When bismuth-substituted iron garnet (RBi)IG single crystals grown from liquid-phase-epitaxy (LPE) are magnetized, the magnetic domain structure gradually evolves. However the domain-wall nucleation that occurs during the demagnetization is not yet understood clearly. This phenomenon is important, because the working of some optical devices that utilize (RBi)IG single crystals depends directly on the formation of domains of opposite direction. In this paper, the authors present a theoretical description of domain-wall nucleation in LPE-grown (RBi)IG single crystals. It was found that the nucleation field is determined by the strength of the microscopic domain-wall which is fixed on the surface of the crystal. Furthermore, the nucleation field is modified by the addition of a magnetic field.©1997 American Institute of Physics.