We performed ultrafast time-resolved near-infrared pump, resonant soft x-ray diffraction probe measurements to investigate the coupling between the photoexcited electronic system and the spin cycloid magnetic order in multiferroic $\mathrm{TbMn}{\mathrm{O}}_3$ at low temperatures. We observe melting of the long range antiferromagnetic order at low excitation fluences with a decay time constant of $22.3\pm 1.1\mathrm{ps}$, which is much slower than the ∼1 ps melting times previously observed in other systems. To explain the data, we propose a simple model of the melting process where the pump laser pulse directly excites the electronic system, which then leads to an increase in the effective temperature of the spin system via a slower relaxation mechanism. Despite this apparent increase in the effective spin temperature, we do not observe changes in the wave vector $q$ of the antiferromagnetic spin order that would typically correlate with an increase in temperature under equilibrium conditions. We suggest that this behavior results from the extremely low magnon group velocity that hinders a change in the spin-spiral wave vector on these time scales.

• Received 26 June 2015
• Revised 20 October 2015

DOI:https://doi.org/10.1103/PhysRevB.92.184429