June 2005
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4 Reads
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2 Citations
A Heisenberg ferromagnet (F) with spin S=1/2, found in a spin-liquid (SL) state at temperatures above the Curie point ΤC, 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 ΤC+, 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 (Τ⩽ΤC, spin-wave theory) and in the SL state (Τ⩾ΤC+) is made. It is shown that ΤC+>ΤC, and a modification of spin-wave theory in which ΤC reaches the value ΤC+ 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 ∝ 1/Τ. The heat capacity of the ferromagnet at Τ↠ΤC goes to infinity, while in the SL state the heat capacity remains finite at the point ΤC+ and falls off for Τ≫ΤC+ in proportion to 1/Τ2. The susceptibility obeys the Curie-Weiss law.