Ultrafast Magnetic Order Transition Driven by Excited-State Carrier Relaxation in Ferromagnetic Heterostructures

While all-optical control of magnetism offers great potential for developing ultrafast spintronic devices, achieving rapid generation of transient magnetic states with long-term stability remains a significant challenge. Here, we propose that the relaxation of photoexcited electrons in a type-II spin-antiparallel heterostructure can give rise to multiple stable magnetic states. Photoinduced spin-dynamics simulations of the CrI₃/CrBr₃ heterostructure reveal an ultrafast phase transition from the antiferromagnetic (AFM) to the ferrimagnetic (FiM) state within 826 fs. The relaxation pathways and rates are found to be critically determined by the competition between electron-phonon coupling (EPC) and spin-orbit coupling (SOC). When electrons are initially excited to higher-energy states, both EPC and SOC strengths are enhanced 3-fold, thereby accelerating the phase transition to 346 fs. These findings advance our understanding of the dynamic coupling among spin, charge, and lattice degrees of freedom in ultrafast magnetic order transition, paving the way for the development of advanced nonvolatile memory and neuromorphic computing systems.