# Dynamic pathway of the photoinduced phase transition of TbMnO$_3$

## TL;DR

This study uses time-resolved resonant soft x-ray diffraction to explore how photoexcitation affects the magnetic and orbital orders in TbMnO₃, revealing a transient spin density wave phase and the coupling between magnetic and orbital orders.

## Contribution

It demonstrates the disentanglement of different magnetic orders in TbMnO₃ using polarized x-ray diffraction and identifies the transient magnetic phase after photoexcitation.

## Key findings

- Transient spin density wave retains cycloidal wave vector.
- Optical excitation energy does not significantly alter magnetic melting dynamics.
- Orbital reconstruction disappears on similar timescale as cycloidal order.

## Abstract

We investigate the demagnetization dynamics of the cycloidal and sinusoidal phases of multiferroic TbMnO$_3$ by means of time-resolved resonant soft x-ray diffraction following excitation by an optical pump. Using orthogonal linear x-ray polarizations, we suceeded in disentangling the response of the multiferroic cycloidal spin order from the sinusoidal antiferromagnetic order in the time domain. This 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 timescale. The transient phase is shown to be a spin density wave, as in the adiabatic case, which nevertheless retains the wave vector of the cycloidal long range order. Two different pump photon energies, 1.55 eV and 3.1 eV, lead to population of the conduction band predominantly via intersite $d$-$d$ transitions 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 timescale comparable to that of the cycloidal order, attesting to a direct coupling between magnetic and orbital orders. 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 complex theoretical studies.