The dependence of the in-plane drive field at which bubble domains spontaneously nucleate in field-accessed bubble devices has been investigated as a function of Hk - 4πMs and of spacer thickness between the bubble film and permalloy propagation elements. The experiments were carried out on amorphous GdCoMo bubble films with T-bar and Y-bar structures. For a given structure and spacer thickness the nucleation field increases linearly with Hk - 4πM. Larger spacer thicknesses also lead to increased nucleation fields. A model based on the Stoner-Wohlfarth astroid is compared to these data and found to be useful in explaining the qualitative trends, but to be in poor quantitative agreement. It is concluded that since the drive field required in a device is proportional to 4πMs, Q - 1 = (Hk - 4πMs)/4πMs must be greater than some minimum value for a given device structure and spacer thickness to permit reliable device operation. Copyright © 1976 by The Institute of Electrical and Electronics Engineers, Inc.