Bit size dependencies on recording power in magneto-optic media having exchange coupled double layers

Thermomagnetically recorded domain sizes in exchange-coupled double layers of TbFeCoCr and TbDyFeCo having various interface wall energies are examined by both Kerr hysteresis loop and polarized microscope observations. For specimens with relatively large interface wall energy, the high Curie temperature layer (TbDyFeCo) independently yields larger domain size than that of the low Curie temperature layer (TbFeCoCr) when recorded with relatively low laser power. The situation, however, is not entirely stable, since the recorded domains of TbDyFeCo reform their sizes and become the same sizes as those of TbFeCoCr when magnetic field is applied to the as-recorded specimen during hysteresis loop cyclization. The reform of the domain-sizes of the TbFeCoCr layer is also observed during the hysteresis loop measurements for the specimen with large interface wall energy, although the applied field was well below the coercivity of the TbFeCoCr layer. The result is interpreted in terms of unstable behavior of the interlayer and intralayer domain wall