# Physics:ANNNI model

Short description: Variant of the Ising model

In statistical physics, the axial (or anisotropic) next-nearest neighbor Ising model, usually known as the ANNNI model, is a variant of the Ising model in which competing ferromagnetic and antiferromagnetic exchange interactions couple spins at nearest and next-nearest neighbor sites along one of the crystallographic axes of the lattice. The model is a prototype for complicated spatially modulated magnetic superstructures in crystals.

The model was introduced in 1961 by Roger Elliott from the University of Oxford,[1] but only given this name in 1980 by Michael E. Fisher and Walter Selke.[2] It provides a theoretical basis for understanding numerous experimental observations on commensurate and incommensurate structures, as well as accompanying phase transitions, in magnets, alloys, adsorbates, polytypes, multiferroics, and other solids; further applications range from modeling of cortex to quantum information. [3] [4]

## References

1. "Phenomenological discussion of magnetic ordering in the heavy rare-earth metals". Phys. Rev. 124 (2): 346–353. 1961. doi:10.1103/PhysRev.124.346. Bibcode1961PhRv..124..346E.
2. M.E. Fisher and W. Selke (1980). "Infinitely many commensurate phases in a simple Ising model". Phys. Rev. Lett. 44 (23): 1502–1505. doi:10.1103/PhysRevLett.44.1502. Bibcode1980PhRvL..44.1502F.
3. P. Bak (1982). "Commensurate phases, incommensurate phases, and the devil's staircase". Reports on Progress in Physics 45 (6): 587–629. doi:10.1088/0034-4885/45/6/001. Bibcode1982RPPh...45..587B.
4. W. Selke (1988). "The ANNNI model—Theoretical analysis and experimental application". Physics Reports 170 (4): 213–264. doi:10.1016/0370-1573(88)90140-8. Bibcode1988PhR...170..213S.