A pressure study of the antiferromagnetic phase of FePM 2 Cl 2 (PM / pyrimidine) A.U.B. Wolter a , Hans-Henning Klauss a, *, F. Jochen Litterst a , T. Burghardt b , Andreas Eichler b , Ralf Feyerherm c , S. Su ¨ llow a a Institut fu ¨r Metallphysik und Nukleare Festko ¨rperphysik, TU Braunschweig, Mendelssohnstr. 3, D-38106 Braunschweig, Germany b Institut fu ¨r Technische Physik, TU Braunschweig, D-38106 Braunschweig, Germany c Hahn  /Meitner Institut, D-14109 Berlin, Germany Received 6 October 2002; accepted 7 December 2002 Abstract We present pressure experiments on FePM 2 Cl 2 (PM/pyrimidine). This material undergoes a transition into a canted antiferromagnetic state below T N /6.3 K at ambient pressure. We establish the bulk modulus, the anisotropy of the compression and the pressure dependence of the antiferromagnetic transition temperature. Our experiments demonstrate the usefulness of pressure experiments to investigate the properties of molecular magnetic compounds. # 2003 Elsevier Science Ltd. All rights reserved. Keywords: Canted antiferromagnet; Fe complex; Molecular magnet; High pressure; Compressibility; Bulk modulus 1. Introduction Molecular magnets, with their structure consisting of magnetic units assembled within a supermolecular aggregate consisting of organic components, tend to be soft materials. Therefore, they are fairly sensitive to the application of external pressure, as has been demon- strated, for instance, for purely organic magnets [1  /3]. Unusual pressure dependencies have been observed, like periodic up/down variations of ferromagnetic transition temperatures or pressure induced ferro- to antiferro- magnetic transitions. This reflects that applying pressure to a molecular magnet affects its properties by variation of two structural entities: (i) the distance between the magnetic ions and (ii) the geometry of the exchange path. Both the variation of the distance between magnetic ions and the geometry of the exchange path, because of the softness of molecular magnets, are extraordinarily large, and in that sense special, distinguishing these materials from conventional magnets. In consequence, to understand the interplay of structural and magnetic properties in molecular magnets it is necessary to determine the pressure dependence of both structural and magnetic parameters. In a first step towards this goal, here we present a combined structural and magnetic study of the properties of a molecular magnet under externally applied pressure. For our study we have chosen FePM 2 Cl 2 (PM / pyrimidine /C 4 N 2 H 4 ). This material belongs to a series of magnetically ordered transition metal complexes of general formula TX 2 (PM) 2 , whereby T /Fe, Co, Ni and X /Cl or Br [4,5]. These materials crystallize in a tetragonal lattice of space group I 4 1 22. The lattice parameters of Fe system have been determined to be a /7.4292 A ˚ and c /20.364 A ˚ . The transition metal ions are connected via pyrimidine rings, thus forming a three-dimensional network of transition metal  /pyrimi- dine complexes. In consequence, a pyrimidine mediated superexchange yields magnetic coupling between the metal ions, causing transitions into long-range magne- tically ordered states at low temperatures [6]. In case of FePM 2 Cl 2 , the transition into a canted antiferromag- netic state is reported to occur at T N /6.1 K. For Fe and Co compounds, it has been possible to directly * Corresponding author. Tel.:  /49-531-391-5128; fax:  /49-531- 391-5129. E-mail address: h.klauss@tu-bs.de (H.-H. Klauss). Polyhedron 22 (2003) 2139  /2141 www.elsevier.com/locate/poly 0277-5387/03/$ - see front matter # 2003 Elsevier Science Ltd. All rights reserved. doi:10.1016/S0277-5387(03)00182-7