L Journal of Alloys and Compounds 317–318 (2001) 44–51 www.elsevier.com / locate / jallcom 57 ¨ Fe Mossbauer spectroscopy study of the AFe Al intermetallics x 12 2x (A= Y, Tm, Lu and U, 4#x #4.3) a, a a a b b * ´ J.C. Waerenborgh , P. Salamakha , O. Sologub , A.P. Gonc ¸alves , S. Serio , M. Godinho , a M. Almeida a ´ ´ ´ Departamento de Quımica, Instituto Tecnologico e Nuclear, P-2686-953 Sacavem, Portugal b ´ ˆ Departamento de Fısica, Faculdade de Ciencias da Universidade de Lisboa, P-1749-016 Lisboa, Portugal Abstract AFe Al intermetallics (A=Y, Tm, Lu and U, 4#x#4.3) were synthesized as polycrystalline material and as large single crystals by x 122x the Czochralski method from bulk charges containing A, Fe and Al in the atomic ratios 1 / 4 / 8. The purity of the studied samples was checked by powder X-ray diffraction and their final composition was estimated using the Rietveld method of structure refinement. The 57 ¨ Fe Mossbauer spectra of samples with final composition x.4 have revealed that the Fe atoms on the same crystallographic site may have different magnetic moments, m , whose values increase with the number of Fe nearest neighbours. The strong sensitivity of the m Fe Fe on small deviations from the ideal 1 / 4 / 8 stoichiometry has clearly shown that while the single crystals of UFe Al grown by the 4 8 Czochralski method have the expected composition, those grown from YFe Al and LuFe Al bulk charges have actual compositions 4 8 4 8 YFe Al and LuFe Al thus explaining contradictory results that have been previously published.  2001 Elsevier 4.2(1) 7.8(1) 4.2(1) 7.8(1) Science B.V. All rights reserved. ¨ Keywords: Rare-earth iron aluminides; Uranium iron aluminides; Mossbauer spectroscopy; Magnetic properties 1. Introduction the refinement of X-ray and neutron diffraction data [4–8] that in these compounds Fe and Al have a strong prefer- The intermetallic systems AFe Al (A= Y or an f ence for the 8f and 8i sites, respectively, and that the 8j x 122x element and x$4), crystallizing in the ThMn -type struc- sites are shared by the remaining Fe and Al. 12 ture, continue to attract much interest. Compounds such as The study of ideally ordered AFe Al stoichiometric 4 8 DyFe Ta , SmFe Si or UFe Si , with uniaxial compounds, where the sublattices of Fe (magnetic) and Al 11.5 0.5 10 2 10 2 anisotropies and high Curie temperatures, T ¯550–650 (non-magnetic) are clearly separated, is a convenient first ord K, suggest that these materials might be potential candi- approach to the understanding of the coupling between the dates for permanent magnets with the lowest f -element content [1,2]. The f element sublattice provides the re- quired anisotropy while the high T is due to the strong ord Fe–Fe exchange interactions. Besides the potential econ- omical impact, these materials have been extensively studied for the elucidation of the fundamental aspects of magnetism and the understanding of their transport prop- erties on the basis of their electronic structure [1,3]. In the tetragonal body-centered ThMn -type structure, 12 space-group I 4/ mmm, the A atom is located on the origin of the unit cell, equiposition 2a, and the other atoms occupy the 8f,8j and 8i sites (Fig. 1). It is now clear from *Corresponding author. Tel.: 1351-21-994-6220; fax: 1351-21-994- Fig. 1. Crystallographic unit-cell of the ThMn -type structure. The 12 1455. f -element atoms are located at the origin and body-centered positions (2a E-mail address: jcarlos@itn1.itn.pt (J.C. Waerenborgh). sites). The other atoms occupy the 8f,8j and 8i sites. 0925-8388 / 01 / $ – see front matter  2001 Elsevier Science B.V. All rights reserved. PII: S0925-8388(00)01355-4