Observation of thermomagnetically written domains by Lorentz electron microscopy in Tb-Co and Tb-Fe-Co disks

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Abstract
For better understanding of the read/write properties of magneto-optical disks, thermomagnetically written domains were observed by Lorentz electron microscopy. The domains were written in the quadrilayered medium with 25-nm-thick Tb-Co film and in the trilayered medium with 50-nm-thick Tb-Fe-Co film, which were sputtered on pregrooved resinous substrates. Recording power and frequency were varied at intervals of 3.5 mm in a radial direction of the disk, and each recording position was observed individually. Qualitative understanding of the read/write properties, such as carrier-to-noise ratio (C/N) and jitter, were well achieved from Lorentz images of the domains. It is found that the C/N value of the Tb-Fe-Co film with a higher figure of merit (Rθk ) is lower than that of the Tb-Co film with a lower value of Rθk . The reason for this is believed to be due to a larger variation in domain shape in the Tb-Fe-Co film compared to the Tb-Co film. A careful observation based upon the image processing of domain bits reveals that the jitter is mainly caused by the irregularity of the domain shape near the edge in the Tb-Fe-Co film.