Antiferromagnetic ordering in magnetic ionic liquid Emim[FeCl 4 ] I. de Pedro a,n , D.P. Rojas a , Jesu ´ s A. Blanco b , J. Rodrı ´guez Ferna ´ ndez a a CITIMAC, Facultad de Ciencias, Universidad de Cantabria, 39005 Santander, Spain b Departamento de Fı ´sica, Universidad de Oviedo, 33007 Oviedo, Spain article info Available online 19 November 2010 Keywords: Ionic liquids Magnetic ionic Liquids Magnetic properties Raman spectroscopy Emim[FeCl 4 ] Antiferromagnetic ordering abstract The magnetic ionic liquids (MILs) are considered to open up a wide range of applications because of their magnetic and electrochromic switching. Until recently almost all magnetic ionic liquids containing tetrachloroferrate ion FeCl 4  evidenced a paramagnetic temperature dependence of the magnetic susceptibility, with only small deviations from the Curie law at low temperatures. However, 1-ethyl- 3-methylimidazolium tetrachloroferrate, Emim[FeCl 4 ], clearly exhibits a long-range antiferromagnetic ordering below the Neel temperature T N E3.8 K. In addition, the shape of the magnetic ordering depends on the cooling speed, indicating that the magnetic coupling could be modified. & 2010 Elsevier B.V. All rights reserved. 1. Introduction Ionic liquid (IL) refers to a class of liquid that is composed solely of ions [1] that exist in liquid state at room temperature. The increasing interest in room temperature ionic liquids (RTILs) is related to their possible exploitation as environmentally friendly neoteric solvents because of their vanishing vapour pressure, thermal and chemical stability, air and moisture stability, wide liquidus range, solvent capability, etc. Suitable applications of RTILs in synthesis, catalysis, biocatalysis, material science, and chemical engineering have been reported [2] in this decade. The rapidly increase in number and relevance of applications stimulated deeper understanding of the structure of RTILs in terms of inter- molecular interactions [3]. Although the dipolar and/or higher order multipolar interactions dominate in ordinary molecular liquids, Columb interaction plays a major role in ILs. Owing to this interaction, specific liquid structures may exist as ILs that are not expected for molecular liquids. In fact, recent spectroscopic [4] as well as diffraction studies [5] have revealed several new structural information that are indicative of ordered local structures [3,6]. In the case that these structural orderings become extensive enough, they may well cause extraordinary physical properties unique to IL. For instance, if magnetic anions are aligned locally, it would show interesting magnetic properties. Ionic liquids with anions containing transition metal complexes (i.e. magnetic ionic liquids or MILs) are the earliest developed RTILs [7]. Potential applications have been located in MILs in relation with their magnetic properties such as transport and separation of materials [8], magnetic fluids based on nanoparticles [9], acting as an absorbent liquid when applying a magnetic field [10] or in a catalysis process [11]. Another point of considerable interest is that a small change in the molecular structure in the constituent cations and anions results in a marked difference in the macroscopic reaction of the MILs to applied magnetic fields [8,12]. As such, it is necessary to evaluate the magnetic behaviour of a wider range of ionic liquids in order to improve or develop these applications. As liquid, it was generally assumed that the metal centers remained isolated, lacking long-range interaction(s) and coupling with other metal ions in the structure. However, in previous paper, we reported that a RTIL, 1-ethyl-3-methylimidazolium tetrachlorofer- rate Emim[FeCl 4 ], clearly shows a long-range antiferromagnetic ordering [13] when it is frozen. In this work, we present a complete dc and ac magnetic study of Emim[FeCl 4 ] together with the physical characterization. 2. Experimental Emim[FeCl 4 ] is a magnetic ionic liquid (purity above 99% with a nonmagnetic impurity of 1-ehtyl-3-methylimidazolium chloride) with a polycrystalline state below the freezing temperature T f ¼ 280 K. The long-range structural ordering has been confirmed with X-ray diffraction at nitrogen temperature (N 2 ; see Fig. 1(a)), which shows the formation of well defined diffraction peaks. These data were taken in a Bruker D8 diffractometer equipped with an Anton park TTK-450 temperature chamber and using a Cu-K a radiation (l ¼ 1.5418 ˚ A). However, up to now, it has not been possible to determine the crystal structure, which looks quite complex: it must be formed by the periodic arrangement of ion- paired species of Emim[FeCl 4 ] (see Fig. 1(b)). To get a deeper understanding of this issue, X-ray and neutron thermodiffracto- metry are under progress. Contents lists available at ScienceDirect journal homepage: www.elsevier.com/locate/jmmm Journal of Magnetism and Magnetic Materials 0304-8853/$ - see front matter & 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2010.11.016 n Corresponding author. Tel.: + 34 942 201512; fax: + 34 942 201402. E-mail address: depedrovm@unican.es (I. de Pedro). Journal of Magnetism and Magnetic Materials 323 (2011) 1254–1257