Journal of Magnetism and Magnetic Materials 272–276 (2004) 152–153 M . ossbauer studies on the Fe(II) oxalate spin ladder Na 2 Fe 2 ðC 2 O 4 Þ 3 ðH 2 OÞ 2 J. Kreitlow a , D. Baabe a , A.U.B. Wolter a , S. S . ullow a , F.J. Litterst a , D.J. Price b , H.-H. Klauss a, * a Inst. f. Metallphysik und Nukl. Festk . orperphysik, TU Braunschweig, Mendelssohnstr. 3, Braunschweig 38106, Germany b Department of Chemistry, University of Southampton, Southampton SO17 1BJ, UK Abstract We investigated the magnetic correlations in the Fe spin ladder system Na 2 Fe 2 ðC 2 O 4 Þ 3 ðH 2 OÞ 2 by magnetic susceptibility and 57 Fe M . ossbauer spectroscopy between 2 and 310 K: The susceptibility exhibits a broad maximum at about 20 K indicating antiferromagnetic coupling and a S ¼ 0 dimer formation. The M . ossbauer measurements reveal a temperature dependent quadrupole splitting and a non-magnetic ground state. The orbital ground state is identified as a singlet with an excited doublet at 383 K in the T 2g manifold. r 2004 Elsevier B.V. All rights reserved. PACS: 76.80.+y; 75.50.Xx; 75.10.Dg Keywords: Molecular magnets; Spin ladder; M. ossbauer spectroscopy One dimensional spin systems with antiferromagnetic Heisenberg exchange interaction and spin S ¼ 1 2 or 1 have been studied very intensively in recent years. Here we present magnetic susceptibility and M . ossbauer studies on the FeðIIÞðS ¼ 2Þ based 2-leg ladder Na 2 Fe 2 ðC 2 O 4 Þ 3 ðH 2 OÞ 2 : This compound is isostructural with the Co analogue Na 2 Co 2 ðC 2 O 4 Þ 3 ðH 2 OÞ 2 synthe- sized recently [1]. In these systems the magnetic interaction is mediated via the oxalate (C 2 O 4 Þ 2 anion. In the monoclinic crystal structure the ladder frame- works extend along the a-axis. The magnetic susceptibility of polycrystalline powder has been measured using a SQUID magnetometer in 1 T (Fig. 1). The susceptibility exhibits a Curie–Weiss behaviour above 50 K: At E20 K a broad maximum indicates an antiferromagnetic interaction and a S ¼ 0 dimer formation at low temperatures due to a magnetic exchange J rung E9 K on the rungs of the ladder. An upturn of wðT Þ below 5 K stems from a small amount of paramagnetic impurities. A quantitative analysis includ- ing crystal field effects and inter dimer interaction is in progress. 57 Fe M . ossbauer spectroscopy has been per- formed between 2 and 300 K using a 57 Co-in-Rh source. In Fig. 2 we plot the M . ossbauer-spectra of Na 2 Fe 2 ðC 2 O 4 Þ 3 ðH 2 OÞ 2 at 300 and 10 K: At 300 K; we observe a two line spectrum, resulting from a quadru- pole splitting of QS ¼ 1:029ð5Þ mm=s and an isomer shift of IS ¼ 1:087ð5Þ mm=s (relative to the 57 Co-in-Rh source). At 10 K the spectrum exhibits no sign of magnetic order but a large increase of the quadrupole splitting to QS ¼ 2:008ð5Þ mm=s and an isomer shift of IS ¼ 1:221ð5Þ mm=s: These values are typical for high spin Fe(II) [2]. Fig. 3 depicts the temperature dependence of the quadrupole splitting between 2 and 310 K; revealing a saturation at low temperatures. The solid line represents a model fit taking into account a constant lattice contribution to the electrical field gradient (EFG) and a temperature dependent contribution from the Fe 3d shell. The fivefold degeneracy of the 3d orbitals is partially lifted due to the octahedral site symmetry [3]. ARTICLE IN PRESS *Corresponding author. Tel.: +49-531-391-5128; fax: +49- 531-391-5129. E-mail address: h.klauss@tu-bs.de (H.-H. Klauss). 0304-8853/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.12.676