Ultrafast magnetisation dynamics in a chromium-based Prussian blue analogue

Ultrafast transient transmittance (TT) and time-resolved magnetic circular dichroism (TR-MCD) measurements were used to investigate the electronic and magnetisation dynamics in thin films of a mixed-valence chromium Prussian blue analogue. Measurements performed for 800 nm photoexcitation unveil a TT spectrum that consists of a broad, complex, bleach-like feature with some additional excited-state absorption contributions, while the spectral dependence of the TR-MCD closely matched the shape of the static MCD spectrum. Both the TT and TR-MCD response evolved on different timescales (2.5 and 1.5 ps, respectively) and superimposed on this is a slow growth in both the TT and TR-MCD response (220 and 90 ps, respectively). Decomposition analyses of the broadband transient spectrum using temperature-dependent transmittance data indicate that the slow growth in pump-probe response is a result of sample heating following lattice thermalisation. These thermal effects have a much more profound impact on the magnetisation dynamics, particularly at high pump fluences, where the >70% loss in magneto-optical response observed after several hundred ps vastly exceeds the corresponding 4% change in transmittance. These conclusions could only be reached through the determination of both static and dynamic responses of the sample, thus illustrating the power of using complementary spectroscopic methods.