Influence of Co substitution on magnetic properties and thermal expansion of Nd 6 Fe 13 Si intermetallic compound P. Iranmanesh a , N. Tajabor a, * , M. Rezaee Roknabadi a , F. Pourarian b , E. Brück c a Department of Physics, Faculty of Sciences, Ferdowsi University of Mashhad, Mashhad 91775-1436, Iran b Department of Material Science & Engineering, Carnegie Mellon University, Pittsburgh, PA 15219, USA c Fundamental Aspects of Materials and Energy, Faculty of Applied Sciences, Delft University of Technology, Mekelweg 15, 2629 JB Delft, The Netherlands article info Article history: Received 25 November 2010 Received in revised form 9 January 2011 Accepted 17 January 2011 Available online 24 February 2011 Keywords: A. Rare-earth intermetallics B. Magnetic properties B. Thermal properties abstract We studied the effect of Co substitution in Nd 6 Fe 13x Co x Si (x ¼ 0, 1) intermetallic compounds on the crystalline structure, magnetic properties and thermal expansion. The partial Co substitution reduces the lattice parameters of the tetragonal Nd 6 Fe 13 Si structure which enhance (3de3d) exchange interaction and this causes an increase of the Néel temperature (T N ). The spin reorientation temperature (T SR ) of Nd 6 Fe 12 CoSi occurs at a higher temperature with respect to that of the Nd 6 Fe 13 Si compound. Both the linear thermal expansion and the linear thermal expansion coefficient a(T) show remarkable anomalies at T SR and the magnetic ordering temperature (T N ). T SR is associated with a shoulder in the linear thermal expansion and near T N we observe invar-type of behaviour. Both effects are enhanced for the Co substituted sample, giving clear evidence for the enhanced exchange interactions. Ó 2011 Published by Elsevier Ltd. 1. Introduction Intermetallic compounds based on rare-earth (R) and transition (T) metals form a large group of materials which have found many applications and encouraged fundamental research. To improve the properties of the Nd 2 Fe 14 B compound several elements (M), such as Si, Al, Cu, Ga and etc, are added. The pronounced effect of these elements is to increase the coercivity of this compound by forming intergranular Nd 6 Fe 13x M 1þx phases [1e3]. The magnetic and structural properties of R 6 Fe 13x M 1þx (R ¼ light rare-earth, M ¼ Si, Al, Cu, Ga .) compounds and their hydrates that crystallize in the tetragonal Nd 6 Fe 13 Si or La 6 Co 11 Ga 3 structural type (with space group I4/mcm) have attracted the interest of many research groups [4e8]. The Nd 6 Fe 13 Si-type structure can be described as a multilayer structure in which the layers are perpendicular to the tetragonal c-axis. The layers with Fe atoms at 16k,4d, 16l 1 and 16l 2 sites are separated by layers of rare-earth (8f and 16l sites) or layers of M (4a site) [9,10]. Because of competing intralayer and interlayer ordering mechanisms (3de3d, 4fe3d and 4fe4f) and complicated magnetic anisotropy, these R 6 Fe 13x M 1þx intermetallic compounds show a diverse magnetic behaviour with various magnetic ordering schemes [11,12]. The compound Nd 6 Fe 13 Si orders antiferromagnetically around 421 K [12]. A powder neutron diffraction study between 2 and 421 K [13] has shown collinear antiferromagnetic structures with a spin reorientation at about 100 K. Below 100 K, the Nd and Fe magnetic moments are found to be parallel within the basal plane, whereas, above this temperature, they are aligned parallel to the c-axis. The antiferromagnetic structure consists of ferromagnetic blocks, separated by Si layers. Thus Nd and Fe moments are coupled locally ferromagnetic and ferromagnetic moments in each block change their direction collectively when going to the next block separated by the Si layer. All moments in neighbouring blocks are aligned antiferromagnetically [13]. The powder neutron diffraction indicates also a large anisotropic change in the unit cell volume associated with a change in the magnetoelastic coupling at the first-order spin reorientation transition. In addition to the neutron diffraction, the Mössbauer spectral studies of Nd 6 Fe 13 Si between 80 and 295 K [13e15] also have shown spin reorientation at 100 K. The measured magnetisation of Nd 6 Fe 13 Si between 4.2 and 295 K in applied fields of zero to 23 T has shown three regions in the magnetic phase diagram [16]. Below 110 K, the magnetic moments are aligned parallel to the basal plane with two observed critical fields at which a gradual spin canting occurs. Above 160 K, the moments are aligned along the c-axis with only one critical field. Between 110 and 160 K, a complex intermediate or mixed magnetic phase with up to three critical fields is observed. In this work, the influence of Co substitution on the lattice parameters, magnetic properties and thermal expansion of Nd 6 Fe 13 Si especially at the spin reorientation temperature is investigated. * Corresponding author. Tel.: þ 98 511 8817741; fax: þ98 511 8763647. E-mail addresses: tajabor@ferdowsi.um.ac.ir, ntajabor@yahoo.com (N. Tajabor). Contents lists available at ScienceDirect Intermetallics journal homepage: www.elsevier.com/locate/intermet 0966-9795/$ e see front matter Ó 2011 Published by Elsevier Ltd. doi:10.1016/j.intermet.2011.01.006 Intermetallics 19 (2011) 682e687