OBSERVATION OF SPIN-GAP STATE IN 2-DIMENSIONAL SPIN-1 KAGOME ANTIFERROMAGNET M-MPYNN-CENTER-DOT-BF4

Low temperature properties of the organic spin-1 Kagome' antiferromagn et, m-MPYNN . BF4, have been studied by measuring heat capacity and ma gnetic susceptibility down to 35 mK. The heat capacity maximum due to a magnetic short-range order was observed at 1.4 K, which is about hal f of the antiferromagnetic interaction 2\J\/k(B)=3.1 K in the Kagome' lattice. As temperature decreases, the susceptibility begins to decrea se below 0.24 K. The susceptibilities both parallel and perpendicular to the Kagome' lattice collapse to almost zero at the lowest temperatu re, which indicates a nonmagnetic ground state. The temperature depend ence suggests that the gap energy of the magnetic state is about 0.25 K.