  In structurally disordered ferromagnets the weak random dipole-dipole
exchange may transform the polydomain state into a spin-glass one. To some
extent the properties of such phase in disordered isotropic ferromagnet can be
qualitatively described by the spherical model with the short-range
ferromagnetic interaction and weak frustrated infinite-range random-bond
exchange. This model is shown to predict that spin-glass phase substitute the
ferromagnetic one at the arbitrary small disorder strength and that its
thermodynamics has some similarity to that of polydomain state along with some
significant distinctions. In particular, the longitudinal susceptibility at
small fields becomes frozen below transition point at a constant value
depending on the disorder strength, while the third order nonlinear magnetic
susceptibilitiy exhibits the temperature oscillations in small field near the
transition point. The relation of these predictions to the experimental data
for some disordered isotropic ferromagnets is discussed.
