Structural and magnetic properties of UFe 6 Ga 6 A.P. Gonc ¸alves a, * , J.C. Waerenborgh a , S. Se ´rio a , J.A. Paixa ˜o b , M. Godinho c , M. Almeida a a Departamento de Quı ´mica, Instituto Tecnolo ´gico e Nuclear/CFMC-UL, Estrada Nacional, P-2686-953 Sacave ´m, Portugal b Universidade de Coimbra, Faculdade de Cie ˆncias e Tecnologia, Dept. Fı ´sica, CEMDRX, 3004-516 Coimbra, Portugal c CFMC-UL/Dep., Fı ´sica, FCUL, 1749-016 Campo Grande, Lisboa, Portugal Received 11 May 2005; received in revised form 19 July 2005; accepted 11 September 2005 Available online 9 November 2005 Abstract UFe 6 Ga 6 polycrystalline samples were prepared by arc-melting, and single crystals were grown by the Czochralski method. This compound crystallizes in the orthorhombic ScFe 6 Ga 6 -type structure (space group Immm, aZ5.0560(4), bZ8.5484(7) and cZ8.6914(7) A ˚ ), an ordered variant of the ThMn 12 -type structure. A ferromagnetic-type transition at T C Z530(5) K is seen in the magnetization and A.C.-susceptibility measurements, and no other magnetic anomaly is observed down to 5 K. Single crystal magnetization measurements along the three different crystallographic axes indicated a as the easy direction, with a spontaneous magnetization M S Z12.3 m B /f.u. at 5 K. The analysis of the 57 Fe Mo ¨ ssbauer spectroscopy data indicated magnetic hyperfine fields, B hf , significantly lower on 4f sites than on 8k sites, in agreement with the trend already observed on UFe x Al 12Kx , where the average B hf were found to increase with the iron–iron interatomic distances. q 2005 Elsevier Ltd. All rights reserved. Keywords: A. Magnetic intermetallics; B. Crystal chemistry of intermetallics; B. Magnetic properties; Diffraction 1. Introduction Intermetallic compounds of f-elements with the ThMn 12 - type structure and high iron content have been considered good candidates for hard magnetic materials [1,2]. The interaction between the 3d and f electrons in a tetragonal structure frequently gives high uniaxial magnetocrystalline anisotropy, Curie temperature and saturation magnetization. However, binary AFe 12 (AZf-element) compounds do not exist, the partial substitution of iron by a third element being necessary to stabilise the ThMn 12 -type structure. One of the most studied family of compounds with this structure is the AFe 12Kx Al x , (AZf-element) series [3–6]. The aluminium concentration necessary to stabilise the ThMn 12 - type structure is relatively high, usually higher than 50%, but the study of these medium-low iron content compounds is fundamental for a better understanding of the contribution from the different magnetic sublattices to the magnetism. The UFe 12Kx Al x phase relations, previously explored by us, indicate a congruent melting composition range between UFe 3.8 Al 8.2 and UFe 5.8 Al 6.2 [7]. The UFe 6 Al 6 alloy does not melt congruently but can be obtained by thermal treatment of polycrystalline samples. Measurements on UFe 6 Al 6 samples indicate an easy-plane anisotropy and a ferromagnetic character with a Curie temperature T C w300 K [8]. In the closely related UFe 6 Ga 6 compound only preliminary data reporting a Curie temperature of T C Z515 K, much higher than the aluminium equivalent, was previously reported [9]. In order to enlighten the reason for this difference a careful investigation of UFe 6 Ga 6 was performed. In the present paper, we report X-ray and neutron diffraction, 57 Fe Mo ¨ssbauer spectroscopy and magnetisation measurements on this compound. 2. Experimental Samples with UFe x Ga 12Kx (5%x%6.5) nominal compo- sitions were prepared by melting the stoichiometric amount of the elements (with purity of at least 99.9%) in an induction furnace equipped with a levitation cold crucible and under an argon atmosphere. The samples were turned and remelted at least three times in order to ensure a better homogeneity. The final weight losses were less than 0.5 wt%. The microstructural analysis of the samples was performed using a scanning electron microscope (JEOL-JSM 840) on sample pieces embedded in resin and polished using SiC paper down to 4000 mesh. Quantitative analysis of the observed phases was made by energy dispersive spectroscopy (EDS) Intermetallics 14 (2006) 530–536 www.elsevier.com/locate/intermet 0966-9795/$ - see front matter q 2005 Elsevier Ltd. All rights reserved. doi:10.1016/j.intermet.2005.09.002 * Corresponding author. Tel.: C351 1 9946182; fax: C351 21 9941455. E-mail address: apg@itn.pt (A.P. Gonc ¸alves).