Ultrafast signatures of spin and orbital order in antiferromagnetic α-Sr₂CrO₄

The antiferromagnetic Mott insulator α-Sr₂CrO₄ possesses multiple spin and orbital ordered phases, but their unique interplay is still relatively unexplored. Here, we used femtosecond optical spectroscopy to study ultrafast spin and orbital ordering dynamics in α-Sr₂CrO₄ through their non-equilibrium response to photoexcitation. By varying the pump photon energy, we selectively drove inter-site spin hopping between neighboring Cr t_2g orbitals and charge transfer-type transitions between oxygen 2p and Cr e_g orbitals. The resulting transient reflectivity dynamics revealed temperature-dependent anomalies across the Néel temperature for spin ordering as well as the transition temperatures linked to different types of orbital order. Our results reveal distinct relaxation timescales for spin and orbital orders in α-Sr₂CrO₄ and provide experimental evidence for the phase transition at T_O, possibly related to antiferro-type orbital ordering.