Physica B 329–333 (2003) 973–974 1 H-NMR study of the 2D spin-gap system m-MPYNN  BF 4 Yutaka Fujii a, *, Takao Goto b , Wataru Fujita c , Kunio Awaga c a Department of Applied Physics, Faculty of Engineering, Fukui University, Fukui 910-8507, Japan b Graduate School of Human and Environmental Studies, Kyoto University, Kyoto 606-8501, Japan c Graduate School of Science, Nagoya University, Nagoya 464-8502, Japan Abstract The organic radical salt m-MPYNN  BF 4 is characterized as a spin-1 kagom! e antiferromagnet and has been suggested to have a singlet ground state with a finite spin gap. 1 H-NMR measurements for this salt have been carried out in the temperature range down to 0:05 K and under the magnetic fields up to 70 kOe: It is found that nuclear spin– lattice relaxation rate T 1 1 increases with decreasing temperature below about 1 K under magnetic fields below about 8 kOe: Further, a gapped magnetic state is found at higher fields. Our results are interesting from the viewpoint of the crossover between gapless and gapped regimes, which has been studied intensively for quasi-1D spin-gap systems. r 2003 Elsevier Science B.V. All rights reserved. Keywords: Spin gap; Kagom! e antiferromagnet; 2D; Nuclear spin–lattice relaxation time T 1 1. Introduction The organic cation radical salt m-MPYNN  BF 4 is known as a candidate of the kagom ! e antiferromagnet of integer spins ðS ¼ 1Þ [1]. Further, great interest has been devoted to it, since the existence of the spin gap of D=k B C0:25 K between the nonmagnetic ground state and magnetic excited states has been found from the susceptibility measurement [2]. In this salt, each dimer consists of a pair of radicals ðS ¼ 1 2 Þ coupled through the ferromagnetic exchange interaction 2J 1 =k B ¼ 23 K; and the dimers form the kagom ! e lattice with antiferromag- netic interactions 2jJ 2 j=k B ¼ 3:1K: Recently, the hexa- gon singlet model was presented for this 2D spin-gap state [3]. The 2D magnetic layers are well separated from each other by anions and crystal solvents (acetone). It has been an attractive subject how a singlet-ground- state system behaves in high magnetic fields where the spin gap D collapses. Recently, from measurements of nuclear spin–lattice relaxation rates T 1 1 for 1D spin-gap systems [4], it has been found that there exists a Tomonaga–Luttinger liquid (TLL) phase in the gapless regime H > D=gm B : We are interested in studying on dynamical properties from a microscopic viewpoint for a 2D spin-gap system m-MPYNN  BF 4 in the magnetic field. We have measured T 1 1 of 1 H with a collection of tiny crystals ðo1 mm 3 Þ by use of a pulsed NMR (spin-echo) method in the temperature range between 0.05 and 300 K under the applied fields up to 70 kOe: 2. Experimental results and discussion The 1 H-NMR spectrum consists of a broad line, which is shaped by the superposition of resonances from over 20 nonequivalent proton sites and is also related to orientational distribution of microcrystals. The spec- trum is broadened as the temperature decreases and as the applied field is increased, according to the enhance- ment of the induced moment [5]. Fig. 1 shows the temperature dependences of the relaxation rate T 1 1 of 1 H at each constant frequency ranged from 5 to 300 MHz: As shown, the temperature dependency of T 1 1 below 1 K changes around 8 kOe: At lower fields, T 1 1 increases with decreasing temperature. Such a behavior of T 1 1 resembles those in *Corresponding author. E-mail address: yfujii@quantum.apphy.fukui-u.ac.jp (Y. Fujii). 0921-4526/03/$-see front matter r 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0921-4526(02)02039-2