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Intriguing Hysteresis Dynamics in Ultrafast Photo‐Induced Magnetization

Time‐resolved magneto‐optical Kerr effect measurement is carried out to determine ultrafast hysteresis behavior during a photo‐induced demagnetization/remagnetization process for Co/Pt ferromagnetic multilayers. Due to the stroboscopic measurement scheme, hysteresis curve measured by a pump–probe technique exhibits an irreversible behavior, repeatedly reset to a metastable hysteresis state at each stroboscopic measurement, observed as the reduction of the coercivity. It is demonstrated that the stroboscopically measured hysteresis and the coercivity could be a stable parameter in describing the ultrafast photo‐induced spin dynamics for both reversible and irreversible behaviors. Moreover, it is found that an unusual coercivity behavior exists together with a nontrivial magnetization change under external fields comparable to the coercivity of the sample.