XMCD for Monitoring Exchange Interactions. The Role of the Gd 4f and 5d Orbitals in Metal-Nitronyl Nitroxide Magnetic Chains Guillaume Champion, †,‡ Nikolia Lalioti, §,⊥ Vassilis Tangoulis, §,⊥ Marie-Anne Arrio, | Philippe Sainctavit, ‡,| Franc ¸ oise Villain, †,‡ Andrea Caneschi, § Dante Gatteschi, § Christine Giorgetti, ‡ Franc ¸ ois Baudelet, ‡ Michel Verdaguer, † and Christophe Cartier dit Moulin* ,†,‡ Contribution from the Laboratoire de Chimie Inorganique et Mate ´ riaux Mole ´ culaires, UniVersite ´ Pierre et Marie Curie, Ba ˆ t. F74, Case 42, 4 place jussieu, 75252 Paris Cedex 05, France, LURE, UniVersite ´ Paris-Sud, Ba ˆ t 209D, BP 34, 91898 Orsay Cedex, France, Dipartmento di Chimica, UniVersita ` degli Studi di Firenze, and UdR I.N.S.T.M. di Firenze, Via della Lastruccia 3, 50019 Sesto Fiorentino (Fi), Italy, and Laboratoire de Mine ´ ralogie Cristallographie de Paris, UniVersite ´ Pierre et Marie Curie, Tour 16, 4 place jussieu, 75252 Paris Cedex 05, France Received February 11, 2003; E-mail: cartier@ccr.jussieu.fr Abstract: We report here the X-ray magnetic circular dichroism (XMCD) study at the Gd M4,5- and L2,3- edges of two linear magnetic chains involving Gd(III) cations bridged by nitronyl nitroxide radicals. This spectroscopy directly probes the magnetic moments of the 4f and 5d orbitals of the gadolinium ions. We compare macroscopic magnetic measurements and local XMCD signals. The M4,5-edges results are in agreement with the J values extracted from the fits of the SQUID magnetic measurements. The L2,3-edges signals show that the electronic density in the Gd 5d orbitals depends on the neighbors of the gadolinium cations. Nevertheless, the 5d orbitals do not seem to play any role in the superexchange pathway between radicals through the metal ion proposed to explain the particular magnetic exchange interactions between the radicals in these chains. Introduction One-dimensional systems can in principle give rise to competing interactions when the spins are coupled antiferro- magnetically to their next-nearest neighbors (nnn) independent of the sign of the interaction with the nearest neighbors (nn). To interpret the magnetic properties, it is necessary to take into account the nn and nnn interactions, which can be sometimes predominant. In the course of our investigation of magnetic materials containing exchange-coupled rare-earth ions and organic radicals, 1-7 we synthesized the linear chain compounds Gd(hfac) 3 NITR (hfac ) hexafluoroacetylacetonate; NITR ) 2-(R)-4,4,5,5-tetramethyl-4,5-dihydro-1H-imidazolyl-1-oxy 3-ox- ide) with R ) isopropyl for 1 8-10 and R ) methoxyphenyl for 2. They are one-dimensional alternating spin materials because NITR consists of organic bidentate radicals with spin s ) 1 / 2 which alternate along the chain with rare-earth Gd(III) magnetic ions with spin S ) 7 / 2 (the spin values are given in p units). Despite rather small structural differences between the two radicals, they exhibit different magnetic properties. In 1, the SQUID measurements 9 show a predominant nnn antiferromag- netic coupling between the two radicals (J 2 /k )-4.5 K), despite the long distance between them, a weak nn ferromagnetic coupling between the Gd(III) cation and the radical (J 1 /k ) 1.85 K), and a weak nnn antiferromagnetic coupling between the gadolinium ions (J 3 /k )-0.33 K). The three exchange pathways for 1 and 2 are schematized in Figure 1. The nnn interactions between radicals are predominant, and this situation leads to a magnetic spin frustration because all of † Laboratoire de Chimie Inorganique et Mate ´riaux Mole ´culaires, Uni- versite ´ Pierre et Marie Curie. ‡ Universite ´ Paris-Sud. § Universita ` degli Studi di Firenze. | Laboratoire de Mine ´ralogie Cristallographie de Paris, Universite ´ Pierre et Marie Curie. ⊥ Present address: Department of Materials Science, University of Patras, 26504 Patras, Greece. (1) Caneschi, A.; Gatteschi, D.; Laugier, J.; Rey, P. J. Am. Chem. Soc. 1987, 109, 2191. (2) Caneschi, A.; Gatteschi, D.; Rey, P.; Sessoli, R. Inorg. Chem. 1988, 27, 1756. (3) Caneschi, A.; Gatteschi, D.; Renard, J. P.; Rey, P.; Sessoli, R. Inorg. Chem. 1989, 28, 2940. (4) Benelli, C.; Caneschi, A.; Gatteschi, D.; Pardi, L.; Rey, P. Inorg. Chem. 1989, 28, 275. (5) Benelli, C.; Caneschi, A.; Gatteschi, D.; Sessoli, R. AdV. Mater. 1992, 4, 504. (6) Benelli, C.; Caneschi, A.; Gatteschi, D.; Sessoli, R. Inorg. Chem. 1993, 32, 4797. (7) Gatteschi, D. Magnetic Molecular Materials; NATO ASI Series E Vol. 198; Kluwer: Dordrecht, 1991. (8) Benelli, C.; Caneschi, A.; Gatteschi, D.; Pardi, L.; Rey, P. Inorg. Chem. 1990, 29, 4223. (9) Bartolome ´, F.; Bartolome ´, J.; Benelli, C.; Caneschi, A.; Gatteschi, D.; Paulsen, C.; Pini, M. G.; Rettori, A.; Sessoli, R.; Volokitin, Y. Phys. ReV. Lett. 1996, 77, 382. (10) Affronte, M.; Caneschi, A.; Sussi, C.; Gatteschi, D.; Lasjaunias, J. C.; Paulsen, C.; Pini, M. G.; Rettori, A.; Sessoli, R. Phys. ReV.B 1999, 59, 6282. Published on Web 06/12/2003 10.1021/ja034608u CCC: $25.00 © 2003 American Chemical Society J. AM. CHEM. SOC. 2003, 125, 8371-8376 9 8371