Enhanced magnetic stiffness in optically excited NdNiO₃ thin film heterostructures

NdNiO₃ is a prototypical correlated system, exhibiting a sharp metal-insulator transition and concomitant para-antiferromagnetic transition. Experiments on confined NdNiO₃ heterostructures have shown that magnetic order weakens but persists down to two unit cell thickness, decoupling from the electronic order. A proposed explanation is the interfacial structure frustrates the octahedral breathing distortion which couples to the magnetic order in bulk. Here, we interrogate this picture by studying the ultrafast spin dynamics following infrared optical excitation. Time-resolved soft x-ray magnetic diffraction measurements were conducted on a series of NdNiO₃/NdAlO₃ superlattices with variable interface confinement, determined by the relative thickness of layers. We find the magnetic order parameter exhibits a pronounced stiffness under strong confinement: melting the order requires higher fluence, and recovery is delayed. At short times, the magnetic correlation length is enhanced, particularly in samples with weaker confinement. Energy scans show the dynamics are correlated with charge redistribution between long and short bond sites, relaxing the breathing distortion. These results point to the possibility of engineering femtosecond dynamics through heterostructuring.