1/z expansion for the Ising and Heisenberg models in an external field

Using an expansion in 1/z, where z is the coordination number of the lattice, we examine the effects of fluctuations on the magnetic susceptibility and specific heat of a spin-s ferromagnet in an external field. As expected, the first-order fluctuation correction to the transition temperature calculated from the zero-field susceptibility agrees with previous results obtained from the expansion of the order parameter. The temperature of the peak in the fluctuation specific heat of the Heisenberg model increases linearly with the external field.