Intriguing Hysteresis Dynamics in Ultrafast Photo-Induced Magnetization

Abstract

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.