Title: ORDER PARAMETER FOR FERROMAGNETIC PHASE TRANSITION IN THE TWO-PARTICLE MAGNETIC CLUSTER SYSTEM
Author(s): N.A.Korynevskii (Institute for Condensed Matter Physics of the National Academy of Sciences of Ukraine, 1 Svientsitskii Str., 79011 Lviv, Ukraine; Institute of Physics, University of Szczecin, 15 Wielkopolska Str., 70451 Szczecin, Poland)

Integration is performed over collective variables in the partition function functional of the two-particle magnetic cluster system using the fourth basic measure density. To rigorously consider the Gaussian and the non-Gauss\-ian fluctuations of the order parameter when dipole-dipole intercluster interactions take place, a two stage layer-by-layer method of integration is applied. To get the equation for the order parameter, the procedure of minimizing the integrand with the last (corresponding to zero values of quasimomentum and Matsubara's frequency) collective variable is used. Solutions of this equation in the phase transition point neighbourhood are found.

Key words: ferromagnets, cluster systems, functional, order parameter
PACS: 05.60.+W, 05.70.Ln, 05.20.Dd, 52.25.Dg, 52.25.Fi