(Received November 15, 1969)
A theory of a dilute Heisenberg ferromagnet is studied in two ways; the molecular field approximation and the Tyablikov approximation. By considering the Heisenberg model in the molecular field approximation, an expression for the averaged moment at any lattice site is obtained by the use of the diagram method. For a simple cubic crystal, numerical calculations of the averaged moment with a spin ½ host are presented. The Curie temperature depending on the concentration of magnetic atoms is presented. In the case of the molecular field approximation, it is shown that the critical concentration at which the ferromagnetic state disappears may not exist. By the use of the Green's function another method based on the Tyablikov approximation is formulated. The theory is straightforward generalization of the previous paper published in Prog. Theor. Phys. 42 (1969), 477, to finite temperature range. The existence of the critical concentration is also indicated.
URL : http://ptp.ipap.jp/link?PTP/44/328/
DOI : 10.1143/PTP.44.328