(Received September 13, 1976)
The Kerr readout characteristics of MnCuBi thin films coated with dielectric SiO layer were studied experimentally. When the incident light intensity I0 is small, the readout signal-to-noise ratio S/N depends on the reflectivity R, the Kerr rotation angle θK and I0 as S/N∝(RI0)0.6θK. When I0 is large, S/N shows a maximum. The maximum S/N of 4 is obtained when I0 is about a half of the threshold laser intensity for the writing. This maximum seems to be associated with the decrease of the magnetization caused by the instantaneous increase of the temperature during the readout. In order to improve the S/N by the Kerr-effect enhancements, various dielectrics for the coating are theoretically compared, concluding that SiO seems to be appropriate.