Description of a Heisenberg ferromagnet above the Curie point as a spin liquid

A Heisenberg ferromagnet (F) with spin S=1/2, found in a spin-liquid (SL) state at temperatures above the Curie point tau(C), is considered. In this spin-liquid state there is no long-range magnetic order but the short-range order is preserved, and the spin correlation functions are isotropic. The spin liquid is described in the framework of a second-order theory by the method of temperature Green functions. The main thermodynamic characteristics of the spin liquid are found as the result of a self-consistent numerical solution of a system of three integral equations. The Curie point tau(+)(C), at which the dc magnetic susceptibility at wave vector q=0 diverges, is determined. A comparison of the thermodynamic characteristics of the system in the F state (tau <=tau(C), spin-wave theory) and in the SL state (tau >=tau(+)(C)) is made. It is shown that tau(+)(C)>tau(C), and a modification of spin-wave theory in which tau(C) reaches the value tau(+)(C) is indicated. At the point of the F-SL phase transition the spin correlation functions suffer a finite discontinuity, and with increasing temperature they fall off proportional to 1/tau. The heat capacity of the ferromagnet at tau ->tau(C) goes to infinity, while in the SL state the heat capacity remains finite at the point tau(+)(C) and falls off for tau >tau(+)(C) in proportion to 1/tau(2). The susceptibility obeys the Curie-Weiss law. (C) 2005 American Institute of Physics.