By Daniel Mazzone, PSI
Based on article published in Proceedings of the National Academy of Sciences
Ultrafast manipulation of magnetic states holds great promise for progress in our understanding of new quantum states and technical applications. Increasing experimental evidence has shown in recent years that magnetism is suppressed by photodoping, but the exact nature of the spin configuration in such transient states and its evolution in time is largely unclear. This holds back progress in our understanding of magnetic states out of thermal equilibrium. Here we overcome these limitations using time-resolved resonant x-ray scattering, enabling studies of transient magnetic correlations throughout the entire Brillouin zone at ultrafast timescales.
Using Sr3Ir2O7 as a model material we demonstrated that femtosecond laser pulses can excite transient magnons at large wavevectors in gapped antiferromagnets and that they persist for several picoseconds . This incoherent transient behavior is fundamentally different to what is observed in nearly gapless magnets such as Sr2IrO4  – an isotropic Heisenberg-like material with comparable Néel temperature and similar energy scales. Here transient correlations are apparent only at the center of the magnetic Brillouin zone but not at its boundary.
Fig.: Schematic illustration of a transient magnetic state in Sr3Ir4O7 created by the impact of a femto-second laser pulse. Image credit: Cameron Dashwood, University College London
We interpret our results in the context of a spin-bottleneck scenario. The magnetic correlations in Sr2IrO4 feature a steep magnetic dispersion ranging from high-energy magnons at the magnetic zone boundary to low-energy excitations at the zone center, allowing for a highly efficient decay of transient magnons. In contrast, the large magnon gap and moderate dispersion of Sr3Ir2O7 leads to transient spin waves that are trapped in the entire Brillouin zone. Our work, thus, suggests that materials with isotropic magnetic interactions are preferred to achieve rapid manipulation of magnetism.
 D. G. Mazzone et al. Proc. Natl. Acad. Sci. U.S.A. 118, e2103696118 (2021).
 M. P. M. Dean et al. Nat. Mater. 15, 601 (2016).