31]). Scattering occurs in the ab-plane for this azimuth of the sample. (c) (0 $q$ 0) integrated intensity as a function of temperature for $\pi$- (black squares) and $\sigma$- (red circles) polarization. Magnetic transition temperatures are shown as vertical dashed lines.

Figure 2

(0 $2q$ 0) intensity for $\pi$ (black squares) and $\sigma$ (red circles) polarization, as a function of (a) momentum transfer, at 10 K and 653 eV; (b) energy, at 10 K and at the peak position shown in (a); (d) temperature, at 653 eV (integrated intensity). Magnetic transition temperatures are shown as vertical dashed lines in (d). (c) Intensity maps of the (0 $2q$ 0) reflection recorded at 653 eV with incoming (top) $\pi$ and (bottom) $\sigma$ polarization as functions of temperature and momentum transfer (in 1/Å on the left and in reciprocal lattice units on the right, using the lattice constants from Blasco et al. [31]). Scattering occurs in the $ab$ plane for this azimuth of the sample.

Figure 3

Time-resolved change in diffracted intensity of the (0 $q$ 0) reflection recorded at 12 K at the Mn $L_2$ edge for different x-ray probe polarizations, following photoexcitation at 1.55 eV with 1.3 mJ/${\mathrm{cm}}^2$ absorbed fluence. $\pi$-polarization data and fit are shown as black squares and a full black line. $\sigma$-polarization data and fit are shown as red circles and a dashed red line.

Figure 4

Time-resolved change in diffracted intensity of the (0 $q$ 0) reflection recorded at the Mn $L_2$ edge for $\pi$ polarization of the x-ray probe, following photoexcitation at 1.55 eV. 12 K data and fit are shown as black squares and a full black line. 30 K data and fit are shown as blue diamonds and a dashed blue line. The absorbed fluence was 1.3 mJ/${\mathrm{cm}}^2$ at 12 K and 0.85 mJ/${\mathrm{cm}}^2$ at 30 K, chosen to achieve a similar maximum decrease in intensity.

Figure 5

Time-resolved change in diffracted intensity of the (0 $q$ 0) reflection recorded at 12 K at the Mn $L_2$ edge for different x-ray probe polarizations, following photoexcitation at 3.1 eV with 0.8 mJ/${\mathrm{cm}}^2$ absorbed fluence. $\pi$-polarization data and fit are shown as black squares and a full black line. $\sigma$-polarization data and fit are shown as red circles and a dashed red line.

Figure 6

Time-resolved change in diffracted intensity of the (0 $2q$ 0) reflection recorded at 12 K at the Mn $L_2$ edge for different x-ray probe polarizations, following photoexcitation at 1.55 eV with 1.3 mJ/${\mathrm{cm}}^2$ absorbed fluence. $\pi$-polarization data and fit are shown as black squares and a full black line. $\sigma$-polarization data and fit are shown as red circles and a dashed red line.