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The TbFeCo magneto-optical media with the coercivity of bigger than 1 kOe are used for the investigation of ultrafast magnetic switching and thermal demagnetization. The magnetic marks induced by 25 fs laser pulses with circular polarization have been observed. The thermal demagnetization spots are measured based on their digital images for the estimation of laser fluence. Based on the experimental parameters, a theoretical calculation for estimation of the maximum applicable effective magnetic field is conducted, where the 25 fs laser pulses with wavelength of 800 nm, NA of 0.6 and circular polarization is applied. The results show that, with the maximum applicable laser fluence, the effective magnetic field on the outer boundary of the laser spot defined by 1/e electrical field amplitude is about 0.52 kOe. It implies that the spin flips can be implemented by a small effective magnetic field, which can be explained with the analysis of spin temperature.

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