The Magneto-Optical Kerr Microscope(MOKM) was constructed to obtain informations about the ultrafast spin dynamics of magnetized thin films. The MOKM consists of two parts. One is the laser source part. This part consists of ~ 400nm laser pulse obtained by the second harmonic generation(SHG) for the mode-locked ~800nm pulse from Ti:sapphire crystal. And the other is the detection part. This part consists of a polarizer, prism and the differential amplifier to detect the signal by Kerr effect between the magnetization of sample and the laser pulse. Using this system, we measured the Kerr angle according to the magnetic field on the CoPt sample for polar direction.
In this study, we used the $Nd:YVO_4$ which is linearly polarized light as pumping laser source and obtained 780nm CW laser by passing through the Ti:sapphire crystal. Then in order to get large energy in short time scale, the CW laser beam is converted to pulse beam operating in 82MHz repetition rate and about 30fs FWHM by using the mode-locking technique. And we used the BBO crystal to make the SHG pulse(410nm) as probe beam. Through the Glan Thomson polarizer, horizontal polarization is changed to 45˚ direction. The horizontal and vertical polarization intensities of probe beam which is incident on the CoPt thin film, that is magnetized by the electromagnet, are changed by Kerr effect. Using the Wollaston prism, this changed beam is decomposed into horizontal and vertical polarization again. In this case, one intensity is different from the other by Kerr effect. These two signals are subtracted each other using the differential amplifier. Since this value is proportional to the Kerr angle, we could obtain the Kerr angle curve directly according to the magnetic polar field of the sample.