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In magneto-optical recording using sputtered TbFeCo, it was observed that writing sensitivity appreciably increases after writing/erasing repetitions. To clarify this phenomena, magnetic properties in TbFeCo were evaluated before and after heat treatment below crystallization temperature. It was found that the Kerr rotation, coercive force, and perpendicular magnetic anisotropy decrease after heat treatment. The writing sensitivity increase can be explained by the coercive force decrease after writing/erasing repetitions. To examine the magnetic property change, mechanical and structural properties in TbFeCo were investigated. After heat treatment, Young's modulus decreases and internal stress is drastically reduced to near O. A high resolution image of the as-sputtered TbFeCo cross section reveals that sputtered TbFeCo is comprised of crystal-like micrograins, about 20Å in diameter, and amorphous regions. As the causes of property changes, the oxidation and the structural relaxation can be considered. After heat treatment, the compensation composition and the compensation temperature didn't change in the present experiment. Thus the possibility of the oxidation can be ruled out. From the drastic decrease of the internal stress, it is considered that the structural relaxation occurs in amorphous regions after heat treatment.