Physica B 320 (2002) 236–238 Site-diluted quantum Heisenberg spin model applied to the magnetic properties of Fe–Al disordered alloys M. Salazar a , Ligia E. Zamora a , G.A. P ! erez Alcazar a , J.A. Plascak b , W.R. Aguirre a,c, * a Departamento de F! ısica, Universidad del Valle, A. A. 25360 Cali, Colombia b Departamento de F! ısica, Universidad Federal de Minas Gerais, C.P. 602 Belo Horizonte, Brazil c Departamento de Ciencias B ! asicas, Pontificia Universidad Javeriana, A. A. 26239 Cali, Colombia Abstract In this work we present a theoretical study of the magnetic behavior of disordered Fe–Al alloys on the basis of a simple-diluted quantum Heisenberg spin model with the assumption that the exchange interaction J depends on the Al concentration q: We calculated the critical temperature and exponents through the mean field renormalization group method. An acceptable fit to the experimental phase diagram for Al concentration in the range 0:30pqp0:45 is obtained. r 2002 Elsevier Science B.V. All rights reserved. Keywords: Fe–Al alloys; Heisenberg model; Dilution 1. Introduction Some theoretical efforts have been developed in order to interpret the reported experimental results for the magnetic properties of ordered Fe–Al alloys. These experimental results [1,2] showed that the critical temperature T C decreases with the Al concentration q and presents an anomalous decrease near q ¼ 0:3: The same behavior was obtained for the mean hyperfine field (H). The first theoretical model was proposed by Sato and Arrot [3] and they proposed an antiferromagnetic behavior near the anomalous region, which was not revealed by neutron scattering experiments [4]. Shukla and Wortis [5] and Grest [6] did their estimates assuming a spin glass state near the critical Al concentration and good agreement with the experimental data has been achieved. Recently, some experimental results have been reported for disordered Fe–Al alloys [7,8] and these results showed that the T C vs. q curve presents the anomaly but it was not found for the H vs. q curve. In the interpretation of these results a theoretical model was proposed [9,10] using a simple diluted Ising model. With this model good accord was obtained for the T C vs. q curve and an excellent accord for the H vs. q for all the composition range. More recently, Moreno and Montenegro [11] reported an investigation of the ferromagnetic disordered Fe 1x Al x alloys by magnetization measurements. They obtained the critical tempera- tures and critical exponents for different alloys. The values obtained for the exponents satisfy the scaling relations and are relatively near the *Corresponding author. Fax: +57-2-33-93237. E-mail address: wrac@puj.edu.co (W.R. Aguirre). 0921-4526/02/$ - see front matter r 2002 Elsevier Science B.V. All rights reserved. PII:S0921-4526(02)00690-7