Physical origin of transient Kerr peak induced by circularly polarized laser in ferromagnetic films

Abstract

The magnetization dynamics of in-plane anisotropic FePt and perpendicularly anisotropic GdFeCo films induced by circularly polarized laser excitation is investigated using femtosecond time-resolved pump-probe Kerr spectroscopy. It is found that a transient Kerr peak occurs near zero-time delay, respectively for each sample. The origin of the peaks is analyzed. It is pointed out that the transient Kerr peaks are not related to ferromagnetism and may originate from paramagnetic magnetization of free electrons in the samples, while the magnetic field that magnetizes the free electrons comes from inverse Faraday effect of the circularly polarized pump laser. The calculation results based on the paramagnetic magnetization model support such a viewpoint. Based on our points, the duration of magnetic pulses induced by inverse Faraday effect should be the same as that of the pump laser pulses.