On the lower critical dimensionality of the Ising model in a random field

The stability of ferromagnetism is studied in the three-dimensional Ising model, in the presence of a static, spatially random magnetic field. An approximation is introduced in which regions with a given spin direction are assumed not to contain smaller clusters of the reverse magnetisation. A combination of the Peierls argument with a rescaling transformation leads to a lower bound for the magnetisation, which is non-zero at low temperatures, provided the random field is not too strong.