Publication date: 21st July 2025
We present an in-situ Lorentz Transmission Electron Microscopy (LTEM) investigation into the cumulative effects of pulsed laser excitation on the metamagnetic transition in freestanding FeRh thin films. Our experiments demonstrate that repeated ultrafast laser pulses progressively introduce defects and associated residual strains, which promote the ferromagnetic phase and change the nucleation process from a homogeneous to a heterogeneous mechanism. This evolution is marked by a significant 20 K reduction in transition temperature, a nearly 50% decrease in the laser fluence threshold required for nucleation, and the emergence of magnetic vortices as dominant nucleation sites that emanate from strain fields of dislocation networks. Complementary thermal modeling and defect analyses identify laser-induced strain and rapid thermal quenching as critical factors promoting defect formation and stabilization. We also conducted ultrafast pump-probe LTEM experiments to further explore the nature of the unique antiferromagnetic to ferromagnetic transition in FeRh. These UTEM experiments aim to solve a long-standing fundamental "chicken-and-egg" dilemma about whether the magnetic order change initiates the structural lattice transformation or structural rearrangements trigger the magnetic phase change. These findings establish a clear connection between defect formation, nucleation energetics, and the microscopic structure of the emerging ferromagnetic phase, offering valuable insights for ultrafast stroboscopic imaging and defect-driven phase transitions in functional materials.
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