EXPERIMENTAL-EVIDENCE OF A HALDANE-GAP IN AN S=2 QUASI-LINEAR-CHAIN ANTIFERROMAGNET

The magnetic susceptibility of the S = 2 quasi-linear-chain Heisenberg antiferromagnet (2,2'-bipyridine)trichloromanganese(III) has been mea sured from 1.8 to 300 K with the magnetic field H parallel and perpend icular to the chains. The analyzed data yield g approximate to 2 and J approximate to 35 K. The magnetization M has been studied at 30 mK an d 1.4 K in H up to 16 T. No evidence of long-range order is observed. Depending on crystal orientation, M approximate to 0 at 30 mK until a critical field is achieved (H-c parallel to = 1.2 +/- 0.2 T and H-c pe rpendicular to = 1.8 +/- 0.2 T), where M increases continuously as H i s increased. These results are interpreted as evidence of a Haldane ga p.