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S. Khmelevskyi, P. Mohn:
"The ANNNI model and the first-order magnetic phase transition in UNi₂Si₂ compound triggered by correlation effects in the system of Ising spins";
Vortrag: Seminar Institut für Allgemeine Physik (IAP), TU Wien; 15.06.2004.

The introduction into the theory of modulated magnetic material and applications of Axial Next Nearest Neighbour Ising (ANNNI) model will be presented.
Recently, we have shown that the sequence of the magnetic phase transitions observed in UNi2S2 intermetallic compound can be described within pure axial Ising model with three competing interactions along one crystal axis. Such description can be achieved only by inclusion of correlation effects beyond the mean-field approximation. To treat the effects of non mean-field fluctuations we use here a cumulant expansion of the free energy up to third order term (Kirkwood approximation).It appears that first order transition between uncompensated antiferromagnetic state (UAF) and ordinary AF state occurring in UNi2Si2 at elevated temperature can be regarded as macroscopic manifestation of the correlation effects in the simplest (Ising) model of magnetism