Field Behavior of Spin Models with Competing Interactions

Abstract

Our interest in the study of spin hamiltonians with competing interactions has been stimulated by recent measurements for the field-temperature phase diagram of compounds which exhibit modulated phases^1,2. In the first part of this talk we report mean-field calculations³ for the global T-p-H phase diagram of the ANNNI model (where p=-J₂/J₁ gives the strength of the axially competing interactions). At high temperatures we obtained several analytic results, but at low temperatures the modulated phases in a field were studied numerically. The analysis in the neighborhood of the critical surface was also supplemented⁴ by an ε-expansion renormalization-group treatment which will be reported in part II. We finally talk about an XY spin hamiltonian with competing interactions⁵ which is suitable for explaining the Lifshitz point in MnP.