Demagnetizing fields near implanted/non-implanted boundaries have a significant effect upon the magnetization directions in contiguous disk devices. A new critical curve for the stable magnetization directions in an implanted garnet film is derived, including the effect of a uniform demagnetizing field. The magnitude of the in-plane field on the critical curve boundary is increased by the presence of the demagnetizing field. For a long, narrow implanted region, aligned along one three-fold easy axis, the magnitude of the in-plane field on the curve along the other two easy axes is ≊Hk1/3+H D/2. Approximating the implanted region between minor loops by a long ellipsoid, the minimum drive field for reliable propagation is predicted to be ≊Hk1/3+HD/2, in reasonable agreement with experiment.