Field Behavior of Spin Models with Competing Interactions
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 phases1,2. 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.
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