Journal of Magnetism and Magnetic Materials 310 (2007) 709–711 The microscopic magnetisation of the superconductor PuCoGa 5 A. Hiess a,Ã , A. Stunault a , E. Colineau b , J. Rebizant b , F. Wastin b , N. Kernavanois a , G.J. McIntyre a , M.T. Fernandez-Diaz a , E. Lelie`vre-Berna a , J.A. Paixa˜o c , G.H. Lander b a Institut Laue Langevin, BP 156, 38042 Grenoble, France b European Commission, JRC-Institute for Transuranium Elements, Postfach 2340, 76125 Karlsruhe, Germany c Physics Department, University of Coimbra, Coimbra 3004-516, Portugal Available online 3 November 2006 Abstract The intermetallic compound PuCoGa 5 is a superconductor with a surprisingly high critical temperature of T sc ¼ 18 K in which the superconductivity has been suggested to arise from magnetic fluctuations. We investigated the microscopic magnetic properties by polarised neutron diffraction on a single crystal of 242 PuCoGa 5 . An induced moment is observed on both the Pu and the Co sites. The total magnetisation is significantly smaller than previously reported in agreement with recent magnetisation measurements. This result challenges the currently discussed models for the superconductivity in this weakly magnetic compound. r 2006 Elsevier B.V. All rights reserved. PACS: 71.27+a; 74.20.Mn; 75.25.+z; 75.40.Cx Keywords: Magnetic superconductor; Neutron diffraction; Strongly correlated electron system 1. Introduction PuCoGa 5 is one of several isostructural compounds for which superconductivity has been observed. It is a super- conductor with a critical temperature of T sc ¼ 18 K and an upper critical field B c2 estimated to exceed 70 T; both parameters are records in this class of intermetallic strongly correlated electron systems [1]. The superconductivity is thought to be unconventional and carried by strongly correlated electrons involving the 5f electrons of plutonium [2]. No magnetic order has been observed in PuCoGa 5 down to T ¼ 1 K, the lowest temperature measured. Those measurements suggest that the bulk magnetic susceptibility follows a modified Curie–Weiss behaviour in the normal state with an effective moment m eff ¼ 0:68 m B . The induced magnetic moment at T ¼ 20 K has been proposed to correspond to about m ¼ 0:006 m B per Tesla [1]. To investigate the microscopic magnetic properties further and possibly to shed light on the relevance of the magnetism for the superconductivity, we report here on experiments using polarised neutron diffraction. This experimental technique is sensitive to periodically arranged magnetic moments only and is insensitive to any magnetic contribution from random impurities or the sample mount. 2. Experimental PuCoGa 5 crystallises in the P4/mmm tetragonal crystal structure. The neutron absorption cross section of the most common isotope 239 Pu is too high for magnetic neutron scattering experiments. The present experiments were possible thanks to a crystal of sufficient size (160 mg) grown at JRC-ITU, Karlsruhe with the rare 242 Pu isotope. It was grown from the melt using 242 PuSb instead of metallic 239 Pu as starting material. For this reason the crystals contain traces of Sb (o1%) and the super- conducting transition temperature is slightly reduced to about T ¼ 15 K. Bulk properties in the normal state such as specific heat and magnetisation do not differ with respect to crystals grown from metallic 239 Pu at ITU [3]. ARTICLE IN PRESS www.elsevier.com/locate/jmmm 0304-8853/$ - see front matter r 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2006.10.275 Ã Corresponding author. Tel.: +33 476 207 165; fax: +33 476 483 906. E-mail address: hiess@ill.fr (A. Hiess). URL: http://www.ill.fr.