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A ferromagnetic sphere is uniformly magnetized in a large field. As the field decreases, there is a critical field the nucleation field at which the magnetization becomes nonuniform. The initial deviation from uniform magnetization the nucleation mode is either coherent rotation or curling. By contrast, a ferromagnetic cube is never uniformly magnetized, and theorists have disagreed on whether nucleation theory for a sphere can be generalized to a cube. A three-dimensional numerical model of a cube is used to show there is a bifurcation at which a curl appears in the magnetization. This bifurcation is a generalized curling-mode nucleation. The nucleation field for a small cube fits the equation for a sphere of the same volume with a reduced demagnetizing factor. The nucleation mode is well approximated by the curling mode for the sphere. The magnetization changes continuously, so the nucleation field is not equal to …