Observation of spin-gap state in two-dimensional spin-1 Kagome antiferromagnet m-MPYNN center dot BF4

Low temperature properties of the organic spin-1 Kagome' antiferromagnet, m-MPYNN . BF4, have been studied by measuring heat capacity and magnetic 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 half of the antiferromagnetic interaction 2\J\/k(B)=3.1 K in the Kagome' lattice. As temperature decreases, the susceptibility begins to decrease below 0.24 K. The susceptibilities both parallel and perpendicular to the Kagome' lattice collapse to almost zero at the lowest temperature, which indicates a nonmagnetic ground state. The temperature dependence suggests that the gap energy of the magnetic state is about 0.25 K.