Journal of Magnetism and Magnetic Materials 272–276 (2004) 894–895 1 H-NMR of S ¼ 1 antiferromagnets with bond alternation H. Yamamoto*, H. Kikuchi, Y. Fujii, M. Chiba Department of Applied Physics, Fukui University, Fukui 910-8507, Japan Abstract The nuclear spin–lattice relaxation times T 1 of 1 H in three model substances of the S ¼ 1 Heisenberg antiferromagnetic chains with bond alternation were measured at temperatures between 1.7 and 180 K: At low temperatures, T 1 1 of two compounds with a finite spin gap decreased exponentially with decreasing temperature reflectingthepresenceofthespingap,whereasthefield-andtemperaturedependenciesof T 1 1 ofthegaplesscompound were completely different from gapped compounds. r 2003 Elsevier B.V. All rights reserved. PACS: 75.10.Jm; 75.50.Ee; 76.60.k; 76.60.Es Keywords: S ¼ 1 bond alternating system; Spin gap; NMR; Spin–lattice relaxation time 1. Introduction S ¼ 1Heisenbergantiferromagneticchainswithbond alternation ðaJ  J  aJ Þ; where J is the exchange constant of the strong bond and a is the bond alternating ratio, have attracted much interest in theoretical and experimental studies. Most interesting pointinthe S ¼ 1bondalternatingsystemisaquantum mechanical phase transition from the gapped singlet dimer phase ð0pao0:6Þ to the gapped Haldane phase ð0:6oap1Þ via the gapless phase at a ¼ 0:6 [1]. Several model substances including Ni 2 ðMedptÞ 2 ðm-C 2 O 4 Þ ðm-N 3 ÞðClO 4 Þð1Þ; Nið333-tetÞðm-NO 2 ÞðClO 4 Þð2Þ and Nið333-tetÞðm-N 3 ÞðClO 4 Þð3Þ were found. From magnetic susceptibility and magnetization measurements, the a’s and J =k B ’s of these three compounds were determined to be 0.3, 44:2 K for compound 1, 0.45, 54:2 K for compound 2 and0.6,110Kforcompound 3 [2,3]. 1 and 2 had the energy gap, whereas 3 with a ¼ 0:6 was gapless, consistent with the theoretical expectation. Whether the spin gap is present or not will give substantial influence on dynamic properties. In order to studythespindynamics,wehavemeasuredtemperature and frequency dependencies of spin–lattice relaxation rates T 1 1 of 1 H in powder samples of 1, 2 and 3 by ARTICLE IN PRESS 1 10 10 2 10 3 0.01 0.1 1 10 (2) = 0.45 (1) = 0.3 (3) = 0.6 T 1 -1 (s -1 ) k T/ J B 42 MHz α α α Fig. 1. Temperature dependence of T 1 1 of the 1 H in powder samples of Ni 2 ðMedptÞ 2 ðm-C 2 O 4 Þðm-N 3 ÞðClO 4 Þð1Þ; Nið333-tetÞ ðm-NO 2 ÞðClO 4 Þð2Þ and Ni ð333-tetÞðm-N 3 ÞðClO 4 Þð3Þ measured at about 42 MHz: Temperature is normalized by J of each compound. *Corresponding author. Tel.: +81-776-27-8655; fax: +81- 776-27-8750. E-mail address: h-yamamoto@curie.apphy.fukui-u.ac.jp (H. Yamamoto). 0304-8853/$-see front matter r 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.jmmm.2003.11.364