Ising model with isotropic competing interactions in the presence of a field

Summary We consider a spin system with competing interactions isotropic with respect to the axes of a cubic lattice in the presence of an external field. We show that for small values of the external fieldH, the paramagnetic to modulated phase transition is fluctuation-induced first order, while for larger fields, such transition changes to continuous at a tricritical point. Applications for fluids systems are proposed. Paper presented at the I International Conference on Scaling Concepts and Complex Fluids, Copanello, Italy, July 4–8, 1994.