An exactly solvable modification of the planar Ising ferromagnet is proposed which has a roughening transition below the Curie temperature. The computation confirms the de Gennes-Fisher scaling theory of correlations with homogeneous surface fields, giving an exponent value ${\Delta }_1=\frac{1}{2}$.

• Received 26 December 1979

DOI:https://doi.org/10.1103/PhysRevLett.44.1165