Coercivity dynamics and origin of time-delayed magneto-optical hysteresis loops in pump-probe Kerr spectroscopy

Abstract

Time-resolved magneto-optical Kerr effect (TR-MOKE) is used to measure hysteresis loops of a magnetic film at a different delayed time after femtosecond laser excitations. Coercivity dynamics of the magnetic film is obtained from TR-MOKE loops as usual. However, we point out that the dynamics of coercivity does not reflect the evolution of initial coercivity shown by the initial hysteresis loop with no laser excitation, but that of the final coercivity under laser excitation, where the final coercivity means one given by the hysteresis loop measured at a negative pump-probe delay time under pump excitation. TR-MOKE loops usually cannot track the evolution of coercivity of a magnetic system upon strong laser shot, and can do that only when the magnetic system is reversible under a low excitation. The oscillatory recovery behavior of the final coercivity is observed, and is ascribed to metastability of zero magnetization state.

Effect of magnetic anisotropy relaxation on laser-induced magnetization precession in thin galfenol films

“…We use two optical pump-probe setups with experimental geometries to measure the longitudinal and polar Kerr effects. The setup of the time-resolved longitudinal magnetooptical Kerr effect (TR-LMOKE) is used to obtain the hysteresis loops at different time delays after the pump pulse excitation, and then to determine the temporal evolution of the saturation magnetization M S and the uniaxial K U and cubic K C anisotropy parameters, similar to works [34,35]. The setup of the time-resolved polar magneto-optical Kerr effect (TR-PMOKE) is used to detect the magnetization precession after the pump pulse excitation.…”

Effect of magnetic anisotropy relaxation on laser-induced magnetization precession in thin galfenol films

“…We use two optical pump-probe setups with experimental geometries to measure the longitudinal and polar Kerr effects. The setup of the time-resolved longitudinal magnetooptical Kerr effect (TR-LMOKE) is used to obtain the hysteresis loops at different time delays after the pump pulse excitation, and then to determine the temporal evolution of the saturation magnetization M S and the uniaxial K U and cubic K C anisotropy parameters, similar to works [34,35]. The setup of the time-resolved polar magneto-optical Kerreffect (TR-PMOKE) is used to detect the magnetization precession after the pump pulse excitation.…”

Intriguing Hysteresis Dynamics in Ultrafast Photo‐Induced Magnetization

“…For instance, coercivity, saturation field, remanence, and saturation magnetization provide basic characteristic parameters in analyzing overall ferromagnetic features. However, it has been known that magnetic hysteresis, measured during the photo‐induced demagnetization/remagnetization process, is quite different from static hysteresis curves with much reduced coercivity . The coercivity reduction is explained based on metastable states during the photo‐induced demagnetization/remagnetization dynamics, leading to a so‐called irreversible process, arising from the magnetization state change in metastable states .…”

“…However, it has been known that magnetic hysteresis, measured during the photo‐induced demagnetization/remagnetization process, is quite different from static hysteresis curves with much reduced coercivity . The coercivity reduction is explained based on metastable states during the photo‐induced demagnetization/remagnetization dynamics, leading to a so‐called irreversible process, arising from the magnetization state change in metastable states . To avoid the irreversible process issue in pump‐probe stroboscopy measurement for ultrafast spin dynamics, external magnetic field cycling in synchronization with the same frequency as the stroboscopic repetition frequency was adopted in TR‐MOKE setup .…”

“…On the other hand, interestingly, Shufa et al have recently reported that coercivity measured by the pump‐probe stroboscopy can be also reversible, where the field‐dependent Kerr signals are observed to be reversible near coercivity . Thus, hysteresis behavior such as coercivity reduction in ultrafast spin dynamics still remains a scientific interest to further explore.…”