Characteristics of bubble memory devices using multilayer permalloy films

We have investigated the bias field and detector's signal characteristics of a bubble memory device using multilayer Permalloy interleaved with Si spacers. Coercivity of two-layer Permalloy stripe pattern becomes minimum in the range from 3 to 5 nm of the Si spacer thickness, and is reduced to approximately half that with a single-layer Permalloy. In the three-layer Permalloy device, the bias field margin increases 2.7 Oe on the average, and the amplitude of signal increases by 30%, the maximum of noise amplitude is only about one-tenth, and the drive field causing maximum noise level is reduced to half, compared with a single-layer Permalloy device. The calculated decrease in potential well of three-layer Permalloy pattern is in good agreement with increase in bias field margin.