Journal of Magnetism and Magnetic Materials 290–291 (2005) 985–988 Exchange coupling in spins quartet of dimeric polyoxoanions [M 4 (H 2 O) 2 (GeW 9 O 34 ) 2 ] 12 (M ¼ Mn 2+ , Cu 2+ ) S. Isber a,Ã , M. Tabbal a , T. Christidis a , F. Terki b , S. Charar b a Department of Physics, American University of Beirut, Bliss Street, P.O. Box 11-0236, Beirut, Lebanon b Groupe d’Etudes des Semiconducteurs (GES), Universite´Montpellier II, CC 075, 34095 Montpellier cedex 5, France Available online 9 December 2004 Abstract Magnetization and magnetic susceptibility of dimeric polyoxoanions [M 4 (H 2 O) 2 (GeW 9 O 34 ) 2 ] 12 (M ¼ Mn 2+ , Cu 2+ ) were investigated using an isotropic exchange coupling model with two coupling constant (J and J 0 ) within the spin quartet formed by M 4 O 16 (M ¼ Mn 2+ , Cu 2+ ). The exchange couplings were found to be antiferromagnetic for both samples with the values (J/k B ¼0.9170.04K and J 0 /k B ¼1.1870.02K) and (J/k B ¼13.570.5K and J 0 / k B ¼6674 K) for the samples containing Mn 2+ and Cu 2+ , respectively. Furthermore, X- and Q-Band powder electron paramagnetic resonance (EPR) measurements on the sample containing Cu 2+ were investigated and the experimental spectra were simulated using a triplet spin state S ¼ 1 and I ¼ 3; and including a small axial distortion D term. This leads to spin-Hamiltonian parameters of: |D| ¼ 0.4470.01GHz, g || ¼ 2.41570.005, g ? ¼ 2.07070.005, A || ¼ (4071)G. r 2004 Elsevier B.V. All rights reserved. PACS: 76.30. Da; 75.10. Dg; 75.50. Pp Keywords: Magnetization; Exchange coupling; EPR; Spin clusters 1. Introduction A magnetic cluster is a group of magnetic ions that interact with each other but which do not interact with other magnetic ions [1,2]. The polyoxometalate com- plexes are well known for their ability to capsulate magnetic clusters of transition metal ions between diamagnetic fragments of metal oxides [3,4]. Transition metal-substituted polyoxometalates have attracted cur- rent attention in analytical chemistry and materials science. These compounds have structures that contain more than one paramagnetic transition metal ion in close proximity, which may exhibit exchange-coupled spins leading to large spin ground states [5,6] and are considered as model systems for the study of magnetic exchange interactions in clusters incorporated in the polyoxometalate matrix as a diamagnetic host. The magnetic clusters are usually classified by size. However, the shape of the cluster is crucial in studying magnetic properties, and clusters up to four spins have been reported with different strengths of exchange couplings [1,4]. X-ray single-crystal diffraction of Na 12 [Mn 4 (- H 2 O) 2 (GeW 9 O 34 ) 2 ]  38H 2 O, (Na 12 -1 hereafter) and Na 11 Cs 2 [Cu 4 (H 2 O) 2 (GeW 9 O 34 ) 2 ]Cl  31H 2 O, (Na 11 Cs-2 hereafter) showed that the exchange interaction between the individual metal ions of each tetrameric Mn 4 O 16 unit in Na 12 -1 and Cu 4 O 16 unit in Na 11 Cs-2 is mediated through M–O–M (M ¼ Mn 2+ , Cu 2+ ) linkages. In each ARTICLE IN PRESS www.elsevier.com/locate/jmmm 0304-8853/$-see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2004.11.305 Ã Corresponding author. Tel.: +9611350000x4295; fax: +9611744461. E-mail address: si00@aub.edu.lb (S. Isber).