NMR study of the doped chain compounds

¹ Institute “Jožef Stefan” - Jamova 39, 1000 Ljubljana, Slovenia
² Faculty of Mathematics and Physics, University of Ljubljana Jadranska 19, 1000 Ljubljana, Slovenia
³ Institute of Electronic Structure & Laser Foundation for Research & Technology-Hellas P.O. Box 1527, 711 10 Heraklion, Crete, Greece

Received: 28 July 2003
Accepted: 23 October 2003

Abstract

NMR has been measured in polycrystalline chain compounds _x () between room temperature and 4. A broad strongly shifted resonance has been found in all three cases, suggesting that tetrahedra serve as a superexchange bridge between chains. The temperature dependence of the first moment of the NMR lines demonstrates the existence of a spin-gap down to very low temperatures even in doped samples. On the other hand, the large inhomogeneous broadening of the NMR spectra and the spin-lattice relaxation data suggest that short-range antiferromagnetic correlations already develop at and coexist with the 1D magnetic excitations in the -substituted derivatives. In addition, we propose that not all NMR centres remain equivalent after doping. Namely, in -doped samples we find an intense, narrow NMR line whose origin is compatible with the deformed tetrahedra.